摘要:

M. EBERT AND J. W. BOAG 203 THE EFFECT OF THE ENERGY OF THE IONIZING ELECTRON ON THE YIELD IN IRRADIATED AQUEOUS SYSTEMS BY T. J. HARDWICK Chemistry Branch, National Research Council of Canada, Chalk River, Ontario Received 29th January, 1952 A study of the indirect action of X- and y-radiation on aqueous ferrous and ceric sulphates shows that the reaction yield varies with the initial energy of the ionizing electrons. This effect can be explained on the basis of existing theories of the primary action of ionizing radiation in water. The interpretation in terms of the theory advanced by Gray permits the calculation of the reaction probabilities of H + H - t H z HO + OH -+ H202 and as a function of the instantaneous electron energy. It has been established for some time that the chemical effects produced by K-particles on water and aqueous systems differ quantitatively, if not qualitatively, from those produced by y-rays, hard X-rays or fast electrons.This effect has been recognized and partly explained on the basis of a difference in the specific ionization density of a-particles and electrons.1 However, in systems where electrons are the ultimate ionizing particles, e.g. irradiation with X-rays, y-rays or with dissolved beta-emitting nuclides, most comparisons of results have ignored the energy of the ionizing electrons, with consequent unexplainable anomalies in the results. ANOMALIES IN THE IRRADIATION OF CERIC SULPHATE.-AS a case in point, con- sider the investigations on the reduction of ceric sulphate in 0.8 N sulphuric acid solutions by light particle radiation.Clark and Coe 2 irradiated ceric sulphate in air-saturated sulphuric acid solutions with 50 kVp X-rays * from both tungsten * kVp is the kilovoltage peak of the X-ray power supply; keV as applied here to X-rays is the mean energy of the X-rays absorbed by the solution.204 YIELD I N IRRADIATED AQUEOUS SYSTEMS and copper targets. The yield in 0.8 N sulphuric acid, expressed in terms of G (ions reduced per 100 eV absorbed) was 5-5. Boiling the solution previous to irradiation (i.e. removing a large fraction of dissolved oxygen) had no marked effect on the yield. Haissinsky 3 irradiated ceric sulphate in 0-8 N sulphuric acid with 14 keV X-rays * from a molybdenum target. In air-free solution G = 5.2 ; in oxygenated solutions G increased to 6.2; the addition of hydrogen decreased the value of G to 4.5.With radon cc-particles plus active deposit in a substantially air-free system, G was 3.5. The author has investigated the reduction of ceric ion in 0-8 N sulphuric acid solutions by C060 y-rays.4 The yields in air-free and air-saturated solution were the same (G = 3.2); the addition of hydrogen gas to air-free solutions increased G to 6.2. The amount of hydrogen produced from air-free solutions was less than 5 % of the oxygen produced. In preliminary experiments made by the author, in which tritium water was added to the air- saturated ceric ion solution, G = 5.5 & 0.5. Although analysis of the gaseous products was not made in this case, Challenger and Rollefson 5 in a similar experi- ment report the evolution of hydrogen gas.I I I I 1 1 1 1 I I 1 1 1 1 1 1 20- - _ ~ _ _ _ FIG. 1.-The yield in the ferrous and ceric sulphate systems as a function of initial energy of the ionizing electron. The only apparent reason for these anomalies is that the energy of the ionizing electrons was different. The average energy of electrons produced by the ab- sorption of C060 y-rays in water can be estimated; the average P-energy per disintegration of tritium has been measured.6.7 However, with X-ray irradiation, particularly where an unfiltered, full-wave rectified power supply is used, the average energy of electrons resulting from photon absorption in the solution is not easily determined. Most investigators fail to give sufficient experimental details for an estimate of the mean electron energy to be made.This latter point is important. An X-ray tube with a tungsten target operating with an unfiltered full-wave rectified power supply (typical of most commercial models) at a peak voltage of 150 kV has been found to produce electrons in water of 0.5 cm depth with an average energy from 11 to 13 keV. The same machine produces the same average electron energy when operated at 30 kV peak. EFFECT OF INITIAL ELECTRON ENERGY ON THE YIELD IN FERROUS AND CERIC SULPHATE soLuTIoNs.-There are only two systems, the reduction of ceric ion in 0.8 N sulphuric acid and the oxidation of ferrous ion in air-saturated 0-8 NT. J . HARDWICK 205 sulphuric acid, where sufficient experimental work has been done to examine the effect of the energy of the ionizing electron on the reaction.The yields G for the two systems have been plotted as a function of the average initial energy of the ionizing electrons in fig. 1. The numbers on the graph refer to the source of the information which is reported in table 1. For no. 5, 6, 13 and 14, all X-ray irradiations, arbitrary values of the average initial electron energy had to be estimated from a consideration of the irradiation technique described. However, much wider limits of error are indicated by the use of areas, rather than points. The discrepancies between point no. 9 and others (no. 1, 2, 3) will be discussed by N. Miller. The data represented by dotted circles were obtained by comparing the rate of reaction for the ferrous and ceric systems under identical conditions of irradi- ation.The source used was an X-ray tube with a tungsten target, using an unfiltered, full-wave rectified power supply operated at various peak voltages. The average initial energy of the electrons produced in the sample was obtained by a crude analysis of the spectrum and a calculation of the stopping effect of filters and the sample. If we assume the interpolation of the ferrous yields in fig. 1 to be reasonable, then the yields obtained with the ceric system are those shown by the dotted circles (no. 16-19). TABLE EF EFFECT OF INITIAL ELECTRON ENERGY ON THE YIELD IN THE FERROUS AND CERIC ION SYSTEMS point 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16-19 average initial yield. source of ionizing e ~ ~ ~ ~ ~ f Ions changed remarks electrons, p~;~,boPb,edv electrons keV.Oxidation of Fez+ in air-saturated 0.8 N sulphuric acid P32 betas 697 21.1 dissolved as P32043- Co60 y -630 20.4 ion chamber comparison Ra Y -450 20.3 ion chamber comparison S35 betas 45.8 20.2 dissolved as S35042- 250 kVp X-rays 25-30 20.3 ion chamber comparison 220 kVp X-rays 20-25 19.5 ion chamber comparison H3 betas 5.69 15.6 dissolved as HTO H3 betas 5.69 15.4 dissolved as HTO Co60 y 630 15.7 calorimetric measurement Reduction of Ce4-' in air-saturated 0.8 N sulphuric acid Co60 y -630 3.2 comparison with Fez+ Ra Y -450 3.2 comparison with Fez+ S35 betas 45.8 3.2 dissolved as S35042- 50 kVp X-ray - 14 5.5 ion chamber comparison 14 kV X-ray 8-1 1 5-2 ion chamber comparison X-rays (see text) comparison with Fez+ H3 betas 5.69 5.5 dissolved as HTO references author 8 author 9 10 author 8 11 12 author 13 14 author 4 author 4 author 8 2 3 author author The data plotted in fig.1 show that as the initial energy of the ionizing electron EO decreases below about 50 keV, the yield in the ferrous system decreases, while that in the ceric system inceases. These results may be explained by the free radical distribution at the end of electron tracks. EXISTING THEORIES OF RADICAL DIsTRIBuTIoN.-Many lines of investigation. both theoretical and experimental, have contributed to an explanation of the events occurring as an ionizing particle is slowed to rest in an absorbing medium, Allen 1 has explained the effects of different types of radiation on water on the basis of the ion (and subsequent free radical) density along the particle tracks. With high energy electrons (regions of low ion density) recombination of unlike radicals to form water, or reaction with solutes, will be the chief fate of the radicals.Regions of high ion density occur only at the end of the tracks (hot spots). In such regions radical-radical reactions occur to form hydrogen gas and hydrogen206 YIELD I N IRRADIATED AQUEOUS SYSTEMS peroxide. The effect on solutes under these conditions will not be as great as for low ion density. The picture of the ionic distribution in ionizing particle tracks has been developed by Lea15 and Gray16 from a consideration of the physics of the slowing down of charged particles in water. Assuming that the initial distribution of free radicals will correspond to that of the parent ions, probabilities of inter- action between radicals can be calculated for various energies of ionizing particles.This calculation is not very precise, for the diffusion laws for the migration of free radicals contain several arbitrary constants. Interradical reactions interfere with normal migration. The reaction of radicals with solute molecules occurs under a condition where the solute molecules are uniformly distributed, but the radicals are in a large concentration gradient. In practice the agreement between these two approaches has been observed qualitatively. However, no data has heretofore existed which would link these two approaches quantitatively. From the data in fig. 1, it is possible to determine the extent to which the reactions H f H - t H z (1) and HO + OH -+ H202 (2) occur at any particular energy of the ionizing electron.The results have been extended to an explanation of heavy particle irradiation on aqueous systems. In so doing, it is unavoidable that some comparisons and explanations made by other authors be repeated, but as these explanations are now on a quantitative basis such inclusions are made for the sake of completeness. The postulate of significant energy " bursts " in particle tracks, and the subsequent effect on chemical reactions will be discussed. The formation of radicals from ions has been described in detail elsewhere.ll179 18 However, it is important to note that these radicals are formed in an excited state. Degradation to the ground state will take place by reaction, by photon emission, or by deactivating collisions with the water molecules.In regions of high ion density the average kinetic energy of the water molecules in and surrounding the track will be greatly increased, making possible reactions not otherwise occurring at ordinary temperatures. REACTIONS BETWEEN LIKE RADICALS.-The reaction H + H + H 2 (1) will take place on collision, the excess energy being removed by the medium. Hydroxyl radicals may react in two ways : OH + OH + H202 (2) OH + OH -+ H20 + 0 (3) From considerations of dipole attraction, reaction (3) is more probable between radicals in thermal equilibrium with the solvent,lg and there is evidence that this is the case.4 The activation energy of reaction (2) has been estimated at 5.5 kcall mole.1 The formation of hydrogen peroxide can occur in regions of high free radical density for two reasons: (i) as free radicals are formed in the excited state, the probability of collisions between hydroxyl radicals before both have been de- activated will be greater at high concentrations; (ii) in the region of high ion density the kinetic energy of the solvent molecules is increased to the point where the activation energy is supplied by the solvent.A schematic picture of the OH-OH reaction as a function of energy is shown in fig. 2. At low electron energies (region L) collision between hydroxyl radicals will occur before significant outward migration begins. In the region of high ion density (hot spot) the product will be almost completely hydrogen peroxide.AtT. J. HARDWICK 207 slightly higher electron energies (- 5-10 keV) there is migration from the tracks, but collisions between OH radicals will occur within a short interval of time. There is still sufficient energy available from the medium for hydrogen peroxide forma- tion (region M). Beyond a certain distance from the track there will no longer be sufficient energy available for reaction (2) ; the reaction between two hydroxyl radicals will form water and oxygen (3). When a solute capable of reacting with hydroxyl radicals is present in region L, practically no solute-radical reaction occurs. In regions M and N, solute-radical reactions take place, but in the former there is probably a concentration effect of the solute. Region N is the region of concentration independence.Special mention must be made of dissolved hydrogen gas. In region N the hydroxyl radicals will be converted quantitatively to hydrogen atoms (4) H2 + OH -+ H20 + H which are then free to react with themselves or with any solute. In region M any hydrogen atoms produced by reaction (4) will react with OH radicals because of the relatively high concentration of the latter, the net result being the elimination of two hydroxyl radicals from the system. Y rom cenira / where reachon ( B ) becomes more probable Man T / reochon (A) Lmyy of Iemzihy dectron - FIG. 2.-Schematic representation of the OH-OH reaction at various energies of the ionizing electron. EFFECTS CAUSED BY THE SLOWING OF FAST ELEcTRoNs.-(i) At energies greater than 15 keV.-When the instantaneous energy Ex of the ionizing electron is greater than 15 keV successive radical pairs are formed relatively far apart H20 -+ H + OH.(R) These radicals diffuse out of the tracks and are all available for reaction with solute molecules. In the absence of a solute, recombination of unlike radicals to form water occurs. A portion of the electron energy is released in '' bursts " of several hundred electron volts,l giving a region of high specific energy release. This results in the formation of H2 and H202 directly, either through immediate radical com- bination, or by excitation rearrangement 1 3 ~ 2 0 2H20 -+ H2 + H202. (F) The magnitude of this effect has been measured by Hart, by Hochanadel and by Johnson on a variety of systems.The results are recorded in table 2. Thus, in the tracks of high energy electrons, 20-25 % of the water molecules which de- compose form H2 and H202 directly, the remaining 75-80 % forming free radicals which are free to migrate.208 YIELD IN IRRADIATED AQUEOUS SYSTEMS (ii) At energies less than 15 keV.-As the energy of the ionizing electron is decreased below 15 keV, both the number of energy bursts and the amount of energy released per burst becomes smaller. Any contribution from this source to the overall reaction will not be significant; in the following discussion on the effect of low energy electrons any products from bursts are ignored. As Ex decreases below 15 keV, the initial distribution of the radicals becomes such that the probability of like radicals reacting within the track increases. This will occur first between hydroxyl radicals, because of their higher local con- centrations.For reasons outlined previously the product will be hydrogen peroxide. The reaction of hydrogen atoms to form hydrogen gas within the track becomes significant at Ex < 4 keV. TABLE 2.-EXTENT OF IMMEDIATE HYDROGEN AND HYDROGEN PEROXIDE FORMATION IN FAST ELECTRON TRACKS relative number of water molecules product source of decomposed by ref. measured irradiation reaction system irradiated R F HCOOH in 02-saturated solution H2 COG0 y 0.79 0-21 20 dilute Orfree KBr solution H202 7, 0.75 0.25 14 ferrous ion in air-saturated 0.8 N &So4 H2 ?, 0-78 0.22 14 H202 in KBr and KI in water H2 2 MeV electrons 0.80 0.20 21 This picture of the behaviour of free radicals explains the decrease in yield in the ferrous system at low Eo.The formation of hydrogen peroxide from two hydroxyl radicals will not affect the yield, as in each case reaction with ferrous ion gives the same products. The decrease in yield must be ascribed to the in- creased formation of hydrogen at the end of the tracks. The increase in yield in the ceric system may be explained by the increased formation of hydrogen peroxide at Ex < 15 keV. As neither ceric nor cerous ions react appreciably with hydroxyl radicals, the increase in yield will be due to the increase of hydrogen peroxide formed within the tracks plus that formed near the tracks after outward diffusion has begun (region L and M, fig. 2). At Ex < 4 keV increased hydrogen gas formation will cause a decrease in yield.A maximum in the yield would therefore be expected at low values of Eo. PROBABILITY OF LIKE RADICAL REACTIONS AS A FUNCTION OF &.-In practice one cannot measure the results of an instantaneous radical distribution at a particular Ex, but must measure the sum of all the effects which occur as an electron of initial energy Eo is slowed to rest. This complicates the problem and renders any interpretation of the results less certain. However, from a knowledge of the mechanism and yield of the ferrous oxidation and ceric reduction one can create a picture of the reactions occurring in the radical tracks. The magnitude of the interradical reactions can be calculated from the change in yield measured at different values of Eo.From the data on the ferrous system (curve A, fig. 1) the extent of hydrogen gas formation can be found ; from data on the ceric system (curve B, fig. 1) the hydrogen peroxide production may be calculated. (i) The reaction H + H --f Hz.-The fraction of the hydrogen atoms combining to form hydrogen gas (FH) will increase as the instantaneous energy of the ionizing electron El decreases, (FH = j { E I ) ] . The area under this curve, f(Er)dEr, will be proportional to the decrease in yield found in the air-saturated ferrous oxida- tion at low values of Eo. If, in general, two ferrous ions are oxidized for each J",T . J . HARDWICK 209 H atom formed,* the value of [: f(E)dEI can be calculated (see appendix). J Co Furthermore, from estimates of the initial H distribution in the track, one would not expect hydrogen formation within the track at energies greater than 4 keV ; below this energy FH would increase gradually.Applying these limitations to the value of 1 f(Er)dEr (= 0.70) one obtains curve A in fig. 3. (ii) The reaction HO + OH -+ H202.-The extent of formation of hydrogen peroxide from hydroxyl radicals at low values of EI may be calculated from in- crease in yield in the ceric ion system as EO decreases. The equation FOH = fQ3) will express the fraction interacting as a function of instantaneous electron energy. The area under this curve f(Er)dEI may be calculated from the variation in yield 0 EO 0 s Eo FIG. 3.-Probability of like radical reactions as a function of the instantaneous energy of the ionizing electron.with EO (curve B, fig. 1). Applying the limiting conditions described below, curve B (fig. 3) is obtained. s( f(EI)dEI = 6.0). ) 0 EO There are three limiting conditions to the equation FOH =f(EI). (i) At low Er the formation of hydrogen gas within the tracks causes a de- (ii) The radicals are assumed to have a high probability of interreaction in (iii) Both the hydrogen peroxide formed within the tracks and after the onset This latter process will predominate at higher crease in yield as fewer H atoms will be available for reaction with ceric ion. this region, hence FOH will approach unity. of migration will be measured. * The quantity 2 is the average yield for the two sets of reactions H + 0 2 + HO2 H + 0 2 ++ HO2 H02 + H -+ H202 H+ + HO2 + Fez+ + H202 + Fe3+ j' H202 + 2Fe2+ +- 2Fe3+ + 20H- '(H02 + H02 +- H202 t 0 2 Fe3+ formed ratio -_____ : 3 H used H202 + 2Fe2i.--f 2Fe3-k + 20H- Fe3 formed ratio = 1. H used The use of any other quantity between these limits does not greatly affect the value of210 YIELD I N IRRADIATED AQUEOUS SYSTEMS E,, but as it involves a diffusion mechanism before reaction, the increase in the amount of hydrogen peroxide formed will vary only gradually with EI. The shape of curve B at high El is therefore somewhat uncertain. Although the curves in Fig. 3 follow the restrictions imposed, the loci are somewhat speculative. However, values of G at various EO calculated from the area under the curve (or portion thereof) agree with those obtained from the curves in fig.1. From the shape of curves A and B in fig. 3, one would expect hydrogen gas and hydrogen peroxide to be the products in regions of much higher ion density, for example, in a-particle tracks. When pure water is irradiated with a-particles, hydrogen peroxide and hydrogen gas are formed initialIy in high yield (GH2 = 0.9-2-5).22-25 The formation of oxygen does not occur until a consider- able concentration of hydrogen peroxide has been built up. APPLICATION To OTHER SYSTEMS.-(I) Irradiation of water.-The reactions of water under various types of radiation have been successfully explained by application of Allen’s “ hot-spot ” theory.l.26 When the above results are applied to the irradiation of pure water qualitative agreement is found in all cases. Sufficient experimental data on the reaction of low energy X-rays on pure water are not available for quantitative comparison.(ii) Irradiation of the ceric and ferrous systems with a-particles.-The reactions of solutes in solutions irradiated with a-particles will be mainly those of hydrogen peroxide and hydrogen. The yield in the ceric system has been found to be about the same as with y-rays, although reduction occurs through hydrogen peroxide (GP = 3*2,4 G, = 3.5 3). The irradiation of gas-free ferrous solutions with a-particles should give a similar yield ; experimentally G = 3.05.28 (iii) Irradiation of Iiydrogen-saturated cerric ion solutions.-The discrepancy which occurs on irradiation air-free ceric ion solutions containing dissolved hydrogen (first section) is explained by the various reactions occurring at different electron energies (fig.3). With high energy electrons the reaction H2 + OH -+ H2O + H (4) provides a new reducing atom which results in an increase in yield. With low energy electrons hydrogen reacts with those OH radicals which diffuse a short way from the track, but which would ultimately form hydrogen peroxide. The presence of one hydrogen molecule in this region results in two hydroxyl radicals reacting with it to form water, with consequently fewer ceric ions reduced. The yields in the system, (i) ferrous ion in air-saturated 0.8 N sulphuric acid and (ii) ceric ion in air-saturated 0.8 N sulphuric acid under light particle irradi- ation have been examined as a function of the initial energy of the ionizing electron.From the change in yield occurring at known initial electron energies, the fraction of like radicals reacting has been determined as a function of the instantaneous electron energy. When the predictions inherent in this picture have been com- pared with experimental results obtained on a variety of systems, including those irradiated by a-particles, satisfactory agreement has been found. CoNcLusIoNs.---The results place Allen’s “ hot-spot ” theory of reactions in particle tracks on a quantitative basis. They confirm that the initial ionic dis- tribution proposed by Gray may be used to predict the probability of solute- radical and radical-radical reactions in an irradiated aqueous system.T. J . HARDWICK 21 1 A P P E N D I X 0 Calculation of S, f(Er)dEl.Reaction H + H --f Hz. From curve A, fig. 1 : EO > 100 keV EO = 10 keV G = 17.6 hG = 2.8 2.8 Fractional change in G __ = 0.14. 20.4 Fraction of H atoms combining to form H2 = 0.07 G = 20.4 ferrous ions oxidized per 100 eV. Y Y Y, 9 , Y Y Y, 2, 9 , Y Y Each H atom removed by H2 formation results in two less ferrous ions oxidized. f(EJdE1 = 0.7. Reaction HO + OH* -+ H202. From curve B, fig. 1 : EO = 10 keV G = 4.8 ceric ions reduced per 100 eV. EO > 100 keV G = 3.2 Y Y Y, 9 , Y Y AG = 1.6 Y, Y, 9 , ,, 1.6 3.2 Fractional change in G - = 0-50. Each OH radical forming H202 causes the reduction of one ceric ion. The formation of H2 at the end of the electron tracks decreases the number of ceric ions reduced. A correction (0.07(v.s.)) must therefore be added to the fractional change in G. Fraction of OH radicals combining to form H202 = 0.50 + 0.07 = 0.57 :o f (Er)dEI = 5.7. Similarly, at Eo = 20 keV, G = 3.95 ceric ions reduced per 100 eV l:,f(EJdEl = 6.0. 1 Allen, J. Physic. Chem., 1948, 52, 479. 2 Clark and Coe, J. Chem. Physia, 1937, 5, 97. 3 Haissinsky, Lefort and Le Bail, J. Chirn. Phys., 1951, 48, 208. 4 Hardwick, Can. J. Chem., 1952, 30, 23. 5 Challenger and Rollefson, AECU-1187. 6 Jenks, Sweeton and Ghormley, ORNL-333. 7 Hanna and Pontecorvo, Physic. Rev., 1949, 75, 983. 8 Hardwick, Can. J. Chem., 1952,30, 39. 9 Hardwick, Can. J . Chem., 1952,30, 17. 10 Miller, J. Chem. Physics, 1950, 18, 79. 11 Todd and Whitcher, AECU-458. 12 Weiss, private communication. 1 3 Gordon, Hart and Walsh, AECU-1534. 14 Hochanadel, paper presented at Amer. Chem. SOC. meeting (Cleveland, April, 1951). 15 Lea, Action of Radiations on Living Cells (Cambridge University Press, 1946), chap. 1 . 16 Gray, J. Chim. Phys., 1951, 48, 172. 17 Weiss, Nature, 1944, 153, 748. 18 Dainton, J. Physic. Chem., 1948, 52, 490. 19 Weiss, Trans. Faraday SOC., 1940, 36, 856. 21 Johnson, J. Chem. Physics, 1951, 19, 1204. 22Duane and Scheuer, Radium, 1913, 10,33. 23 Nurnberger, J. Physic. Chem., 1934, 38, 47. 25 Lanning and Lind, J . Physic. Chem., 1938, 42, 1229. 26 Lefort, J. Chim. Phys., 1950, 47, 624. 27 Allen, Hochanadel, Ghormley and Davis, AECU-1413. 28 Hart, private communication. 20 Hart, ANLJ4636.

ISSN:0366-9033    

DOI:10.1039/DF9521200203

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

THE y-RAY AND X-RAY INDUCED POLYMERIZATION OF AQUEOUS SOLUTIONS OF ACRYLONITRILE BY E. COLLINSON AND F. S. DAINTON The Chemistry Department The University Leeds 2 Received 1 1 th February 1952 Polymerization induced by the action of radium y-rays 220 kV and 50 kV X-rays on aqueous solutions of acrylonitrile has been studied. Radiation dosimetry was by measure- ment of the oxidation of ferrous ions in all cases. Polymerization in D20 solution fol- lowed by infra-red spectroscopic analysis of the polymers produced gave direct proof of the formation of D atoms in the irradiated liquid and confirmed their participation in the polymerization. The kinetics of the polymerization were studied over the available concentration range and with dose rates ranging from 6.6 X 1012 to 33,400 x 1012 ion pairslml min.At low dose rates the kinetics were consistent with a non-uniform distribution of the radicals formed from the water whilst at high dose rates an increasing degree of uni- formity was indicated. There was apparently no difference in the effects produced by the three types of radiation. A possible explanation of these results is that all three radiations give rise to randomly distributed volume elements of radicals in water in which chemical reaction takes place the volume elements being widely separated and independent of each other at low dose rates but tending to overlap at high dose rates. Though this hypothesis is not finally established the viscosity molecular weights of the polymers showed a dependence on dose rates which is in general agreement with it.A useful method for the detection and identification of radicals formed in water is the study of the polymerization of dissolved vinyl compounds which the radicals may initiate. The investigation described in this paper was undertaken as an application of this method to the problem of the number and type of radicals formed when high energy radiation is absorbed by water. Jn the course of the work it became evident that this method might also be used to explore any non-uniformity of the initial distribution of the initiating radicals. This feature arises from the fact that addition polymerization reactions are chain reactions and the expression for their rates depends on more than one rate constant. The kinetics of such reactions are therefore more sensitive to changing conditions of radiation intensity reactant concentration and radical distribution than are the kinetics of reactions which are complete in a single stage.Since the maximum radiation intensity available to us when this work was begun was much smaller than can be used in most photochemical reactions it was necessary to select a monomer having as large a water solubility as possible. Acrylonitrile was therefore used in spite of certain other disadvantages. Experi- ments have also been made with methacrylonitrile and methyl methacrylate but measurements on these substances are not yet complete and are not described here. EXPERIMENTAL RADrATroNs.-Radium y-rays 220 kV X-rays from a G.E.C.Maximar radiotherapy unit operated-at 15 mA and 50 kV X-rays from a Machlett AEG-50-T tube operated at 26 mA were employed. Two radium sources of 400 and 232.5 mcurie strength screened by the equivalent of 0.5 mm of platinum were available. In the majority of experiments 212 21 3 the 220 kV irradiation was unfiltered but in some experiments 0.5 mm of aluminium filtration was employed. The 50 kV radiation was unfiltered. DOSlMETRY.-For each type of radiation the chemical method recommended by Miller 1 was employed. In this method the chemical reaction induced is the oxidation of ferrous to ferric ions in aerated aqueous solutions the solutions being 0.8 N in sulphuric acid and the chosen initial concentration of ferrous ion lying in the range 10-3 to 10-4 M.Under these conditions the rate of oxidation of ferrous ions is independent of their con- centration and the reaction has been calibrated by Miller for the measurement of dose rates in the wavelength region lying between the wavelengths of radium y-rays and 200 kV X-rays. Over this wavelength range the ionic yield remains unchanged and it is also independent of dose rate up to 4200 r/min.2 Though extrapolation of the results to 50 kV X-rays has yet to be justified experimentally the work of Fricke 3 indicates that much the same result is obtained for wavelengths at least as large as those of 100 kV X-rays. Dosimetry of 50 kV X-rays is difficult by any method. The chemical method was therefore considered as likely to give accurate values as any other technique under the conditions employed for polymerization.The water used for preparing the solutions was twice distilled the second distillation being from alkaline potassium permanganate using a Pyrex glass still incorporating an anti-splash column. The still and any other apparatus coming into contact with liquid E. COLLINSON AND F. S. DAINTON < 5.4cm JII FIG. 1 .-The cells used for y-irradiations. to be irradiated was scrupulously cleaned with a nitric + sulphuric acid mixture rinsed with pure water and then treated with steam uncontaminated by organic impurity. Solutions were prepared from A.R. ferrous ammonium sulphate and A.R. sulphuric acid. The concentration of ferrous ion before and after an irradiation was determined by measuring the optical density of the solution at 508 mp after the addition of ortlzo- phenanthroline which forms a complex with ferrous ions.For dosimetry the cells were always irradiated under exactly the same conditions as were to be used for polymerization. The solutions were unstirred during the irradiations and in measuring the integral dose rate care was taken to ensure that the exposure time was below that required either to denude the solution of air (50,000 r) or to reduce the concentration of ferrous ion below lO-4M at the position of highest dose rate in the cell. Irradiation times were also arranged to cause a total concentration change of about one-half the initial concentration.1 On completion of the irradiation a suitable volume of the solution was made up with ortlzo-phenanthroline at a pH neutral to Congo Red to give an optical density of about 0.43 in a 10 cm cell.The optical density was accurately measured by means of a Beckmann DU spectrophotometer. From the change in con- centration and Miller's value of M / N = 6.7," the dose rate in ion pairs i.p./ml min could be calculated. This unit is used throughout and is equivalent to expressing the dose rate in terms of energy absorbed assuming W,,, = 32.5 eV. THE CELLS AND DOSE RATES.-FOr y-irradiations three Pyrex glass cells of differing dimensions (fig. l) used in conjunction with the two radium sources provided four different dose rates viz. 71.8 & 4-1 ; 48.8 rrt 2.5 ; 26.3 i 0-7 ; 6.62 -ir. 0.53 i.p./ml * This value actually applies to 0.8 N sulphuric acid.Allowance for the difference in stopping power between this and water leads to the value of 6-43 for water.2 The dose rates quoted here should be corrected by this factor. POLYMERIZATION OF ACRYLONITRILE FIG. 2.-The dependence of X-ray dose rate on distance from the focus. (i) 220 kV filtered X-radiation (ii) 220 kV unfiltered X-radiation (iii) 50 kV unfiltered X-radiation 0 -physical measurements (3 -chemical measurements (assuming Wwater = 325 eV) 0 -chemical measurements-cell in thermostat. 214 min x 10-12 the errors being expressed as standard deviations of several experimental values. Calculations showed that the largest variations in dose rates obtained was that in cells of type 11. For the integral dose rate of 48.8 i.p./nil niin x 10-12 the maximum dose rate was 294 i.p./ml min x 10-17 and the minimum 13.4 i.p./ml min x 10-12.For 220 kV X-irradiations flat cylindrical Pyrex glass cells 5 cm diam. and 1 cm deep were used. Dose rates were varied by adjusting the distance between the cell irradiated and the focus of the X-ray tube. Estimates of the integral dose rates in the cells were first derived from measurements in air with a Victoreen condenser chamber dosimeter. These measurements indicated that over the range covered the radiation intensity was in accord with the inverse square law. Measurement of the absorption by the glass layers of a cell and the liquid in a cell showed that the integral dose rate was within 0.2 % of that at the centre of the cell and that the variation in dose rate across the cell was ap- proximately 13 % being slightly higher for the unfiltered radiation.Air dose rate measurements at the position of the centre of the cell enabled estimates to be made of the integral dose rates. Comparative results for the physical and chemical measurements are given in fig 2. The differences are not surprising considering the elementary nature of the absorption measurements but the physical measurements were useful in giving estimated values of the dose rates thus simplifying the use of the chemical method. In experiments in which the cells were immersed in a water thermostat the dose rates were noticeably higher than for the same cell position in air. Since the dose rates appeared to be in good accord with the inverse square law the values used in interpretation of the polymerization results were corrected by interpolation on the inverse square law plots.The following considerations determined the design of the cell for 50 kV X-irradi- ations. The cell size was a compromise between the volume requirement to give reason- able weights of polymer a diameter which would be wholly included in the X-ray beam and a depth which would not by absorption of the radiation give rise to a large change in dose rate between the top and bottom of the cell. Moreover the upper surface of the cell had to be as thin as possible to minimize absorption. The cell was of Pyrex glass cylindrical in shape 0.5 cm deep and 2.9 cni diam. The upper glass surface was approximately 0.05 mm thick.Dosimetry of the 50 kV irradiations was exclusively by the chemical method. It was to be expected that there would be an appreciable dose rate variation across the cell and irradiation times for dosimetry were cut down suc- cessively until no further apparent increase in dose rate occurred. This indicated that there was then no lowering of Fez+ concentration below 10-4 M in any part of the cell. In order to estimate the value of the highest and lowest dose rates in the cell measurements E . COLLINSON AND F. S. DAINTON 215 were made with open topped cells of (i) the same and (ii) double the depth of the covered cell. From such measurements the absorption of radiation due to the glass top and to the layer of liquid could be estimated. From the estimate of the absorption coefficient in water the effective wavelength of the radiation was approximately 1.2A.The dosi- metric measurements on 50 kV X-rays are given in fig. 2. The three dose rates employed appear to conform reasonably well with the inverse square law but since this cannot be regarded as established the polymerization dose rates were taken to be as measured. At the maximum integral dose rate of 33.4 x 10-15 i.p./ml min the dose rates at the top and bottom of the cell were 91.6 x 1015 and 7.3 x 1015 i.p.,/ml niin respectively. THE KINETIC STUDIES.-ACrylOnitrile was obtained from 1.C.I. Ltd. or from L. Light Ltd. After drying over anhydrous sodium sulphate the monomer was fractionally distilled through a vacuum-jacketed column filled with single spiral glass helices and having an efficiency of about 15 plates.The distillations were carried out under high vacuum and the middle 50 % of distillate was collected for use. Pure monomer was stored in the dark under vacuum and was distilled off as required. PROCEDURE FOR y- AND 220 kV X-RAY IRRADuTIoNs.-Jn the earlier experiments polymerizations were followed by separation and weighing of the polymer. Later re- actions were followed by means of dilatometers the capillaries being of various diameters depending on the concentration of the monomer. The reaction cell was first connected through its capillary tubing to a glass bulb and the whole thoroughly cleaned. A known volume of water was then pipetted into the bulb and de-aerated by three successive freez- ings in liquid air followed by evacuation and remelting.A sample of distilled monomer was weighed into a clean dry tube which was then attached to the vacuum line. After de-aeration of the monomer by the same procedure the monomer was distilled into the bulb. An appropriate volume of the solution was run into the reaction cell by inverting the bulb and cell and the latter was then sealed off for irradiation. The dilatometer was immersed in a thermostat maintained at a temperature of 25.00 & 0.01" C and the level of liquid in the capillary was read to an accuracy of 0.005 cm by means of a reading microscope. Irradiation was then begun. In the initial stages of an irradiation the cell was removed and examined visually at short intervals of time in order to ascertain the time of onset of polymeriiation.After a suitable irradiation period the cell was replaced in the thermostat and when temperature equilibrium was attained readings were taken. The time required for temperature equilibration was cut down in later experiments by maintaining the cells at 25" C throughout the irradiation. Further irradiations were then given and readings were taken in this way until the end of therun. The dilatometer was then opened to the atmosphere and the polymer was filtered off into a sintered glass crucible of porosity 4 which was left to dry to constant weight in a vacuum desiccator. The final dilatometer contraction and the weight of the polymer made possible the construction of a polymer weight against time curve.PROCEDURE FOR 50 kV X-RAY IRRmrATroNs.-Owing to the mechanical weakness of the upper window of the cell used for these experiments work under vacuum was made difficult. The solutions were therefore de-aerated in a separate vessel and then saturated with nitrogen. For the purpose of filling the cell was attached to the de-aeration bulb by a joint and after blowing a little nitrogen through the system some of the solution was run into the cell which was then sealed to prevent access of air. The disadvantage of this method of preparation was that the solutions were liable to contain small amounts of air and tap grease. Experiments with y-rays indicated that the ultimate rate attained after a longer inhibition period brought about by the oxygen in a solution which had not been de-aerated was within 0.5 % of the rate in a normally de-aerated solution.y-ray runs in the presence of added tap grease showed slightly longer inhibition periods but the ultimate rate was never more than 3 % from the normal rate. Thus although the 50 kV X-ray polymerizations carried out by the above technique were not expected to give the same order of accuracy as those done in IJ~CUO yet the inaccuracies introduced were sufficiently small for the method to give useful results. A sufficient volume of solu- tion could be prepared at one time in the above manner to enable a complete weight polymer against time curve to be plotted each polymerization providing one point on such a curve. INVESTIGATIONS OF THE POLYMER CHAIN LENGTHS.-A~~ such measurements have SO far been viscometric.The solvent employed for polyacrylonitrile was dimethyl form- amide. Some solvent was obtained from E. I. Du Pont de Nemours Inc. and some from I.C.I. Ltd. It was distilled under vacuum using a simple spiral fractionating column. The fraction distilling between 52.5" C and 55" C under a pressure of 19 mm of mercury 216 was collected for use. Its viscosity was 0.8602 centistokes at 25.00 & 0.01" C. The dis- tilled solvent was kept in a glass-capped Pyrex flask. The viscometer was of the Ostwald design and conformed to the specification of type No. 1 (B.S. 188 1937). It was calibrated using pure water. Kinetic energy corrections were applied in the manner described in the above pamphlet. All viscosities were measured at a temperature of 25.00 1 O*Ol" C.For each polymer sample the viscosity of solutions of at least three concentrations were measured the lowest concentration being approximately 0.06 g/100 ml. Extrapolations of plots of both rlsp/c and In ~ J c against concentration to zero concentration were used to give the best estimate of the intrinsic viscosity. Most of these plots and particularly those for the higher molecular weights showed an upward curvature probably due to the fact that the polymers were unfractionated. POLYMERIZATIONS INDUCED BY IONIZING RADIATlONS IN DEUTERIUM OXIDE SOLUTI0N.- The deuterium oxide was 99.75 % DzO. It was distilled under vacuum before use. Tnfra-red spectroscopic analyses were carried out by means of a Perkin Elmer single beam spectrometer.Experiments were performed using both 220 kV and 50 kV X-rays. Preparation of monomer solutions was by the usual techniques but in order to have the best chance of detecting absorption by the end groups i.e. to obtain lowest molecular weight product the highest possible dose rate and lowest possible monomer concentration were used. After completion of the irradiations which were taken to as high a percentage con- version as possible the deuterium oxide was distilled away from the polymer under vacuum and the polymer was gently warmed (to about 35" C) under vacuum for several hours. It was then dissolved in dimethyl formamide to give a limpid yellow solution. A thin film of the polymer was made by putting some of this solution on a rock salt plate and evaporating the solvent under vacuum for 2 days continuously after which the film was covered by another rock salt plate.Control polymers were prepared from solutions in water and from irradiation of the pure monomer. The regions of the infra-red spectrum examined were those in which the characteristic 0-D or C-D bond frequencies might be expected to occur. C-D vibration frequencies generally occur in the region of 2150 to 2200 cm-1 e.g. 2204 cm-1 (CH3D) 2180 cm-1 (CzHsD) 2170 cm-1 (C3H7D) and the 0-D frequency near 2500 cm-1 e.g. 2490 cm-1 (CC13OH).4 PROOF OF INDIRECT ACTION.-h POLYMERIZATION OF ACRYLONITRILE RESULTS view of the much higher concentration of water in the irradiated solutions the polymerizations were almost certain to be due to indirect action.Nevertheless it is known that single monomers can be polymerized by the direct action of radiations 5 and it was important to know whether appreciable percentages of the effects were due to direct action. Benzene apparently has an inappreciable effect on the rate of polymerization of acrylo- nitrile in the pure state.5 Therefore it seems most likely that few radicals initiating poly- merization are produced from benzene and that benzene does not inhibit polymerization. Consequently a reasonable measure of the amount of polymerization occurring at a given concentration due to direct action on the monomer should be obtained by irradiating a solution of monomer of the same concentration in benzene. Using a monomer concentration of 0.485 M in dry de-aerated benzene solution and irradiating by y-rays at a dose rate of approximately 43 x 1012 i.p./ml min in the usual way no visible polymerization occurred for about 60 h.The amount of polymer formed even after 727 h was only equivalent to that formed in aqueous solution in 4.1 h. It is therefore clear that direct action plays a negligible part in the polymerizations. PROCESSES OCCURRING DURING PoLYMmrzATIoN.-In general the water-insoluble polymer was substantially the only product formed from the monomer. This was demonstrated by carrying out prolonged irradiations of monomer solutions. Using monomer concentrations from 0.5 M to 0.1 M it was found possible to polymerize the solutions to at least 95 % and with longer irradiations they would probably have been polymerized further.Even at the lowest concentration used in the kinetic work (i.e. 0.036 M) the percentage polymerization could be taken to at least 89 %. At very high dose rates and low concentrations as will be described later semi-liquid very low molec- ular weight products were formed but such products did not generally occur in any appreciable amounts. 217 Y/ FIG. 3.-The dependence of the inhibition period on dose rate and monomer concentration in the y-ray polymerizations I (i) 0 -6.62 x 1012 i.p./ml. min @ -26.3 x 1012 , , 0 -71.8 x 1012 , , (ii) Values interpolated from (i) at l/(monomer concentration) = 7.5. all the polymerizations small inhibition periods the length of which depended on the dose rate and the monomer concentration were found.These were of the order minutes or seconds whereas irradiation of solutions containing air led to inhibition periods of several hours. It appears therefore that the inhibition periods previously reported,7 and the smaller ones in the present work may have been due to residual oxygen in the solutions. E . COLLINSON AND F . S . DAINTON NO hydrogen peroxide was detected in the experiments using y- or 220 kV X-rays. The reagent used was titanic sulphate in 0.5 N H2SO4 solution the concentration limit of detection with 10 cm cells in the Beckmann spectrophotometer being 3 x 10-7 M. I n the 50 kV experiments hydrogen peroxide was detected in every solution examined. As the de-aeration of these solutions was neither as efficient nor as reproducible as that for y- and 220 kV X-rays estimates of the concentrations of hydrogen peroxide could be made sufficiently accurately by visual comparisons with standard solutions.The following facts were established (i) the production of hydrogen peroxide did not cease at the end of the induction period but continued throughout the whole period of irradiation ; (ii) the rate of hydrogen peroxide production appeared to be independent of the monomer concentration and increased with increase in dose rate. NO attempt has yet been made to detect and measure gaseous products which may be formed during the polymerizations. Preliminary experiments in this direction have shown however that small amounts of gas are evolved.6 (> -48.8 X 1012 , , THE INHIBITION PERIOD.-h Using the standard method of de-aeration the measured inhibition periods for the y-ray polymerizations were found to be proportional to [rnll-l and R-1 where R is the dose rate (fig.3). Owing to the shortness of the times involved and the detection of the onset of polymerization by intermittent visual inspection the precision of these observa- tions was not high. The inhibition periods found in the 220 kV and 50 kV X-ray polymer- izations appeared to be in general agreement with the above relationships but in the former case they were too small (a few seconds) and in the latter case the degree of de-aeration was insufficiently constant to give precise results. THE KINETICS.-AII the weight of polymer against time curves were sigmoid the in- hibition period being followed by a period of acceleration which tended to decrease with increasing concentration or dose rate.Since the duration of this acceleration period appeared to respond to dose rate and concentration changes in a manner similar to that of the inhibition period it seemed most likely that this was caused by oxygen not destroyed 218 in the inhibition period though it could equally well have been due to some other unknown impurity. Using highly purified materials (monomer fractionated three times and water twice distilled under vacuum in addition to the normal procedure) the inhibition period the acceleration period and the rate of polymerization remained almost unchanged. The acceleration may also merely have been that associated with the growth of the con- centration of growing polymer chains to its stationary value the fact that a post- irradiation polymerization was found apparently supporting this view.The magnitude of the post-irradiation polymerization varied from about 4 ”/ of the radiation polymer- ization rate at low dose rates to about 0.7 % at high dose rates. However the observation that there was no period of acceleration when a system was re-irradiated after the post- irradiation reaction had ceased indicates that the acceleration was not due to this cause. In calculating the rate of polymerization appropriate to a given concentration from a polymer weight against time curve the point at which the period of acceleration ended POLYMERIZATION OF ACRYLONITRILE 0 -6.62 x 1012 i.p./mi niin FIG.4.-The polymerization of acrylonitrile by y-rays. 0 -226.3 x 1012 i.p./ml min , C -71.8 x 1012 , 8 -48.8 x 10” , , (assuming Wwater = 32.5 eV). was taken as the starting point of the polymerization. As the polymerization progressed the rate tended to become higher than the rate of a polymerization started at the cor- respondingly lower concentration. Such behaviour could be due either to continued removal of the retarder or to the production of an accelerator. The most likely retarder is oxygen and the only obvious possibility for an accelerator is the polymer itself. No hydrogen peroxide was formed during the y- and 220 kV X-irradiations and it was also confirmed that addition of hydrogen peroxide to the polymerizations gave no change in polymerization rate which was outside experimental error.In order to check the effect of dead polymer on the rate of polymerization a sample of polymer was shaken with pure monomer and with several washings of distilled water and was then added to a solution to be polymerized. Retardation rather than acceleration was experienced but it is possible that an inhibitor was introduced with the polymer in spite of the care taken to ensure its purity. It seems unlikely that an acceleration was caused by coagulation assisting the termination reaction since the continual movement of the cells from their irradiation position to the thermostat invariably gave rise to coagulation at an early stage. The dependence of the rates of polymerization on dose rate and monomer concentra- tion for each type of radiation is presented in fig.4 5 and 6. These results may be summarized as follows. E. COLLINSON A N D F . S . DAINTON 219 The order of reaction with respect to mottomer concentration [ml].-For y-rays in the concentration range 0.038 M to 0.75 M and with dose rates ranging from 6.62 to 71.8 i.p./ml rnin x 10-12 and for 220 kV X-rays in the concentration range 0-04 M to 0.75 M and with dose rates ranging from 30.8 to 11 30 i.p./ml min x 10-12 the reaction is second order. For 50 kV X-rays in the concentration range 0.27 M to 0.57 M the reaction is ap- parently of a slightly lower order being about 1.8 and 1-7 at dose rates of 33,400 and FIG. 5. - The polymerization of acrylonitrile by 220 kV X-rays.0 -30.8 x 1012 i.p./ml. rnin I I (assuming WW = 32.5 eV). , 0 -43.8 X 1012 , , 8 -309 X 1012 , , Q -842 X 1012 , , @-1,130 X 1012 , , (assuming WW, = 32.5 eV). I FIG. 6.-The polymerization of acrylonitriie by 50 kV X-rays. 0 -1,820 x 1012 i.p./ml rnin 0 -7,400 x 1012 i.p./ml min 63 -33,400 x 1012 y 7400 i.p./ml min x 10-12 respectively. These latter values however are not known so accurately owing to the rather lower precision of the measurements and also to the narrow range of monomer concentration. Moreover with y-rays and 220 kV X-rays there is a tendency for maximum values of the second order rate constant to occur at about 0.44 M. If the same tendency arises in the 50 kV results the apparently lower order may be explained since conformity to the second order law is indicated at the lower concentrations.Summarized data on second order rate constants are given in table 1. At monomer concentrations above 0.75 M the reaction rates in the polymer- izations by y- and 220 kV X-rays fell off appreciably below those corresponding to the second order reaction and at concentrations above 1 M the rates became very irre- producible. 220 POLYMERIZATION OF ACRYLONITRILE TABLE 1 .-SUMMARIZED DATA ON THE RADIATION POLYMERIZATION OF ACRYLONITRILE IN AQUEOUS SOLUTION G for monomer concentration AT/ N for monomer concentration = 0.5 M 1.41 , 1.07 , 1.61 x lo4 1.44 , dose rate radiation (i,p.~mlmin 10-~2) average second order rate (Wwater = 32.5 eV) constant k2 (]./mole sec.) radium y-rays 9 9 7.6 , 0.2 , 8-5 rt 0.4 , 1.18 & 0.06 x 10-4 4.2 & 0.1 5.0 A 0.3 x 10-5 1.47 x l o 4 1 a 3 0 2.02 , 1.49 , 6-62 26-3 48.8 71.8 30.8 43.8 309 842 1,130 , 6.3 r 0.3 2.24 & 0.08 x 10-4 4.5 5 0.5 3-7 k 0.2 , = 0.5 M 4-45 x 104 4.43 , 4.34 , 3.29 , 4.52 x 104 4.00 , 9.11 x'iO.7 6.55 x 103 3.08 , 1-00 , 22dkV X-rays ? 3 Y9 3 50 i V X-rays Y Y 39 1,820 7,400 33,400 4-55 X 'iO3 4.83 , 2.96 , 2.12 x 103 1 *oo 3.25 x ';02 FIG.7.-The dependence of rate of polymerization on dose rate for the polymerization of acrylonitrile. The values at each concentration are in- terpolated from fig.3 5 and 6. @ y-ray results 0 220 kV X-ray results 0 50 kV X-ray results. 4.3 ri; 0.5 x 10-4 8.2 0.4 1-20 i 0.06 x YO-3 The dependence of the polymerization rate 4 on the dose rate R.-Fig. 7 shows that there is a continuity in the results obtained for the three types of radiation. There is also a variation in the dependence of 4 on R as the latter increases. At the lowest y-dose rates C$ is proportional to R.95 f 4 5 whilst at the highest 50 kV X-ray dose rate 4 is pro- portional to R.25 f **5.* Between these limits the exponent varies regularly regardless of the radiation quality. The efect of temperature.-Most of the work has been carried out without temperature control the temperatures varying between 8" C and 25" C.In spite of this no appreci- able variation in the rates of polymerization attributable to temperature variation were observed. Moreover some X-ray polymerizations were carried out using the thermostat at fixed temperatures of 25" C and 45" C. Changes in the rates of polymerization were very small and indicated that the temperature coefficient if not zero was negative. * It should be noted that at dose rates greater than about 7000 x 1012 i.p./ml min the ionic yield of the ferrous sulphate radiation tends to fall.2 No quantitative data are yet available but an effect of this kind would imply that the highest dose rates used were in fact larger than given in fig. 7. Hence the true dose rate exponent at these dose rates may be lower than 0-25.E. COLLINSON A N D F . S. DAINTON 221 THE RESULTS OF THE VJSCOMETRIC MEASUREMENTS.-aeCaUSe the molecular weights were expected to be dependent upon monomer concentration each polymer sample chosen for viscosity measurement had been taken to the same percentage conversion as nearly as possible namely 40 %. The variation of intrinsic viscosity 'with dose rate at a fixed initial monomer concentration of 0.5 M is shown in fig. 8. The scatter of these results is in general outside the error of the viscosity measurements and may be due to differences in the rates of coagulation of the polymer and variations in the extent to which the post-irradiation polymerization was allowed to proceed before examining the polymer. POLYMERIZATION IN DEUTERIUM OXIDE soLuTIoN.-It has previously been shown that after polymerization of acrylonitrile in aqueous solutions by X-rays the polymer contains OH groups which can be detected by infra-red analysis.6 This constitutes direct evidence for the production of OH radicals during the irradiation of water by X-rays.There is as yet no direct evidence for the production of H atoms at the same time and it was with this aim that the experiments using DzO were undertaken. If D atoms and OD radicals are formed during the irradiation of D20 by X-rays and if the D atoms initiate or terminate polymer chains by addition it should be possible to detect the characteristic C-D absorption frequency in the infra-red spectrum of the polymer. FIG. 8.-The dependence of intrinsic viscosity on dose rate for polymerizations of acrylonitrile to 40 %.The polymers obtained from the 220 kV X-irradiations were slightly yellow in colour. The starting monomer concentration was 0.066 M and the dose rate was 1130 x 1012 i.p./ml min. Polymerization proceeded up to 86 % conversion. The infra-red analyses gave the following results. (i) There was no evidence of OD absorption in the D20 polymer but in both the D20 and H20 polymers a broad OH absorption existed. The control polymer from pure monomer showed no appreciable OH absorption. (ii) A markedly more pronounced absorption occurred at 2200 cm-1 in the D2O than in the H20 polymer indicating the existence of C-D groups (fig. 9). (iii) In all three polymers an absorption occurred in the region of 1660 cm-1 indicat- ing the presence of C=C bonding.The intensity of this absorption was about the same in both the DzO and H20 polymers and was rather larger in the control polymer. The polymers obtained from the 50 kV irradiations were again obtained from a starting monomer concentration of 0.066 M. The dose rate in this case was 33,400~ 1012 i.p./ml min and the polymerizations were taken almost to 100 % conversion. The polymers so obtained were a deeper yellow colour than those from the 220 kV irradi- ations and in addition to the powdery polymer there was some semi-liquid material which was clear and pale brown in colour. These polymers were found to be soluble in acetone and in dimethyl formamide a property clearly indicative of a very low molecular weight.The infra-red analyses of the polymers gave results in agreement with those for the 220 kV X-irradiations (see fig. 9). POLYMERIZATION OF ACRYLONITRILE 222 Extremely careful drying of the polymers had no effect on the absorption due to OH nor did the polymer prepared from pure monomer show OH absorption even when left standing for some days in the open air indicating that the OH absorption of the polymers was not due to absorbed water. FIG. 9.-Infra-red absorption spectra of polyacrylonitrile films. (i) 220 kV X-ray dose rate = 1,130 x 1012 i.p./ml min (ii) 50 kV X-ray dose rate = 33,400 x 1012 i.p./ml min A polymerization in D20 3 H20 of pure monomer. 2'400 2300 2200 2/00 2300 2200 216 I ~ (ii) DISCUSSION B c 9 THE POLYMERIZATIONS IN DzO.-The infra-red spectra of these polymers in- dicate that when water is irradiated by X-rays H atoms are produced which can initiate and/or terminate the polymerization of acrylonitrile.The presence of C=C bonding in the polymers may be due to (i) termination of growing chains by H atom abstraction (ii) chain transfer with monomer involving transfer of a H atom or (iii) termination by disproportionation. The lack of absorption characteristic of the OD groupings would be puzzling if it were not compensated by the presence of the same broad OH absorption in both the D20 and H20 polymers. It appears that any OD end groups must exchange rapidly with any other labile H grouping in their vicinity. Care was taken to prevent access of undried air to the polymer samples as far as possible but if the exchange is sufficiently rapid it may have been achieved.It is unlikely that any of the H atoms in dimethyl formamide itself would exchange but there may have been some small amount of impurity containing labile H atoms in the sample. THE INHIBITION PERIOD (I.p.).-The most likely agent responsible for the I.P. is residual oxygen though this has not been definitely established. Inhibition may occur either by removal of the initiating radicals or by termination of the growing chains. An inverse dependence of I.P. on [nil] implies that the inhibitor is removed to some extent by growing polymer chains and that not all of the radicals formed by irradiation are used in initiating polymerization or removing E.COLLINSON AND F . S . DAINTON (iii) If x > 1 in the above expression then x = 1. (ii) Most of the inhibitor is removed by growing polymer chains. H + 0 2 -+ H02. 223 inhibitor. The relation I.P. a [m&l leads to additional conclusions. Jf we portional to [ml]” where x > 1 then the above relation shows that make the reasonable assumption that the polymerization initiation rate is pro- (i) Only a small proportion of the total radicals formed are used in initiation of polymerization or removal of inhibitor. The relation I.P. cc R-1 leads to the conclusion that the initiation rate is pro- portional to R. This follows from the fact that the inhibitor is removed almost entirely by growing polymer chains i.e. I.P. oc (initiation rate)-1.The above arguments apply only to the y-irradiation work. In the X-ray results the I.P. was so low that the dependence on monomer concentration and dose rate was not accurately ascertained. If the inhibitor concerned were oxygen the above results imply that very little 0 2 is removed in the process THE DEPENDENCE OF THE RATE OF POLYMERIZATION ON THE DOSE RATE AND MONOMER CONCENTRATION.-The fOllOWhg reaction schemes are possible :- kl (1) (2) H 2 0 + 2A(H + OH) A + ml -f Aml* (3) Am:-1 + m1 -f Am ( 5 ) Am + Am%+ Am,m,A (or Am + Am,) (4) Am + A -+ Am,A (6) 2A -f H20 (or H2 or Hz02) ki kT3 ktl k, both assumed to be very rapid k2 where A = H or OH Am = a growing polymer chain Am or Am,A = a dead polymer chain.Chain transfer with water is unlikely in view of the high dissociation energy of water. Chain transfer with monomer may take place but will not affect the dependence of the overall rate on R and [m]. Jf the polymer chains are long and if the effective initiating radicals are dis- tributed uniformly the method of stationary states may be applied to the system and we obtain eqn. (1) and (2) according as reaction (4) or reaction ( 5 ) is the sole termination step Neither of the above expressions is in accord with the results at low dose rates i.e. - d[ml]/dt a R0.95[m#. This difficulty raises the question as to whether the assumption of uniformity is correct or whether the results can be explained on the basis of a non-uniform radical distribution.If for example it is supposed that the polymerizations occur in a large number of independent small volumes the number of which at any instant is proportional to the dose rate and if the rate of polymerization in each such element is given by eqn. (l) then the overall rate of polymerization will be proportional to R[rn@. As the dose rate increases the volume elements will tend to overlap and it might be expected that an increas- ing dose rate would lead to a decreasing exponent of R until at dose rates sufficiently high to give an effectively uniform distribution of radicals the polymerization rate would become independent of R and equal to (k1kp/kt,)[ml]2. It has previously been suggested 8 that the radiation induced polymerization of acrylonitrile could only be explained on the assumption of a degree of non- uniformity of radical distribiition.This conclusion was based on the supposition POLYMERIZATION OF ACRYLONITRILE 224 that acrylonitrile would behave like methyl methacrylate in aqueous polymeriz- ations. For the latter monomer experiments independent of ionizing radiation indicated that either mutual termination or radical termination could take place depending on the rate of radical production.9 However in these experiments the rate of radical production was higher than the overall rate of radical produc- tion in any of the irradiations employed in the present work. Behaviour of a similar nature has been found for acrylonitrile. Using OH radicals generated uniformly throughout the reaction vessel and independently of ionizing radiation but with about the same overall rate of production quite different reaction kinetics are found.10 The rate of polymerization is then proportional to ls[rnl] where Z is the intensity of light absorbed by a solution containing Fe3’0H- ions a result which indicates that mutual termination of the growing polymer chains is pre- dominant.This implies that a higher concentration of radicals is available with ionizing radiations but that the radicals are confined to a limited fraction of the total irradiated volume. THE POLYMER CHAIN LENGTHS.-Though the viscosimetric measurements can only be regarded as a semi-quantitative indication of the molecular weights it can be shown that they are in agreement with the conclusions drawn from the kinetics.On the concept of independent small volumes of radicals it is expected that at low dose rates the polymer molecular weights will be independent of R but proportional to [rnl]. If there is a uniform radical distribution and termin- ation of chains is by radicals the molecular weight may be expected to be pro- portional to [ml] and to R-*. This type of dependence should therefore be ap- proached at high dose rates. The results of the intrinsic viscosity measurements indicate that [77] cc [ml]” where x > 1. In the relation [q] = KMS between intrinsic viscosity and number average molecular weight M for a polymer in solution the value of /i? generally lies between 0-5 and 1.0 [q] being expressed in 100 ml/g. To obey a relation of the type M cx [ml] a value of p of at least unity is required in this case.Such values have been reported for other systems and because no data are available on the value of 18 for this system we shall assume that the relation [q] = KM is applicable. At a given dose rate and assuming no chain transfer to occur we have and assuming - 4 = aP where 4 = rate of polymerization a = rate initiation of chains andP = average polymer chain length. :. log 4 = log Y- + log P log [q] = log k + logP = C + log P [TI = k E :. log + - log [TI = log a + c. Now at constant [ml] the concept of non-uniformity leads to d (log a)/d (log R) = $ or 1 at high and low dose rates respectively. In fig. 10 C + loglo cc (= loglo $ - loglo [q]) has been plotted against R.(3) of appropriate to each value of [y] having been obtained by interpolation on fig. 5. Dependence of a upon R in the expected manner should give rise to a curve in fig. 10 of slope varying from approximately unity at low dose rates to 0.5 at high dose rates. The best line of decreasing slope has been drawn through these points and values interpolated from this curve have been used together to plot the curve in fig. 8. The results show that the viscosity measurements are in general agreement with the hypothesis of non-uniformity based on the kinetic measurements. E . COLLINSON AND F . S . DAINTON c = constant. FIG. 10.-Plot of (c + loglo a) against loglo (dose rate) a = initiation rate 225 OTHER EFFECTS.-The fall of rate below what is expected of a second order reaction when the monomer concentration rises above 0.75 M and the irrepro- ducibility above 1 M is not an effect of the radiation.It has been shown that similar behaviour occurs in alcoholic solutions of styrene which have been studied over the whole concentration range.5 The effect is to be attributed to the lower efficiency of radical generation due to a higher percentage of direct action and promotion of the termination step produced by increased tendency of the chains to coagulate before termination. As judged by the fall of intrinsic viscosity of the polymers formed in this region the latter effect is probably predominant and would explain the irreproducibility. The fact that temperature has so small an effect on the rates of polymerization seems to indicate that the termination step may have an energy of activation which counterbalances the energy of activation of the initiation step.Such an energy of activation may well arise if termination were by H atom abstraction from a growing chain. This would also give rise to double bonds at the ends of the chains in agreement with the occurrence of C=C double bonds as detected by the infra-red analyses. THE IMPL,ICATIONS OF THE PROPOSED mcmNIsM.-It appears that the polymer- ization of acrylonitrjle by X- and y-rays can be given a general interpretation on the postulate of a non-uniform radical distribution at low dose rates becoming more uniform at higher dose rates. The substantial continuity of the polymer- ization results between the three radiations employed suggests that the degree of non-uniformity at a given dose rate is about the same for each.Such a result appears at first sight to be contrary to the accepted view that the higher the electron energy the lower is the ionization density along its path. But this applies only to average values of the specific ionization. It is not impossible that a large proportion of the energy dissipated by any electron in water may ultimately appear as a series of individual groups of ions of approximately the same size. A change in the energy of the incident radiation would then merely alter the spatial dis- tribution of such groups of ions thus giving rise to the change in average track density of ionization due to the primary electrons.From this point of view it is to be remembered that approximately half the total ionization produced by a primary electron occurs as &rays of lower energies the other half occurring as POLYMERIZATION OF ACRYLONITRILE 226 isolated ion clusters of a conjectural size.11 Support is given to this theory by the fact that the ionic yield of the oxidation of ferrous ions in aqueous solution at concentrations below 10-4 M is independent of the dose rate.12 Though the results of these polymerizations can be explained in a general way on this concept of non-uniformity there are nevertheless certain difficulties in a complete interpretation along these lines. Thus in the previous treatment it has been tacitly assumed that stationary state homogeneous kinetics can be applied to each individual volume element.This is not strictly justifiable since the kinetics within a volume element will be non-stationary but a satisfactory non-stationary state treatment has not yet been found possible. Moreover the average lifetime of a polymer of chain length 7000 at a monomer concentration of 0.5 M will be of the order 14 sec if we assume a value of kp = 103 I./mole sec for acrylonitrile. It is difficult to see how such a long living chain can be terminated by radicals produced in the same isolated group as the radical initiating the chain since the initiating radicals would be expected to have a much shorter lifetime. The predominant type of chain termination may however occur by mutual interaction of growing chains and the dependence of rate upon [m# may arise from the non-stationary conditions.In certain other chain reactions induced by ionizing radiations a dependence of the reaction rate on the square root of the dose rate is found e.g. the polymer- ization of acrylonitrile as the pure monomer 13 and the radiolysis of aqueous solutions of hydrogen peroxide.14 In these cases it must be presumed that the reaction chains are terminated in pairs and that each of the chains so terminated was initiated in a different volume element. The problem thus becomes one for which the usual treatment applicable to systems of uniform concentration is valid. In general chain reactions in which the chains are of long life and the chain carriers have a high coefficient of diffusion are likely to fall into this category.These requirements are both easily satisfied in the H202 radiolysis. The authors desire to express their thanks to I.C.I. Ltd. for gifts of acrylouitrile and dimethyl formamide to Dr. N. Sheppard and Mr. J. K. Brown of the Colloid Science Department Cambridge for carrying out the infra-red spectroscopic analyses and advising on interpretation of the results to Professor J. S. Mitchell of the Department of Radiotherapeutics Cambridge for placing 220 kV and 50 kV X-ray sets and a Beckmann spectrophotometer at their disposal to Messrs. R. S. Quick and J. Haybittle of the Department of Radiotherapeutics Cambridge for assistance and technical advice in connection with X-ray apparatus and physical dosimetry and to the Department of Scientific and Industrial Research for an apparatus grant and a maintenance grant to one of us (E.C.). 1 Miller J. Chem. Physics 1950 18 79. 2 Miller N. private communication. 3 Fricke and Morse Phil. Mag. 1929 7 129. 4 Randall Fowler Fuson and Dangl Infra-red Determination of Organic Strrrctirres (Van Nostrand New York 1st ed. 1949). 5 Chapiro J. Chim. Phys. 1950 47 747. 6 Dewhurst H. A. private communication. 7 Dainton J. Physic. Chem. 1948 52 490. 8 Dainton Nature 1947 160 268. 9 Baxendale Evans and Park Trans Faraday Soc. 1946,42 155. 10 James D. G. L. unpublished work. 11 Lea Actions of Radiations on Living Cells (Cambridge University Press 1946 chap. 1). 12 Sutton H. C. unpublished work. 13 Chapiro Cousin Landler and Magat Rec.trav. chim. 1949 68 1037. 14 Rowbottom J. unpublished work. THE y-RAY AND X-RAY INDUCED POLYMERIZATION OF AQUEOUS SOLUTIONS OF ACRYLONITRILE BY E. COLLINSON AND F. S. DAINTON The Chemistry Department The University Leeds 2 Received 1 1 th February 1952 Polymerization induced by the action of radium y-rays 220 kV and 50 kV X-rays on aqueous solutions of acrylonitrile has been studied. Radiation dosimetry was by measure-ment of the oxidation of ferrous ions in all cases. Polymerization in D20 solution fol-lowed by infra-red spectroscopic analysis of the polymers produced gave direct proof of the formation of D atoms in the irradiated liquid and confirmed their participation in the polymerization. The kinetics of the polymerization were studied over the available concentration range and with dose rates ranging from 6.6 X 1012 to 33,400 x 1012 ion pairslml min.At low dose rates the kinetics were consistent with a non-uniform distribution of the radicals formed from the water whilst at high dose rates an increasing degree of uni-formity was indicated. There was apparently no difference in the effects produced by the three types of radiation. A possible explanation of these results is that all three radiations give rise to randomly distributed volume elements of radicals in water in which chemical reaction takes place, the volume elements being widely separated and independent of each other at low dose rates but tending to overlap at high dose rates. Though this hypothesis is not finally established the viscosity molecular weights of the polymers showed a dependence on dose rates which is in general agreement with it.A useful method for the detection and identification of radicals formed in water is the study of the polymerization of dissolved vinyl compounds which the radicals may initiate. The investigation described in this paper was undertaken as an application of this method to the problem of the number and type of radicals formed when high energy radiation is absorbed by water. Jn the course of the work it became evident that this method might also be used to explore any non-uniformity of the initial distribution of the initiating radicals. This feature arises from the fact that addition polymerization reactions are chain reactions and the expression for their rates depends on more than one rate constant.The kinetics of such reactions are therefore more sensitive to changing conditions of radiation intensity reactant concentration and radical distribution than are the kinetics of reactions which are complete in a single stage. Since the maximum radiation intensity available to us when this work was begun was much smaller than can be used in most photochemical reactions it was necessary to select a monomer having as large a water solubility as possible. Acrylonitrile was therefore used in spite of certain other disadvantages. Experi-ments have also been made with methacrylonitrile and methyl methacrylate, but measurements on these substances are not yet complete and are not described here. EXPERIMENTAL RADrATroNs.-Radium y-rays 220 kV X-rays from a G.E.C.Maximar radiotherapy unit operated-at 15 mA and 50 kV X-rays from a Machlett AEG-50-T tube operated at 26 mA were employed. Two radium sources of 400 and 232.5 mcurie strength screened by the equivalent of 0.5 mm of platinum were available. In the majority of experiments 21 E. COLLINSON AND F. S. DAINTON 21 3 the 220 kV irradiation was unfiltered but in some experiments 0.5 mm of aluminium filtration was employed. The 50 kV radiation was unfiltered. DOSlMETRY.-For each type of radiation the chemical method recommended by Miller 1 was employed. In this method the chemical reaction induced is the oxidation of ferrous to ferric ions in aerated aqueous solutions the solutions being 0.8 N in sulphuric acid and the chosen initial concentration of ferrous ion lying in the range 10-3 to 10-4 M.Under these conditions the rate of oxidation of ferrous ions is independent of their con-centration and the reaction has been calibrated by Miller for the measurement of dose rates in the wavelength region lying between the wavelengths of radium y-rays and 200 kV X-rays. Over this wavelength range the ionic yield remains unchanged and it is also independent of dose rate up to 4200 r/min.2 Though extrapolation of the results to 50 kV X-rays has yet to be justified experimentally the work of Fricke 3 indicates that much the same result is obtained for wavelengths at least as large as those of 100 kV X-rays. Dosimetry of 50 kV X-rays is difficult by any method. The chemical method was therefore considered as likely to give accurate values as any other technique under the conditions employed for polymerization.The water used for preparing the solutions was twice distilled the second distillation being from alkaline potassium permanganate using a Pyrex glass still incorporating an anti-splash column. The still and any other apparatus coming into contact with liquid < 5.4cm FIG. 1 .-The cells used for y-irradiations. JII to be irradiated was scrupulously cleaned with a nitric + sulphuric acid mixture rinsed with pure water and then treated with steam uncontaminated by organic impurity. Solutions were prepared from A.R. ferrous ammonium sulphate and A.R. sulphuric acid. The concentration of ferrous ion before and after an irradiation was determined by measuring the optical density of the solution at 508 mp after the addition of ortlzo-phenanthroline which forms a complex with ferrous ions.For dosimetry the cells were always irradiated under exactly the same conditions as were to be used for polymerization. The solutions were unstirred during the irradiations and in measuring the integral dose rate care was taken to ensure that the exposure time was below that required either to denude the solution of air (50,000 r) or to reduce the concentration of ferrous ion below lO-4M at the position of highest dose rate in the cell. Irradiation times were also arranged to cause a total concentration change of about one-half the initial concentration.1 On completion of the irradiation a suitable volume of the solution was made up with ortlzo-phenanthroline at a pH neutral to Congo Red, to give an optical density of about 0.43 in a 10 cm cell.The optical density was accurately measured by means of a Beckmann DU spectrophotometer. From the change in con-centration and Miller's value of M / N = 6.7," the dose rate in ion pairs i.p./ml min could be calculated. This unit is used throughout and is equivalent to expressing the dose rate in terms of energy absorbed assuming W,,, = 32.5 eV. THE CELLS AND DOSE RATES.-FOr y-irradiations three Pyrex glass cells of differing dimensions (fig. l) used in conjunction with the two radium sources provided four different dose rates viz. 71.8 & 4-1 ; 48.8 rrt 2.5 ; 26.3 i 0-7 ; 6.62 -ir. 0.53 i.p./ml * This value actually applies to 0.8 N sulphuric acid.Allowance for the difference in stopping power between this and water leads to the value of 6-43 for water.2 The dose rates quoted here should be corrected by this factor 214 POLYMERIZATION OF ACRYLONITRILE min x 10-12 the errors being expressed as standard deviations of several experimental values. Calculations showed that the largest variations in dose rates obtained was that in cells of type 11. For the integral dose rate of 48.8 i.p./nil niin x 10-12 the maximum dose rate was 294 i.p./ml min x 10-17 and the minimum 13.4 i.p./ml min x 10-12. For 220 kV X-irradiations flat cylindrical Pyrex glass cells 5 cm diam. and 1 cm deep were used. Dose rates were varied by adjusting the distance between the cell irradiated and the focus of the X-ray tube.Estimates of the integral dose rates in the cells were first derived from measurements in air with a Victoreen condenser chamber dosimeter. These measurements indicated that over the range covered the radiation intensity was in accord with the inverse square law. Measurement of the absorption by the glass layers of a cell and the liquid in a cell showed that the integral dose rate was within 0.2 % of that at the centre of the cell and that the variation in dose rate across the cell was ap-proximately 13 % being slightly higher for the unfiltered radiation. Air dose rate measurements at the position of the centre of the cell enabled estimates to be made of the integral dose rates. Comparative results for the physical and chemical measurements are given in fig 2. The differences are not surprising considering the elementary nature FIG.2.-The dependence of X-ray dose rate on distance from the focus. (i) 220 kV filtered X-radiation (ii) 220 kV unfiltered X-radiation (iii) 50 kV unfiltered X-radiation 0 -physical measurements (3 -chemical measurements (assuming Wwater = 325 eV) 0 -chemical measurements-cell in thermostat. of the absorption measurements but the physical measurements were useful in giving estimated values of the dose rates thus simplifying the use of the chemical method. In experiments in which the cells were immersed in a water thermostat the dose rates were noticeably higher than for the same cell position in air. Since the dose rates appeared to be in good accord with the inverse square law the values used in interpretation of the polymerization results were corrected by interpolation on the inverse square law plots.The following considerations determined the design of the cell for 50 kV X-irradi-ations. The cell size was a compromise between the volume requirement to give reason-able weights of polymer a diameter which would be wholly included in the X-ray beam, and a depth which would not by absorption of the radiation give rise to a large change in dose rate between the top and bottom of the cell. Moreover the upper surface of the cell had to be as thin as possible to minimize absorption. The cell was of Pyrex glass cylindrical in shape 0.5 cm deep and 2.9 cni diam. The upper glass surface was approximately 0.05 mm thick. Dosimetry of the 50 kV irradiations was exclusively by the chemical method.It was to be expected that there would be an appreciable dose rate variation across the cell and irradiation times for dosimetry were cut down suc-cessively until no further apparent increase in dose rate occurred. This indicated that there was then no lowering of Fez+ concentration below 10-4 M in any part of the cell. In order to estimate the value of the highest and lowest dose rates in the cell measurement E . COLLINSON AND F. S. DAINTON 215 were made with open topped cells of (i) the same and (ii) double the depth of the covered cell. From such measurements the absorption of radiation due to the glass top and to the layer of liquid could be estimated. From the estimate of the absorption coefficient in water the effective wavelength of the radiation was approximately 1.2A.The dosi-metric measurements on 50 kV X-rays are given in fig. 2. The three dose rates employed appear to conform reasonably well with the inverse square law but since this cannot be regarded as established the polymerization dose rates were taken to be as measured. At the maximum integral dose rate of 33.4 x 10-15 i.p./ml min the dose rates at the top and bottom of the cell were 91.6 x 1015 and 7.3 x 1015 i.p.,/ml niin respectively. THE KINETIC STUDIES.-ACrylOnitrile was obtained from 1.C.I. Ltd. or from L. Light Ltd. After drying over anhydrous sodium sulphate the monomer was fractionally distilled through a vacuum-jacketed column filled with single spiral glass helices and having an efficiency of about 15 plates.The distillations were carried out under high vacuum and the middle 50 % of distillate was collected for use. Pure monomer was stored in the dark under vacuum and was distilled off as required. PROCEDURE FOR y- AND 220 kV X-RAY IRRADuTIoNs.-Jn the earlier experiments polymerizations were followed by separation and weighing of the polymer. Later re-actions were followed by means of dilatometers the capillaries being of various diameters depending on the concentration of the monomer. The reaction cell was first connected through its capillary tubing to a glass bulb and the whole thoroughly cleaned. A known volume of water was then pipetted into the bulb and de-aerated by three successive freez-ings in liquid air followed by evacuation and remelting. A sample of distilled monomer was weighed into a clean dry tube which was then attached to the vacuum line.After de-aeration of the monomer by the same procedure the monomer was distilled into the bulb. An appropriate volume of the solution was run into the reaction cell by inverting the bulb and cell and the latter was then sealed off for irradiation. The dilatometer was immersed in a thermostat maintained at a temperature of 25.00 & 0.01" C and the level of liquid in the capillary was read to an accuracy of 0.005 cm by means of a reading microscope. In the initial stages of an irradiation the cell was removed and examined visually at short intervals of time in order to ascertain the time of onset of polymeriiation. After a suitable irradiation period the cell was replaced in the thermostat and when temperature equilibrium was attained, readings were taken.The time required for temperature equilibration was cut down in later experiments by maintaining the cells at 25" C throughout the irradiation. Further irradiations were then given and readings were taken in this way until the end of therun. The dilatometer was then opened to the atmosphere and the polymer was filtered off into a sintered glass crucible of porosity 4 which was left to dry to constant weight in a vacuum desiccator. The final dilatometer contraction and the weight of the polymer made possible the construction of a polymer weight against time curve. PROCEDURE FOR 50 kV X-RAY IRRmrATroNs.-Owing to the mechanical weakness of the upper window of the cell used for these experiments work under vacuum was made difficult.The solutions were therefore de-aerated in a separate vessel and then saturated with nitrogen. For the purpose of filling the cell was attached to the de-aeration bulb by a joint and after blowing a little nitrogen through the system some of the solution was run into the cell which was then sealed to prevent access of air. The disadvantage of this method of preparation was that the solutions were liable to contain small amounts of air and tap grease. Experiments with y-rays indicated that the ultimate rate attained after a longer inhibition period brought about by the oxygen in a solution which had not been de-aerated was within 0.5 % of the rate in a normally de-aerated solution. y-ray runs in the presence of added tap grease showed slightly longer inhibition periods but the ultimate rate was never more than 3 % from the normal rate.Thus although the 50 kV X-ray polymerizations carried out by the above technique were not expected to give the same order of accuracy as those done in IJ~CUO yet the inaccuracies introduced were sufficiently small for the method to give useful results. A sufficient volume of solu-tion could be prepared at one time in the above manner to enable a complete weight polymer against time curve to be plotted each polymerization providing one point on such a curve. far been viscometric. The solvent employed for polyacrylonitrile was dimethyl form-amide. Some solvent was obtained from E. I. Du Pont de Nemours Inc. and some from I.C.I. Ltd. It was distilled under vacuum using a simple spiral fractionating column.The fraction distilling between 52.5" C and 55" C under a pressure of 19 mm of mercury Irradiation was then begun. INVESTIGATIONS OF THE POLYMER CHAIN LENGTHS.-A~~ such measurements have S 216 POLYMERIZATION OF ACRYLONITRILE was collected for use. Its viscosity was 0.8602 centistokes at 25.00 & 0.01" C. The dis-tilled solvent was kept in a glass-capped Pyrex flask. The viscometer was of the Ostwald design and conformed to the specification of type No. 1 (B.S. 188 1937). It was calibrated using pure water. Kinetic energy corrections were applied in the manner described in the above pamphlet. For each polymer sample the viscosity of solutions of at least three concentrations were measured the lowest concentration being approximately 0.06 g/100 ml.Extrapolations of plots of both rlsp/c and In ~ J c against concentration to zero concentration were used to give the best estimate of the intrinsic viscosity. Most of these plots and particularly those for the higher molecular weights showed an upward curvature probably due to the fact that the polymers were unfractionated. The deuterium oxide was 99.75 % DzO. It was distilled under vacuum before use. Tnfra-red spectroscopic analyses were carried out by means of a Perkin Elmer single beam spectrometer. Experiments were performed using both 220 kV and 50 kV X-rays. Preparation of monomer solutions was by the usual techniques but in order to have the best chance of detecting absorption by the end groups i.e.to obtain lowest molecular weight product the highest possible dose rate and lowest possible monomer concentration were used. After completion of the irradiations which were taken to as high a percentage con-version as possible the deuterium oxide was distilled away from the polymer under vacuum and the polymer was gently warmed (to about 35" C) under vacuum for several hours. It was then dissolved in dimethyl formamide to give a limpid yellow solution. A thin film of the polymer was made by putting some of this solution on a rock salt plate and evaporating the solvent under vacuum for 2 days continuously after which the film was covered by another rock salt plate. Control polymers were prepared from solutions in water and from irradiation of the pure monomer. The regions of the infra-red spectrum examined were those in which the characteristic 0-D or C-D bond frequencies might be expected to occur.C-D vibration frequencies generally occur in the region of 2150 to 2200 cm-1 e.g. 2204 cm-1 (CH3D) 2180 cm-1 (CzHsD) 2170 cm-1 (C3H7D) and the 0-D frequency near 2500 cm-1 e.g. 2490 cm-1 (CC13OH).4 All viscosities were measured at a temperature of 25.00 1 O*Ol" C. POLYMERIZATIONS INDUCED BY IONIZING RADIATlONS IN DEUTERIUM OXIDE SOLUTI0N.-RESULTS PROOF OF INDIRECT ACTION.-h view of the much higher concentration of water in the irradiated solutions the polymerizations were almost certain to be due to indirect action. Nevertheless it is known that single monomers can be polymerized by the direct action of radiations 5 and it was important to know whether appreciable percentages of the effects were due to direct action.Benzene apparently has an inappreciable effect on the rate of polymerization of acrylo-nitrile in the pure state.5 Therefore it seems most likely that few radicals initiating poly-merization are produced from benzene and that benzene does not inhibit polymerization. Consequently a reasonable measure of the amount of polymerization occurring at a given concentration due to direct action on the monomer should be obtained by irradiating a solution of monomer of the same concentration in benzene. Using a monomer concentration of 0.485 M in dry de-aerated benzene solution and irradiating by y-rays at a dose rate of approximately 43 x 1012 i.p./ml min in the usual way no visible polymerization occurred for about 60 h.The amount of polymer formed even after 727 h was only equivalent to that formed in aqueous solution in 4.1 h. It is therefore clear that direct action plays a negligible part in the polymerizations. PROCESSES OCCURRING DURING PoLYMmrzATIoN.-In general the water-insoluble polymer was substantially the only product formed from the monomer. This was demonstrated by carrying out prolonged irradiations of monomer solutions. Using monomer concentrations from 0.5 M to 0.1 M it was found possible to polymerize the solutions to at least 95 % and with longer irradiations they would probably have been polymerized further. Even at the lowest concentration used in the kinetic work (i.e. 0.036 M) the percentage polymerization could be taken to at least 89 %.At very high dose rates and low concentrations as will be described later semi-liquid very low molec-ular weight products were formed but such products did not generally occur in any appreciable amounts E . COLLINSON AND F . S . DAINTON 217 NO hydrogen peroxide was detected in the experiments using y- or 220 kV X-rays. The reagent used was titanic sulphate in 0.5 N H2SO4 solution the concentration limit of detection with 10 cm cells in the Beckmann spectrophotometer being 3 x 10-7 M. I n the 50 kV experiments hydrogen peroxide was detected in every solution examined. As the de-aeration of these solutions was neither as efficient nor as reproducible as that for y- and 220 kV X-rays estimates of the concentrations of hydrogen peroxide could be made sufficiently accurately by visual comparisons with standard solutions.The following facts were established : (i) the production of hydrogen peroxide did not cease at the end of the induction (ii) the rate of hydrogen peroxide production appeared to be independent of the NO attempt has yet been made to detect and measure gaseous products which may Preliminary experiments in this direction have period but continued throughout the whole period of irradiation ; monomer concentration and increased with increase in dose rate. be formed during the polymerizations. shown however that small amounts of gas are evolved.6 I Y/ FIG. 3.-The dependence of the inhibition period on dose rate and monomer concentration in the y-ray polymerizations (i) 0 -6.62 x 1012 i.p./ml.min @ -26.3 x 1012 , ,, (> -48.8 X 1012 , , 0 -71.8 x 1012 , ,, (ii) Values interpolated from (i) at l/(monomer concentration) = 7.5. THE INHIBITION PERIOD.-h all the polymerizations small inhibition periods the length of which depended on the dose rate and the monomer concentration were found. These were of the order minutes or seconds whereas irradiation of solutions containing air led to inhibition periods of several hours. It appears therefore that the inhibition periods previously reported,7 and the smaller ones in the present work may have been due to residual oxygen in the solutions. Using the standard method of de-aeration the measured inhibition periods for the y-ray polymerizations were found to be proportional to [rnll-l and R-1 where R is the dose rate (fig.3). Owing to the shortness of the times involved and the detection of the onset of polymerization by intermittent visual inspection the precision of these observa-tions was not high. The inhibition periods found in the 220 kV and 50 kV X-ray polymer-izations appeared to be in general agreement with the above relationships but in the former case they were too small (a few seconds) and in the latter case the degree of de-aeration was insufficiently constant to give precise results. THE KINETICS.-AII the weight of polymer against time curves were sigmoid the in-hibition period being followed by a period of acceleration which tended to decrease with increasing concentration or dose rate. Since the duration of this acceleration period appeared to respond to dose rate and concentration changes in a manner similar to that of the inhibition period it seemed most likely that this was caused by oxygen not destroye 218 POLYMERIZATION OF ACRYLONITRILE in the inhibition period though it could equally well have been due to some other unknown impurity.Using highly purified materials (monomer fractionated three times and water twice distilled under vacuum in addition to the normal procedure) the inhibition period, the acceleration period and the rate of polymerization remained almost unchanged. The acceleration may also merely have been that associated with the growth of the con-centration of growing polymer chains to its stationary value the fact that a post-irradiation polymerization was found apparently supporting this view.The magnitude of the post-irradiation polymerization varied from about 4 ”/ of the radiation polymer-ization rate at low dose rates to about 0.7 % at high dose rates. However the observation that there was no period of acceleration when a system was re-irradiated after the post-irradiation reaction had ceased indicates that the acceleration was not due to this cause. In calculating the rate of polymerization appropriate to a given concentration from a polymer weight against time curve the point at which the period of acceleration ended FIG. 4.-The polymerization of acrylonitrile by y-rays. 0 -6.62 x 1012 i.p./mi niin 0 -226.3 x 1012 i.p./ml min 8 -48.8 x 10” , , C -71.8 x 1012 , ,, (assuming Wwater = 32.5 eV). was taken as the starting point of the polymerization.As the polymerization progressed, the rate tended to become higher than the rate of a polymerization started at the cor-respondingly lower concentration. Such behaviour could be due either to continued removal of the retarder or to the production of an accelerator. The most likely retarder is oxygen and the only obvious possibility for an accelerator is the polymer itself. No hydrogen peroxide was formed during the y- and 220 kV X-irradiations and it was also confirmed that addition of hydrogen peroxide to the polymerizations gave no change in polymerization rate which was outside experimental error. In order to check the effect of dead polymer on the rate of polymerization a sample of polymer was shaken with pure monomer and with several washings of distilled water, and was then added to a solution to be polymerized.Retardation rather than acceleration was experienced but it is possible that an inhibitor was introduced with the polymer in spite of the care taken to ensure its purity. It seems unlikely that an acceleration was caused by coagulation assisting the termination reaction since the continual movement of the cells from their irradiation position to the thermostat invariably gave rise to coagulation at an early stage. The dependence of the rates of polymerization on dose rate and monomer concentra-tion for each type of radiation is presented in fig. 4 5 and 6. These results may be summarized as follows E. COLLINSON A N D F . S . DAINTON 219 The order of reaction with respect to mottomer concentration [ml].-For y-rays in the concentration range 0.038 M to 0.75 M and with dose rates ranging from 6.62 to 71.8 i.p./ml rnin x 10-12 and for 220 kV X-rays in the concentration range 0-04 M to 0.75 M and with dose rates ranging from 30.8 to 11 30 i.p./ml min x 10-12 the reaction is second order.For 50 kV X-rays in the concentration range 0.27 M to 0.57 M the reaction is ap-parently of a slightly lower order being about 1.8 and 1-7 at dose rates of 33,400 and FIG. 5. - The polymerization of acrylonitrile by 220 kV X-rays. 0 -30.8 x 1012 i.p./ml. rnin 0 -43.8 X 1012 , ,, 8 -309 X 1012 , ,, Q -842 X 1012 , ,, @-1,130 X 1012 , ,, (assuming WW, = 32.5 eV). I I I FIG. 6.-The polymerization of acrylonitriie by 50 kV X-rays. 0 -1,820 x 1012 i.p./ml rnin 0 -7,400 x 1012 i.p./ml min 63 -33,400 x 1012 y , (assuming WW = 32.5 eV).7400 i.p./ml min x 10-12 respectively. These latter values however are not known so accurately owing to the rather lower precision of the measurements and also to the narrow range of monomer concentration. Moreover with y-rays and 220 kV X-rays there is a tendency for maximum values of the second order rate constant to occur at about 0.44 M. If the same tendency arises in the 50 kV results the apparently lower order may be explained since conformity to the second order law is indicated at the lower concentrations. Summarized data on second order rate constants are given in table 1. At monomer concentrations above 0.75 M the reaction rates in the polymer-izations by y- and 220 kV X-rays fell off appreciably below those corresponding to the second order reaction and at concentrations above 1 M the rates became very irre-producible 220 POLYMERIZATION OF ACRYLONITRILE TABLE 1 .-SUMMARIZED DATA ON THE RADIATION POLYMERIZATION OF ACRYLONITRILE IN AQUEOUS SOLUTION AT/ N G for monomer (Wwater = 32.5 eV) constant k2 (]./mole sec.) concentration concentration = 0.5 M dose rate radiation (i,p.~mlmin 10-~2) average second order rate for monomer = 0.5 M radium y-rays 6-62 1.18 & 0.06 x 10-4 1.61 x lo4 4-45 x 104 9 26-3 4.2 & 0.1 , 1.44 , 4.43 ,, 9 48.8 7.6 0.2 , 1.41 , 4.34 ,, 22dkV 71.8 8-5 rt 0.4 , 1.07 , 3.29 ,, X-rays ? 3 Y9 3, 50 i V X-rays Y Y 39 30.8 43.8 309 842 1,130 1,820 7,400 33,400 5.0 A 0.3 x 10-5 1.47 x l o 4 6.3 r 0.3 , 1 a 3 0 2.24 & 0.08 x 10-4 4-55 X 'iO3 4.5 5 0.5 , 4.83 ,, 3-7 k 0.2 , 2.96 ,, 4.3 ri; 0.5 x 10-4 2.12 x 103 8.2 0.4 1 *oo 1-20 i 0.06 x YO-3 3.25 x ';02 4.52 x 104 2.02 ,, 4.00 ,, 1.49 9.11 x'iO.7 6.55 x 103 3.08 ,, 1-00 ,, The dependence of the polymerization rate 4 on the dose rate R.-Fig.7 shows that there is a continuity in the results obtained for the three types of radiation. There is also a variation in the dependence of 4 on R as the latter increases. At the lowest y-dose rates C$ is proportional to R.95 f 4 5 whilst at the highest 50 kV X-ray dose rate 4 is pro-portional to R.25 f **5.* Between these limits the exponent varies regularly regardless of the radiation quality. FIG. 7.-The dependence of rate of polymerization on dose rate for the polymerization of acrylonitrile.The values at each concentration are in-terpolated from fig. 3 5 and 6. @ y-ray results 0 220 kV X-ray results 0 50 kV X-ray results. The efect of temperature.-Most of the work has been carried out without temperature control the temperatures varying between 8" C and 25" C. In spite of this no appreci-able variation in the rates of polymerization attributable to temperature variation were observed. Moreover some X-ray polymerizations were carried out using the thermostat at fixed temperatures of 25" C and 45" C. Changes in the rates of polymerization were very small and indicated that the temperature coefficient if not zero was negative. * It should be noted that at dose rates greater than about 7000 x 1012 i.p./ml min the ionic yield of the ferrous sulphate radiation tends to fall.2 No quantitative data are yet available but an effect of this kind would imply that the highest dose rates used were in fact larger than given in fig.7. Hence the true dose rate exponent at these dose rates may be lower than 0-25 E. COLLINSON A N D F . S. DAINTON 221 THE RESULTS OF THE VJSCOMETRIC MEASUREMENTS.-aeCaUSe the molecular weights were expected to be dependent upon monomer concentration each polymer sample chosen for viscosity measurement had been taken to the same percentage conversion as nearly as possible namely 40 %. The variation of intrinsic viscosity 'with dose rate, at a fixed initial monomer concentration of 0.5 M is shown in fig.8. The scatter of these results is in general outside the error of the viscosity measurements and may be due to differences in the rates of coagulation of the polymer and variations in the extent to which the post-irradiation polymerization was allowed to proceed before examining the polymer. POLYMERIZATION IN DEUTERIUM OXIDE soLuTIoN.-It has previously been shown that after polymerization of acrylonitrile in aqueous solutions by X-rays the polymer contains OH groups which can be detected by infra-red analysis.6 This constitutes direct evidence for the production of OH radicals during the irradiation of water by X-rays. There is as yet no direct evidence for the production of H atoms at the same time and it was with this aim that the experiments using DzO were undertaken.If D atoms and OD radicals are formed during the irradiation of D20 by X-rays and if the D atoms initiate or terminate polymer chains by addition it should be possible to detect the characteristic C-D absorption frequency in the infra-red spectrum of the polymer. FIG. 8.-The dependence of intrinsic viscosity on dose rate for polymerizations of acrylonitrile to 40 %. The polymers obtained from the 220 kV X-irradiations were slightly yellow in colour. The starting monomer concentration was 0.066 M and the dose rate was 1130 x 1012 i.p./ml min. Polymerization proceeded up to 86 % conversion. The infra-red analyses gave the following results. (i) There was no evidence of OD absorption in the D20 polymer but in both the D20 and H20 polymers a broad OH absorption existed.The control polymer from pure monomer showed no appreciable OH absorption. (ii) A markedly more pronounced absorption occurred at 2200 cm-1 in the D2O than in the H20 polymer indicating the existence of C-D groups (fig. 9). (iii) In all three polymers an absorption occurred in the region of 1660 cm-1 indicat-ing the presence of C=C bonding. The intensity of this absorption was about the same in both the DzO and H20 polymers and was rather larger in the control polymer. The polymers obtained from the 50 kV irradiations were again obtained from a starting monomer concentration of 0.066 M. The dose rate in this case was 33,400~ 1012 i.p./ml min and the polymerizations were taken almost to 100 % conversion. The polymers so obtained were a deeper yellow colour than those from the 220 kV irradi-ations and in addition to the powdery polymer there was some semi-liquid material which was clear and pale brown in colour.These polymers were found to be soluble in acetone and in dimethyl formamide a property clearly indicative of a very low molecular weight. The infra-red analyses of the polymers gave results in agreement with those for the 220 kV X-irradiations (see fig. 9) 222 POLYMERIZATION OF ACRYLONITRILE Extremely careful drying of the polymers had no effect on the absorption due to OH, nor did the polymer prepared from pure monomer show OH absorption even when left standing for some days in the open air indicating that the OH absorption of the polymers was not due to absorbed water. 2'400 2300 2200 2/00 2300 2200 216 (ii) ~ FIG.9.-Infra-red absorption spectra of polyacrylonitrile films. (i) 220 kV X-ray dose rate = 1,130 x 1012 i.p./ml min (ii) 50 kV X-ray dose rate = 33,400 x 1012 i.p./ml min A polymerization in D20 B 3 H20 c 9 of pure monomer. I DISCUSSION THE POLYMERIZATIONS IN DzO.-The infra-red spectra of these polymers in-dicate that when water is irradiated by X-rays H atoms are produced which can initiate and/or terminate the polymerization of acrylonitrile. The presence of C=C bonding in the polymers may be due to (i) termination of growing chains by H atom abstraction (ii) chain transfer with monomer involving transfer of a H atom or (iii) termination by disproportionation. The lack of absorption characteristic of the OD groupings would be puzzling if it were not compensated by the presence of the same broad OH absorption in both the D20 and H20 polymers.It appears that any OD end groups must exchange rapidly with any other labile H grouping in their vicinity. Care was taken to prevent access of undried air to the polymer samples as far as possible, but if the exchange is sufficiently rapid it may have been achieved. It is unlikely that any of the H atoms in dimethyl formamide itself would exchange but there may have been some small amount of impurity containing labile H atoms in the sample. THE INHIBITION PERIOD (I.p.).-The most likely agent responsible for the I.P. is residual oxygen though this has not been definitely established. Inhibition may occur either by removal of the initiating radicals or by termination of the growing chains.An inverse dependence of I.P. on [nil] implies that the inhibitor is removed to some extent by growing polymer chains and that not all of the radicals formed by irradiation are used in initiating polymerization or removin E. COLLINSON AND F . S . DAINTON 223 inhibitor. Jf we make the reasonable assumption that the polymerization initiation rate is pro-portional to [ml]” where x > 1 then the above relation shows that : (i) Only a small proportion of the total radicals formed are used in initiation of polymerization or removal of inhibitor. (ii) Most of the inhibitor is removed by growing polymer chains. (iii) If x > 1 in the above expression then x = 1. The relation I.P. cc R-1 leads to the conclusion that the initiation rate is pro-portional to R.This follows from the fact that the inhibitor is removed almost entirely by growing polymer chains i.e. I.P. oc (initiation rate)-1. The above arguments apply only to the y-irradiation work. In the X-ray results the I.P. was so low that the dependence on monomer concentration and dose rate was not accurately ascertained. If the inhibitor concerned were oxygen the above results imply that very little 0 2 is removed in the process The relation I.P. a [m&l leads to additional conclusions. H + 0 2 -+ H02. THE DEPENDENCE OF THE RATE OF POLYMERIZATION ON THE DOSE RATE AND MONOMER CONCENTRATION.-The fOllOWhg reaction schemes are possible :-(1) H 2 0 + 2A(H + OH) kl (2) A + ml -f Aml* ki (3) Am:-1 + m1 -f Am kT3 (4) Am + A -+ Am,A ktl ( 5 ) Am + Am%+ Am,m,A (or Am + Am,) k,, (6) 2A -f H20 (or H2 or Hz02) k2 both assumed to be very rapid, where A = H or OH Am = a growing polymer chain Am or Am,A = a dead polymer chain.Chain transfer with water is unlikely in view of the high dissociation energy of water. Chain transfer with monomer may take place but will not affect the dependence of the overall rate on R and [m]. Jf the polymer chains are long and if the effective initiating radicals are dis-tributed uniformly the method of stationary states may be applied to the system and we obtain eqn. (1) and (2) according as reaction (4) or reaction ( 5 ) is the sole termination step Neither of the above expressions is in accord with the results at low dose rates, i.e. - d[ml]/dt a R0.95[m#.This difficulty raises the question as to whether the assumption of uniformity is correct or whether the results can be explained on the basis of a non-uniform radical distribution. If for example it is supposed that the polymerizations occur in a large number of independent small volumes, the number of which at any instant is proportional to the dose rate and if the rate of polymerization in each such element is given by eqn. (l) then the overall rate of polymerization will be proportional to R[rn@. As the dose rate increases the volume elements will tend to overlap and it might be expected that an increas-ing dose rate would lead to a decreasing exponent of R until at dose rates sufficiently high to give an effectively uniform distribution of radicals the polymerization rate would become independent of R and equal to (k1kp/kt,)[ml]2.It has previously been suggested 8 that the radiation induced polymerization of acrylonitrile could only be explained on the assumption of a degree of non-uniformity of radical distribiition. This conclusion was based on the suppositio 224 POLYMERIZATION OF ACRYLONITRILE that acrylonitrile would behave like methyl methacrylate in aqueous polymeriz-ations. For the latter monomer experiments independent of ionizing radiation indicated that either mutual termination or radical termination could take place depending on the rate of radical production.9 However in these experiments the rate of radical production was higher than the overall rate of radical produc-tion in any of the irradiations employed in the present work.Behaviour of a similar nature has been found for acrylonitrile. Using OH radicals generated uniformly throughout the reaction vessel and independently of ionizing radiation, but with about the same overall rate of production quite different reaction kinetics are found.10 The rate of polymerization is then proportional to ls[rnl] where Z is the intensity of light absorbed by a solution containing Fe3’0H- ions a result which indicates that mutual termination of the growing polymer chains is pre-dominant. This implies that a higher concentration of radicals is available with ionizing radiations but that the radicals are confined to a limited fraction of the total irradiated volume. THE POLYMER CHAIN LENGTHS.-Though the viscosimetric measurements can only be regarded as a semi-quantitative indication of the molecular weights it can be shown that they are in agreement with the conclusions drawn from the kinetics.On the concept of independent small volumes of radicals it is expected that at low dose rates the polymer molecular weights will be independent of R but proportional to [rnl]. If there is a uniform radical distribution and termin-ation of chains is by radicals the molecular weight may be expected to be pro-portional to [ml] and to R-*. This type of dependence should therefore be ap-proached at high dose rates. The results of the intrinsic viscosity measurements indicate that [77] cc [ml]”, where x > 1. In the relation [q] = KMS between intrinsic viscosity and number average molecular weight M for a polymer in solution the value of /i? generally lies between 0-5 and 1.0 [q] being expressed in 100 ml/g.To obey a relation of the type M cx [ml] a value of p of at least unity is required in this case. Such values have been reported for other systems and because no data are available on the value of 18 for this system we shall assume that the relation [q] = KM is applicable. At a given dose rate and assuming no chain transfer to occur we have - 4 = aP, where 4 = rate of polymerization a = rate initiation of chains andP = average polymer chain length. and assuming :. log 4 = log Y- + log P, [TI = k E log [q] = log k + logP = C + log P, :. log + - log [TI = log a + c. (3) Now at constant [ml] the concept of non-uniformity leads to d (log a)/d (log R) = $ or 1 at high and low dose rates respectively.In fig. 10 C + loglo cc (= loglo $ - loglo [q]) has been plotted against R. of appropriate to each value of [y] having been obtained by interpolation on fig. 5. Dependence of a upon R in the expected manner should give rise to a curve in fig. 10 of slope varying from approximately unity at low dose rates to 0.5 at high dose rates. The best line of decreasing slope has been drawn through these points and values interpolated from this curve have been used together to plot the curve in fig. 8. The results show that the viscosity measurements are in general agreement with the hypothesis of non-uniformity based on the kinetic measurements E . COLLINSON AND F . S . DAINTON 225 OTHER EFFECTS.-The fall of rate below what is expected of a second order reaction when the monomer concentration rises above 0.75 M and the irrepro-ducibility above 1 M is not an effect of the radiation.It has been shown that similar behaviour occurs in alcoholic solutions of styrene which have been studied over the whole concentration range.5 The effect is to be attributed to the lower efficiency of radical generation due to a higher percentage of direct action and promotion of the termination step produced by increased tendency of the chains to coagulate before termination. As judged by the fall of intrinsic viscosity of the polymers formed in this region the latter effect is probably predominant, and would explain the irreproducibility. The fact that temperature has so small an effect on the rates of polymerization seems to indicate that the termination step may have an energy of activation which counterbalances the energy of activation of the initiation step.Such an energy of activation may well arise if termination were by H atom abstraction FIG. 10.-Plot of (c + loglo a) against loglo (dose rate) a = initiation rate c = constant. from a growing chain. This would also give rise to double bonds at the ends of the chains in agreement with the occurrence of C=C double bonds as detected by the infra-red analyses. THE IMPL,ICATIONS OF THE PROPOSED mcmNIsM.-It appears that the polymer-ization of acrylonitrjle by X- and y-rays can be given a general interpretation on the postulate of a non-uniform radical distribution at low dose rates becoming more uniform at higher dose rates.The substantial continuity of the polymer-ization results between the three radiations employed suggests that the degree of non-uniformity at a given dose rate is about the same for each. Such a result appears at first sight to be contrary to the accepted view that the higher the electron energy the lower is the ionization density along its path. But this applies only to average values of the specific ionization. It is not impossible that a large proportion of the energy dissipated by any electron in water may ultimately appear as a series of individual groups of ions of approximately the same size. A change in the energy of the incident radiation would then merely alter the spatial dis-tribution of such groups of ions thus giving rise to the change in average track density of ionization due to the primary electrons.From this point of view it is to be remembered that approximately half the total ionization produced by a primary electron occurs as &rays of lower energies the other half occurring a 226 POLYMERIZATION OF ACRYLONITRILE isolated ion clusters of a conjectural size.11 Support is given to this theory by the fact that the ionic yield of the oxidation of ferrous ions in aqueous solution at concentrations below 10-4 M is independent of the dose rate.12 Though the results of these polymerizations can be explained in a general way on this concept of non-uniformity there are nevertheless certain difficulties in a complete interpretation along these lines. Thus in the previous treatment it has been tacitly assumed that stationary state homogeneous kinetics can be applied to each individual volume element.This is not strictly justifiable since the kinetics within a volume element will be non-stationary but a satisfactory non-stationary state treatment has not yet been found possible. Moreover the average lifetime of a polymer of chain length 7000 at a monomer concentration of 0.5 M will be of the order 14 sec if we assume a value of kp = 103 I./mole sec for acrylonitrile. It is difficult to see how such a long living chain can be terminated by radicals produced in the same isolated group as the radical initiating the chain since the initiating radicals would be expected to have a much shorter lifetime. The predominant type of chain termination may however, occur by mutual interaction of growing chains and the dependence of rate upon [m# may arise from the non-stationary conditions. In certain other chain reactions induced by ionizing radiations a dependence of the reaction rate on the square root of the dose rate is found e.g. the polymer-ization of acrylonitrile as the pure monomer 13 and the radiolysis of aqueous solutions of hydrogen peroxide.14 In these cases it must be presumed that the reaction chains are terminated in pairs and that each of the chains so terminated was initiated in a different volume element. The problem thus becomes one for which the usual treatment applicable to systems of uniform concentration is valid. In general chain reactions in which the chains are of long life and the chain carriers have a high coefficient of diffusion are likely to fall into this category. These requirements are both easily satisfied in the H202 radiolysis. The authors desire to express their thanks to I.C.I. Ltd. for gifts of acrylouitrile and dimethyl formamide to Dr. N. Sheppard and Mr. J. K. Brown of the Colloid Science Department Cambridge for carrying out the infra-red spectroscopic analyses and advising on interpretation of the results to Professor J. S. Mitchell of the Department of Radiotherapeutics Cambridge for placing 220 kV and 50 kV X-ray sets and a Beckmann spectrophotometer at their disposal to Messrs. R. S. Quick and J. Haybittle of the Department of Radiotherapeutics Cambridge, for assistance and technical advice in connection with X-ray apparatus and physical dosimetry and to the Department of Scientific and Industrial Research for an apparatus grant and a maintenance grant to one of us (E. C.). 1 Miller J. Chem. Physics 1950 18 79. 2 Miller N. private communication. 3 Fricke and Morse Phil. Mag. 1929 7 129. 4 Randall Fowler Fuson and Dangl Infra-red Determination of Organic Strrrctirres (Van Nostrand New York 1st ed. 1949). 5 Chapiro J. Chim. Phys. 1950 47 747. 6 Dewhurst H. A. private communication. 7 Dainton J. Physic. Chem. 1948 52 490. 8 Dainton Nature 1947 160 268. 9 Baxendale Evans and Park Trans Faraday Soc. 1946,42 155. 10 James D. G. L. unpublished work. 11 Lea Actions of Radiations on Living Cells (Cambridge University Press 1946 chap. 1). 12 Sutton H. C. unpublished work. 13 Chapiro Cousin Landler and Magat Rec. trav. chim. 1949 68 1037. 14 Rowbottom J. unpublished work

ISSN:0366-9033    

DOI:10.1039/DF9521200212

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

SOME ASPECTS OF THE RADIATION CHEMISTRY OF ORGANIC SOLUTES BY GABRIEL STEIN Department of Physical Chemistry, Hebrew University, Jerusalem Received 3 1 st January, 1952 The reactions leading to the eventual formation of free radicals in irradiated aqueous solutions are discussed. It is concluded that, particularly for sparsely ionizing radiations, electron ejection and capture processes are of importance for the understanding of the resultant chemical changes. Reactions of organic substrates, especially some dyes, in aqueous and non-aqueous systems are described, and it is shown that reversible reduction processes occur. Their mechanism is discussed with reference to th& possible biological significance. Other reactions of organic substrates, especially simple aromatic substances, are described which prove the formation and the role of the OH radical.The specific functions of molecular oxygen in the radiation chemistry of organic substrates are shown to depend on the nature of the substrate. Regarding the mode of action of ionizing radiations on dilute aqueous solutions, Weiss’s explanation 1 has been accepted in general. According to this, the solvent molecules are split, to yield in the end neutral fragments : H20 %%%*%+ H + OH (1) The individual processes leading to this final result have been the subject of some discussion. Lea 2 considered ionization to be the main primary process leading to observable chemical action, the chemical effects due to energy absorbed in excitation being negligible. The reason for this is that, in processes involving ionization, according to H2O %-%%%% + H20+ + e, the electron ejected will react at a site remote from the place of the original ioniza- tion, so that the radicals formed by processes like (3) and (4) (44 will have a smaller chance to recombine than those formed by a direct dissociation as in reaction 1, where the solvent cage will facilitate recombination to some ex- tent.394 Recently, it has been suggested5 that most of the chemical effects of alpha particles may be explained on the basis of the emission of ultra-violet quanta, which would cause chemical effects due to reaction 1.It has been assumed that, in this case, the charges produced in the primary reaction 2, by the alpha particles, recombine by virtue of coulombic attraction.On the other hand, attention has been drawn recently 6 to the possible significance of unstable ionic intermediates, other than those appearing in reactions 1-4, for the chemical processes. The suggestion has also been made 7 that, in the electron capture process 4, molecular hydrogen is produced directly according to 227 (2) H20+ + aq -+H& + OH H20- + aq -+ H + OH;, H20 + e -+ H2O- H20 + e -+ H2 + 0- (4b)228 ORGANIC SOLUTES This process was assumed to have a significant probability to account for a number of the chemical reactions observed. In the following, some r a n t experimental results involving organic substrates are discussed which may have a bearing on these problems. processes involved in the radiation chemistry of aqueous systems have often been considered in the light of experience gained from work with gaseous systems.It might be advantageous to consider the unique properties of aqueous solutions and, in other cases, to compare processes with effects in solids, to which many of the properties of aqueous systems are akin. Thus, whenever ionization processes occur in aqueous solution, sclvation will occur, owing to the rearrangement of the solvent dipoles in the field of the freshly produced charge. This rearrangement reduces the free energy of the system to a very great extent, so that reaction 3 will take place and be sufficiently exothermic to cause the formation of a neutral free radical and a hydrated proton. The field due to this will be much smaIler than could be deduced from the value of the original ionization potential of the HzO molecule.Whilst the depth of the freshly formed electron trap is thus decreased, simultaneously the free energy of the liberated electron is also decreased, owing to the polarization it causes in the medium. By the time the electron is slowed down to thermal energies, it may come under the influence of the electron affinity of the aqueous system itself or that of other suitable acceptors present in solution. A quantity related to the energy gained when an electron attaches itself to liquid water has been recently estimated from photochemical data.8 There is a likelihood therefore that, in aqueous solutions, the back reaction THE IONIZATION PROCESS AND THE REACTIONS OF THE CHARGED ENTITIES.-The H20f + e -+ H20 will occur to a much smaller extent than either in the gas phase or in most other solvents.The situation is somewhat modified with densely ionizing radiations, where the sites of subsequent ionizations along the track are near enough to each other to permit combination between an electron originating from a preceding site and a positive trap formed farther along the track. Thus, whilst the recombination between the or&inaZ partners seems unlikely, there is an increased likelihood, with densely ionizing particles only, of ionic recombination along the track. The fact that, even in gases, the rate of decrease of ionization due to recombination is proportional to n2, where n is the number of ion pairs present per unit volume, indicates that recombination is mainly between positive ions and electrons which do not originate from the same event.Thus, ionic recombination in aqueous systems irradiated with gamma or hard X-rays will not play a large role. With alpha particles such processes, as well as others due to the high ionization density, e.g. interaction of radicals,9 may play a role. They may account for the ultra- violet emission in this case, as well as for the reduced chemical efficiency of alpha particles, compared with gamma and X-rays, which has been observed in some cases. 10 It is thus to be expected that, in aqueous systems irradiated with sparsely ionizing radiations, reactions 3 and 4 will predominate. Other positive ions, which may be formed instead of H20+, e.g. OH+, may be expected to undergo a similar fate. Owing to the very high energy of hydration they will yield the hydrated proton and the corresponding neutral entity before other chemical reactions can occur, unlike the situation in the gas phase.In the following, the action of sparsely ionizing radiations, e.g. gamma and X-rays, will be considered only. ejected electron in the gas phase have been discussed by Burton and Magee.3 In aqueous solution, the electron may interact with the solvent which is in great excess, or with other electron acceptors present which have electron affinities (5) CHEMICAL ACTIONS DUE TO THE EJECTED ELECTRONS.-The reactions due to theGABRIEL STEIN 229 exceeding that of water. In addition to some inorganic positive ions which may possibly play such a role, in acid solutions the hydrated proton may interact either according to ~ 3 6 (6) in an exothermic process or, if reactions 4 and 4a have aIready taken place, accord- ing to Weiss’s suggestion 11 (7) This latter ion will then presumably serve as an electron acceptor, yielding molec- ular hydrogen.In both cases it will be expected that the evolution of hydrogen and the decomposition of water will be dependent on the pH, whilst in the case of direct formation of H2 according to 4b it would presumably be independent of it. In fact it has been found that, whilst neutral, pure water yields only a small quantity of hydrogen gas,l2 the decomposition of water and formation of hydrogen gas are greatly increased by the presence of Hf ions originating from acids, which themselves are not attacked, and which do not enter chemical reactions.13 This dependence on the pH seems to indicate that the importance of reactions of the type of 6 and 7 outweighs that of 46.The experiments of Dainton and James 6 support this view. It is possible that 4b is responsible for the constant, relatively small amount of H2 found in the decomposition of dilute aqueous solutions, even in that of H2O2.14 If this view is correct, it would be expected that, under the influence of sparsely ionizing radiations, under suitable experimental conditions, reduction processes will proceed readily, caused either by the ejected electron, or the H atom resulting from it. Recently it was found that, when methylene blue is irradiated in aqueous systems,l~~ 16 different phenomena may occur, according to the experimental conditions.If the irradiation is carried out whilst molecular oxygen is present in the system, irreversible reactions may take place. These involve the destructive oxidation of the dye with a relatively low yield, depending on the concentration of the dye.17 If, however, the irradiation is carried out in the absence of molec- ular oxygen, a reversible decoloration can take place with high efficiency. The decoloration can be reversed by the admission of molecular oxygen.16 The methylene blue may be incorporated into a gel, containing 1 to 10 % of gelatine or agar and only 0.003 % of the dye. Under these conditions, the re- versible decoloration will proceed readily in the absence of 0 2 and the reduction yield can be further increased by the incorporation of substances like benzoic acid in the system.The role of this would be to take up the OH radicals formed, which otherwise would reoxidize the dye.16 If the irradiation is carried out in the presence of 0 2 , there will be no decolora- tion up to a certain dose, beyond which decoloration will proceed readily. If the irradiation is carried out in the presence of carbon dioxide, a partial inhibition of the decoloration is observed. These results can be explained on the assumption that the dye interacts with either the electrons or with the H atoms formed from the electrons, according to H& + e -+ H ~ , + g + H -+ H L ~ dye + e -+ dye- dye + H --+ dyeH. In aqueous solutions we have been unable to differentiate between these two pro- cesses. In the presence of 0 2 , by virtue of its greater electron affinity, the reaction 0 2 + e --f 0 2 - , (9) or the corresponding reaction with the H atom 0 2 + H -+ HO2 ( 9 4 will take place in preference to reactions 8, 8a.to take place, and 0 2 admitted afterwards, the reaction If reactions 8, 8a are permitted (10) dye- + 0 2 --f dye + 0-2230 ORGANIC SOLUTES will take place involving a complete or partial electron transfer. C02 competes with the dye 16a but less efficiently than 0 2 and is itself reduced according to c02 + e' --f c02-- (1 1) CO2 + H -+ HC02. (1 l a ) That the original assumption of the role of C02 is correct6 is supported by the recent important experiments of Garrison and co-workers 18 which show the reduction of C02 in the absence of 0 2 and the formation of formic acid under the influence of ionizing radiations.The gel system described above manifests several interesting properties. In its composition it is not unlike biological systems and, in it, one particular substance, present in very small quantities only, is preferentially affected through a series of reversible oxidation-reduction processes which are initiated by the reducing entity formed by the radiations. In such a fairly complex system, the molecule with the greatest electron a f i i t y will retain in the end all those electrons or H atoms which have not undergone other, irreversible processes. The system may therefore serve as a schematic example of highly organized biological structures, and its behaviour may be used in the interpretation of the great radiation sensitivity of some living organisms in which one important component, say an enzyme, may be preferen- tially affected, although present in minute quantities only.These effects of the reducing entity may also be compared with the experiments of Forssberg.19 The enzyme catalase, in aqueous solution, in the absence of 0 2 , is inactivated on ir- radiation. This has been attributed to the action of the reducing entity, and this interpretation was supported by the fact that only those added substances provided a protection against inactivation which themselves were potential acceptors of the reducing entity. A reinterpretation of the experiments of Zimmer 1% may serve as further support for the conclusions reached here regarding the importance of electron (or H atom) trapping centres.He found that methylene blue is decolorized not only in aqueous, but also in glycerol solution. When this glycerol solution of the dye was under- cooled to - 70" C, forming a rigid glass containing the dye in solution, and was then irradiated, decoloration did not take place. If, however, this irradiated glass was allowed to warm up to room temperature without further irradiation, de- coloration occurred. Zimmer interpreted his results in terms of the splitting of the solvent into unspecified active radicals, which cannot diffuse away at - 70" C, but do so when the temperature is raised. Were the situation such, it would be more reasonable to expect recombination of the rigidly held, bulky radicals as soon as the temperature is raised.It is more likely that ionization takes place as the primary process, the electron which has been ejected travelling in the solid. It may then be trapped by (i) the solvent, (ii) the dye or (iii) positive ions created through irradiation according to I = Cl[S11 + C2[S21 + C3[S31 +- . . . (1 2) where [Sj] denotes the concentration of electron acceptors Sj, ci is a proportionality factor related to its electron affinity and I is the total number of electrons liberated by the radiation. Even though the electron affinity of the solvent is comparatively small, at these low temperatures the velocity of the subsequent reaction involving the release of a captured electron from a trap - d[Si-]/dt = Aj exp (- Ei/RT) [Si-] (1 2 4 will be greatly decreased.Consequently, electrons once trapped will be preserved in this state for longer periods than at room temperature. Therefore, since the number of solvent electron traps greatly exceeds the number of other traps which compete with it for the electrons, decoloration will not occur. This picture will be correct, if the probability of electron capture by one particular type of trap is proportional to the number of such traps and if the factor of proportionality CiGABRIEL STEIN 23 I is not a sensitive function of the temperature. On warming, reaction (12a) sets in, involving electron release from the solvent traps and retrapping by the dye. Alternatively, reactions analogous to reaction 4a can take place and normal decoloration can be observed. If this interpretation is correct, electron trapping should occur at very low temperatures even in solid water, and ice should show e.g. paramagnetic resonance after irradiation, owing to the presence of odd ions. Whilst this experiment has not been carried out yet owing to experimental complications, recently somewhat similar experiments were performed supporting the views detailed above and bringing the study of these processes more in line with previous knowledge derived from sirmlar processes in solids.The bulk of such information has hitherto been derived from the study of irradiated ionic crystals, where trapping was mainly by irregularities, the state and distribution of which was not always precisely known or controllable. How- ever, certain dyes, e.g. Sudan 111, when incorporated in plastics, form a solid system in which the nature and numbers of at least some of the trapping centres are known. On irradiation with X- or gamma-rays,20 these coloured plastics exhibit a deepening of colour, quite unlike their behaviour on irradiation with ultra-violet light, which results in bleaching.The deepening of the colour is due to the development of a well-defined absorption band in the red portion of the spectrum. Similar to the methylene-blue gels, the formation of this band can be inhibited by the presence of 0 2 in the plastic. If once formed by the irradiation of the plastic in the absence of 0 2 , it can be made partially reversible by the diffusion of 0 2 into the plastic. The irradiated plastics, coloured and uncoloured, show paramagnetic resonance after irradiation 21 which, on detailed investigation, indicates trapping of the ejected electrons partly by the plastic “ lattice ” itself, and partly by the dye.Electron ejection and trapping processes in plastics have previously been considered by Winogradoff.22 Preliminary experiments on the effect of the temperature at which irradiation has been carried out 21b indicate that, at lower temperatures, the relative trapping efficiency of the shallower trap increases greatly and that, on raising the tempera- ture, electron release and re-trapping processes do occur. Thus, when the plastics are irradiated in liquid nitrogen, the pattern of paramagnetic resonance observed at these low temperatures is different from that obtained by irradiating the same plastics at room temperature, and changes into the latter, if the specimen irradiated at low temperatures is allowed to warm up. Related thermal or photochemical processes have been observed previously in other solids 23 indicating release of trapped electrons and retrapping.In these cases, electrons ejected by more energetic radiations and later trapped may be released from the traps by, e.g. infra-red radiation or thermally, and retrapped in other deeper, or shallower, traps. If such reversible electron trapping processes have really some significance in biological systems, then the experiments reported here may bear some relatior, to the recently observed fact that, if irradiation with ionizing radiations of some biological structures is preceded or followed by irradiation in the visible or infra- red, the biological effects may be altered in a manner which cannot be obtained by infra-red irradiation alone.24 REACTIONS OF THE OH RmICALS.-The oxidizing action of the radiations in aqueous solutions may possibly have been explained by the assumption of active entities other than the OH radical, which has been assumed above to result from the ready hydration of the charged positive fragment, as in reaction (3).Recently, experimental evidence has been obtained 253 26 which strongly indicates that the active species is in fact a free radical and, most likely, the OH radical. It has been known that OH radicals formed by other means, e.g. by Fenton’s reagent (H202 + Fez+ salt) according to the Haber-Weiss mechanisms27 are capable of hydroxylating aromatic compounds, e.g.benzene to phenol.28 Reinvestigation of this process using ionizing radiations instead of Fenton’s reagent has shown 26232 ORGANIC SOLUTES that similar processes occur in this case as well. The elucidation of the reaction mechanism has shown that the reaction occurs in two distinct steps. In the first of these a free organic radical is formed from the aromatic molecule, which is dehydrogenated. This indicates a reaction according to The phenyl radical can undergo secondary reactions which differ according to the experimental conditions. In the absence of 0 2 it can react, e.g. (i) with another OH radical forming phenol ; (ii) with another similar organic radical, yielding diphenyl ; (iii) with an H atom re-forming benzene; or (iv) it may attack another benzene molecule, yielding diphenyl that way.In the presence of 0 2 the organic radical will react preferentially with this or with the HO2 radical formed from it. In this manner, phenol formation will be enhanced and diphenyl formation sup- pressed, in the presence of 02.26 These results show that OH radicals are produced in irradiated water. The interaction of these with organic acceptors may lead to the formation of organic free radicals. Further proof for this has been provided in a series of experi- ments25-31 in which the analogous reaction was carried out using mono- substituted benzene derivatives as the substrates. In these, the existing sub- stituent may exert a directive action with respect to a second incoming substituent.If so, the ratios of the three possible di-substituted derivatives will not be in statistical proportions (2 ortho : 2 meta : 1 para). The quantitative study of the formation of the three hydroxylated derivatives obtained from these substrates has shown that, in all these cases, the directive effect of the existing substituent has operated in a manner which is, for such groups as the -N02,296 -COOH,29c -Cl,31 not their usual mode of operation. Namely, in all these cases, the substitution was mainly in the para position, with some ortho and meta formed. Together with the study of the influence of an already existing -OH group 30 where no meta-sub- stituted derivative was formed at all, the results were in agreement with the assump- tion of a free radical mechanism, and with the experimental results of other workers, using free radicals of a different nature and of different origin in similar aromatic substitution reactions.32~ 33 It has also been shown,30 that when the reacting entity is not a free radical, but a charged entity, e.g.the (FeOH)Z+ ion, different substitu- tion ratios may be obtained. On the one hand, this study of the aromatic substitution ratios serves as a confirmation of the role of OH radicals in radiation chemistry; on the other hand, the action of radiations on aqueous solutions provides a suitable method for the study of organic reaction mechanisms involving free radicals. THE ROLE OF MOLECULAR OXYGEN.-It has already been shown that the 0 2 molecule may play an important part in the reactions occurring in irradiated aqueous solutions, owing to its electron affinity 347 6 and to the fact that reaction 10a, the attachment of an H atom to the 0 2 molecule, is both strongly exothermic and does not require any appreciable activation energy.35 Thus in irradiated systems, whenever 0 2 is present or being formed during the irradiation process, the impression may be created, when the behaviour of other solutes is being investigated, that reduction processes are suppressed. In reality these do occur, but it is 0 2 that is being reduced preferentially. In such systems, 0 2 may exert a protective action as with methylene blue,l6 with certain metal-ion solutions 33 and presumably also catalase.19 This will be also the case whenever 0 2 is present in a solution containing an organic substrate which reacts more readily with H atoms than with either OH or HO;! radicals.Thus, in the deamina- tion of certain amino acids,360 it was shown that this process proceeded more readily in the presence of H2 in the solution than either in vacuo or in the presence of 0 2 . The role of H2 is to react according to H2 + OH -+ HOH + H. (14)GABRIEL STEIN 233 The presence of 0 2 results in the removal of H atoms and a protection effect. In other cases, as shown in the preceding section, the presence of 0 2 may lead to the increased formation of one particular product, either through its addition to an organic free radical, which has been formed primarily, or through the intermediary formation of the HO;! radical, which thus removes H atoms from the solution, and can then add to the organic radical.This process may result in the specific forma- tion of one kind of organic product at the expense of another. Thus, the forma- tion of phenol occurs at the expense of the diphenyl formation in the presence of 02.26 In such cases, when one of the products is biologically active, whilst the other is possibly inactive, the presence of 0 2 may effect in this manner the bio- logical action of the radiation. This aspect has been emphasized recently when it was shown that, in the presence of 0 2 , organic products may be formed which are biologically significant and which are not formed at all in the absence of 0 2 . Thus from phenol, o-quinone is formed,30 whilst from amino acids, the cor- responding keto acid is obtained.37 These products resemble those obtained in biological processes.Similar reactions may influence the oxidation yields in inorganic systems. In all such cases, the presence of 0 2 will manifest itself in an increased reaction yield. Finally, it is also possible that the presence of 0 2 will make no difference to the apparent chemical yield, measured on one particular substrate. This may occur whenever H atoms and H02 radicals have similar reactivities with the substrate concerned. Thus it was observed that, with some amino acids, deamination occurred to the same extent under both sets of conditions.36a Similarly, with Ce4+ ions in acid solution, reduction processes will proceed to the same extent in de-aerated or 0 2 containing solutions.38 The influence of 0 2 will therefore be quite different according to the nature of the substrate, the experimental conditions and the particular process being observed.It may manifest itself, according to circumstances, in a protection effect, a poten- tiation, or no effect at all. 1 Weiss, (a) Nature 1944, 153, 748 ; (b) Trans. Faradzy SOC., 1947, 43, 314 ; (c) Brit. J . Radiol., Suppl., 1947, 1, 56. 2 Lea, (a) Actions of Radiations on Living Cells (Cambridge University Press, 1946) ; (6) Brit. J. Radiol., suppl., 1947, 1, 59. 3 cf. Burton, Ann. Rev. Physic. Chem., 1950, 1, 113. 4 Dainton, Brit. J. Radiol., 1951, 24,428. 5 Dee and Richards, Nature, 1951,168,736. 6 cf. Dainton and Collinson, Ann. Rev. Physic. Chem., 1951, 2, 99. 7 Haissinsky and Magat, Compt.rend., 1951, 233, 954. 8 Farkas and Klein, J. Chem. Physics, 1948, 16, 886. 9 Gray, J. Chim. Phys., 1951,48, 172. 10 Dale, Gray and Meredith, Phil. Trans. Roy. Soc. A, 1949,242, 33. 11 Weiss, Nature, 1950, 165, 728. 12 cf. e.g. Allen, J. Physic. Chem., 1948, 52, 479. 13 Milling, Stein and Weiss, to be published. 14 Johnson, J. Chem. Physics, 1951, 19, 1204; Lefort, Compt. rend., 1951, 233, 1194. 15 (a) Gallic0 and Camerino, Experientia, 1948, 4, 110; (b) Zimmer, Naturwiss. 1944, 16 Day and Stein, (a) Nature, 1950, 166, 146 ; (b) Nucleonics, 1951, 8, 11, 34. 17 Shekhtman, Krasnovskii and Vereshchinskii, Dokludy A N . SSSR, 1950, 74, 767 ; cf. also Vereshchinskii, Uspekhi Khim., 1951, 5, 737. Regarding related photo- chemical experiments cf. Schenck and Kinkel, Naturwiss., 1951, 38, 355, 503. 18 Garrison, Morrison, Hamilton, Benson and Calvin, Science, 1951, 114, 416. 19 Forssberg, Nature, 1947, 159, 308. 20 Day and Stein, Nature, 1951, 168, 644. 21 (a) Schneider, Day and Stein, Nature, 1951, 168, 645 ; (b) Day, Schneider and Stein, 22 Winogradoff, Nature, 1950, 165, 123. 32, 375 ; (c) for early work cf. ref. (2a). unpublished results. H234 INACTIVATION OF BACTERIOPHAGE 23 Garlick, Luminescent Materials (Oxford University Press, 1949), p. 35, 163 ; Curie, Nature, 1950, 166, 70. Regarding the role of organic dyes as electron acceptors, cf. e.g. Putseiko and Terenin, Doklady A.N. SSSR, 1950, 70,401 ; Chem. Abstr., 1951,45, 5018. 24 cf. e.g., Yost, Genetics, 1951, 36, 176. 25 Stein and Weiss, Nature, 1950, 166, 1104. 26 Stein and Weiss, (a) Nature, 1948, 161, 650 ; (6) J. Chem. Soc., 1949, 3245. 27 Haber and Weiss, Proc. Roy. SOC. A, 1934, 147, 332. 28 Cross, Bevan and Heiberg, Ber., 1900, 33, 2015. 29 Loebl, Stein and Weiss, (a) J. Chem. SOC., 1949, 2074 ; (6) J. Chem. Soc., 1950, 2704 ; 30 Stein and Weiss, J . Chem. SOC., 1951, 3265. 31 Johnson, Stein and Weiss, J. Chem. SOC., 1951, 3275. 32 Hey, Nechvatal and Robinson, J . Chem. SOC., 1951, 2892. 33 DeTar and Scheifele, J . Arner. Chem. SOC., 1951, 73, 1442. 34 Massey, Negative Ions (Cambridge University Press, 1950), p. 28. 35 Farkas and Sachsse, 2. physik. Chem. B, 1934, 27, 111. 36 Stein and Weiss, J. Chem. Soc., 1949, 3256 ; Stein, Watt and Weiss, to be published. 37 Johnson, Scholes and Weiss, Science, 1951, 114, 412. 38 Milling, Stein and Weiss, to be published. (cl J. Chem. Soc., 1951, 405.

ISSN:0366-9033    

DOI:10.1039/DF9521200227

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

INACTIVATION OF BACTERIOPHAGE INDIRECT INACTIVATION OF BACTERIOPHAGE DURING AND AFTER EXPOSURE TO IONIZING RADIATION BY TIKVAH ALPER Radiotherapeutic Research Unit Hammersmith Hospital Ducane Road W. 12 234 done with the coliphage T3. Received 3 1st January 1952 Bacteriophages S13 and T3 have been irradiated with X-rays y-rays and U.V. light while suspended in varying concentrations in buffer solution. The problems studied included (i) variation of inactivation dose with concentration (ii) shape of survival curves (iii) inactivating effect of H202 (iv) after effect of radiation. Phage particles surviving irradiation (with ionizing rays) in dilute solution were found to be much more susceptible thereafter to the inactivating effect of H202. This change in the phage particles is an indirect effect of radiation but could not be attributed to the action of OH or HO2 radicals.Where material is capable of biological assay radiation effects can be studied at very low concentrations as was pointed out by Dale1 in the classical work in which he discovered the indirect effect of ionizing radiations on various enzymes. Bacterial viruses are commonly assayed by the plaque-counting technique single particles penetrate the host bacteria multiply and produce in a confluent growth of the bacteria areas of lysis which are visible to the naked eye. It is possible therefore to estimate the number of viable particles in very dilute suspensions. Studies on the effects of radiation on such suspensions can yield information both on the mechanism of radical action and on the behaviour during and after ir- radiation of the smallest living organisms which may in some cases be regarded as single macromolecules.EXPERIMENTAL (a) BACTERIOPHAGE PREPARATION.-In most of the investigations to be described bacteriophage S13 was used. This is a dysentery phage active against S. flexneri strain Y6R but for all assays a sensitive E. coli strain was used. Some work has also been TIKVAH ALPER FIG. 1 .-H202 produced in aerated water by X-rays from Maximar 100 no filtration. 235 The diameter of S13 was determined by Elford 2 as being 13-18 mp and was estimated at 16 mp by Lea,3 who used radiations of different ion densities to obtain data which he interpreted in the light of the “one-hit” hypothesis.This good agreement between the size determination by Lea’s methods and others lends S13 a particular interest in radiobiology. Recent electron microscope studies by Elford 4 have revealed that S13 is a spherical virus and have confirmed that the diameter is about 15 mp The coliphage T3 is estimated as being about 45 mp in diameter and is a round phage with no tail.5 S13 stock suspensions were prepared by a method which was intended to reduce as far as possible organic material other than phage. The best stock preparation contained about 1.4 x 10-3 g/ml total solids of which about one part in 5 x 104 consisted of viable phage particles 1010/m1 in number. The lowest concentration of this stock used was a dilution of about 5 x 10-7 containing about 5000 phage particles per ml.In all the experiments suspensions of phage were in 10-3 M phosphate buffer pH 7 made up with Analar chemicals and glass distilled water. All glassware used was chemically clean. (b) IRRADIATION AND DosmmY.-(i) y-ruys.-In several sets of experiments the sources were 1 g radium 200 mg radium and 200 mc C060 the y-ray dose rates being respectively 140 r/min 30 r/min and 6 r/min. With the radium sources dose rates were calculated from the known activity and the geometry of the arrangement. The Co60 was in 15 ml of solution in a thick glass bottle and calibrated small bakelite-graphite ionization chambers were used to measure the dose rate in the phage suspensions which were exposed in ampoules arranged round the C060 container.(ii) X-ruys.-Several sets of experiments were done some with X-ray machines running at about 200 kV with or without external filtration and some with a Maximar 100 beryllium window tube running at 100 kV. The doses received by the phage sus- pensions were measured by suitable dosimetry methods for the conditions of each experi- ment. Dose rates in the X-ray experiments varied from 800 r/mh to 3600 r/min. (iii) Ultra-violet light.-A low pressure discharge lamp running at 3 mA and 9 W was used. No absolute determination was made of the energy absorbed; the quartz test tubes containing the phage suspensions were always exposed in the same position relative to the lamp and the results used for internal comparison. (c) H202 ESTIMATION.-when it became apparent that H202 played an important part in the indirect inactivation of bacteriophage it was necessary to measure the H2Oz produced at the rather low radiation doses which were inactivating the phage.As suggested by Savage,6 the oxidation of iodide to iodine was used to investigate the pro- duction in aerated water of H202 by X-rays emitted by a Maximar 100 beryllium window tube in the range 0-10,000 r total dose. The measurements made by Alper Stein and Wakley,7 are reproduced in fig. 1. 236 I ._ 0 - L 140 r/min. 4,' 8 INACTIVATION OF BACTERIOPHAGE FIG. 2.-Observations in 8 irradiations of S13 1.4 x 10-9 g/ml y-rays at 16 15 19 10 RESULTS SHAPE OF SURVIVAL CURVES.-The primary object of the first experiments (undertaken in collaboration with Dr.D. E. Lea) was to establish the relationship between inactivation dose and solid content of the phage suspension. It was at that time thought that indirect inactivation of phage particles resulted simply from single collisions with active radicals the sort of mechanism which has since been called monotopic by Gray.* Where only this indirect effect of radiation is taking place survival curves should be exponential. However the survival curves for the lowest concentrations of phage exposed to y-rays at 140r/min showed a marked departure from the exponential. Fig. 2 presents the results of eight experiments on suspensions of 1.4 X 10-9 g solids/ml. The survival curves for the whole range of concentrations used up to 3.3 x 10-2 g/ml are presented in fig.3 and 4. As the concentration was increased the curves apparently approached more nearly to the exponential. SURVIVAL CURVES PHAGE IN PRE-IRRADIATED BUFFER.-It Was thought that the non- exponential nature of the curves might be explained by the presence of a protective sub- stance which on combining with active radicals lost its protective action. Such a protective substance could gradually be " cleaned up " by radiation and the survival curve would become exponential after the cleaning-up process was complete. Thus preliminary irradiation of the suspending buffer solution should dispose of such protec- tion Ampoules containing 0.38 ml buffer were exposed to large doses of y-rays (30,000 r or more) and the pre-irradiated buffer was used in the last dilution stage of the phage preparation the standard procedure being to add 0.02 ml of a phage suspension to the Dose in Rontycns X lo3 17 2 - L ._ > 7 - 07 3 I1 12 (3 14 18 irradiated ampoule. The highest phage concentration which could be prepared in this way was therefore 5 x 10-2 of the stock phage preparation. Parallel ampoules were always prepared one to be used as a control for the other which was to undergo further irradiation. Since the number of active phage particles in the controls decreased with time the " surviving fraction " in the irradiated ampoules was taken as the ratio between the number present in the irradiated ampoule and the number in the control at the time of each sampling.The survival curves presented in fig. 5 therefore represent the in- activation due only to the action of the y-rays on the phage in pre-irradiated buffer the action of the irradiated buffer on the phage being automatically allowed for. Comparison of fig. 5 with fig. 3 and 4 shows that the inactivation doses were con- siderably less when the phage was irradiated in pre-irradiated buffer. The survival curves are exponential for the lowest concentrations but depart from the exponential at higher concentrations showing that the cleaning-up hypothesis was not tenable. TEMPERATURE DEPENDENcE.-It was found that the slope of the survival curves was dependent on temperature in the y-ray experiments described the temperature dependence being particularly marked for the suspensions in pre-irradiated buffer (fig.6 and 7). INACTIVATION BY H202.-An extensive series of experiments was undertaken to determine the effect of added H202 on S13 in order to assess the part played by the formed H202 in the irradiation experiments. The results were briefly (i) inactivation of S13 by chemical H202 proceeded exponentially ; (ii) inactivation by H202 was dependent on temperature on H202 concentration and on the solid content of the phage suspension (fig. 8 9 and 10 illustrate these results). 237 IRRADIATION IN PRESENCE OF CATALASL-Since H202 inactivated the phage but re- quired time to do this it seemed possible that the curvature of the semi-logarithmically plotted survival curves might be due to the gradual building up in the phage suspensions of H202 or H202 together with some other persistent toxic product of irradiation.It was thought that some light might be thrown on the action of irradiation-formed H202 if catalase were present in a suspension undergoing irradiation. In case catalase could act as a competitor for radicals a control was used containing in the suspension an equal --I. T A G € [LEA AND SALAMAK] + 5% PEPTONE 'L J<.STOCK ',STOCK PHAGE !06 I / - 5- 10' STOCK + 17- PEPTONE \ 6 4 10 \,37"C 7 1 8 9 5 DOSE ROENTGENS x 103 TIKVAH ALPER 3 iFTtL 2 4 I IO'STOCK 3 2 TIME HOURS 1 3- A I 20°C 0 25oc FIG. 3.-Survival curves S13 in various dilutions of stock preparation 'y-rays at 140 rlmin. FIG. 4.-Survival curves S13 in various dilutions y-rays at 140 r/min.FIG. 5.-SurvivaI curves S13 in various dilutions in pre-irradiated buffer y-rays 140 r/min. FIG. 6.-Temperature dependence of survival curves y-rays 30 r/min. FIG. 7.-Temperature dependence of survival curves phage in pre-irradiated buffer y-rays 140 r/min. FIG. 8.-Survival curves phage exposed to H202 at two temperatures. amount of catalase which had been inactivated by heating for 15 min at 104" C. A phage suspension containing no catalase was irradiated simultaneously with the other two at 6 r/min. The survival curves (fig. 11) demonstrated that the presence of active catalase caused inactivation of the phage to proceed exponentially in contrast with the catalase free suspensions. It seemed probable therefore that where the time taken for irradiation was long enough the gradual build-up of peroxides was responsible for an ever increasing rate of inactivation.SURVIVAL CURVES AT A HIGHER DOSE RATE.-A series of experiments was then per- formed with X-rays at a much higher dose rate (3600 r/min) so that the total irradiation time was too short to allow of much action by the formed H202. It was found (fig. 12) 238 that the survival curves were exponential at all concentrations. It should be noted however that the 3600 r/min dose rate was obtained by the use of soft X-rays whereas y-rays were used in the lower dose rate experiments. It is possible that quality dependence played some part in the shape of the survival curves as M. Ebert 9 has found that H202 production in clean aerated water depends on the quality as well as the type of radiation.Recent irradiations of dilute phage suspensions at about 3000 r/min with 200 kV X-rays (9) I 4 HOUR5 1 2 3 4 5 I 2 3 4 5 . 6 7 OOK ROENTGENS x 103 FIG. 9.-Survival curves phage of solid content 6 x 10-9 g/ml exposed to two different H202 concentrations. FIG. 10.-Survival curves two concentrations of stock S13 preparation in H202 solution INACTIVATION OF BACTERIOPHAGE :.I;- (10) \ m e + INACTIVATED CATALASE P H & E + C A T K * Y 10-4 m. FIG. 11 .-Survival curves phage irradiated in presence and absence of catalase y-rays 6 r/min. FIG. 12.-All survival curves X-ray irradiation of phage at 3600 rlmin. FIG. 13.-" Dilution curve " X-irradiation at 3600 r/min.FIG. 14.-Survival curves phage irradiated with ultra-violet light. have yielded non-exponential survival curves although the curvature is not nearly so pronounced as in the curves at 140 r/min. EFFECT OF DILunoN.-The series of exponential curves at 3600 r/min was used to deter- mine the variation of inactivation dose with concentration of the suspension inactivation dose being defined as that dose needed to reduce the fraction of surviving phage particles to e-1. In fig. 13 inactivation dose has been plotted against total solid content. The pattern is the same as that found for rabbit papilloma virus by Friedewald and Anderson,*() and for tobacco mosaic virus by Lea Smith Holmes and Markham.11 The initial con- stancy of the inactivation dose is well defined up to a total solid content of about 10-6 g/ml.2 3 \ 2 X Id6 STOCK 5 x 10. STOU 239 INACTIVATION BY ULTRA-VIOLET LIGHT.-It may be of interest to compare these results with those for inactivation by ultra-violet light. Inactivation was independent of con- centration in the range tested viz. 1.4 X 10-9 g/ml to 1.4 x 10-4 g/ml and proceeded exponentially at all concentrations (fig. 14). As the irradiation times were short no delayed effect would have shown up in the survival curves and such an effect was not looked for at the time these experiments were done. The fact that inactivation dose was independent of concentration was in contrast with the results for ionizing radiations or for added H202.AFTER-EFFECTS OF RADunoN.-Evidence from the experiments so far described led to the expectation that after the end of X- or y-irradiation phage would continue to be inactivated by the products of the irradiation formed in the suspending medium. This was in fact found to be the case as illustrated in fig. 15 by survival curves after various doses of X-rays. The curves were exponential as far as they went. In subsequent work however greater initial concentrations of phage were used so that inactivation could TIKVAH ALPER FIG. 15.-Inactivation of phage - after various doses of X-rays 36CO R 4 5 0 0 R 160 1 6 0 0 0 R - - - L L I I 20 42 60 80 i00 !20 lo0 MIPJUTES AFTER IRRADIATIOhI COb44EKE3 + Phaye A 6uJu /rruda/ed ;cO & FIG.16.-Survival curves irradiated phage and non-irradiated phage in irradiated buffer and non-irradiated phage in heat-inactivated irradiated phage suspension. be followed for longer times and to smaller surviving fractions. It was then found that the curves departed from the exponential the inactivation rate decreasing with time. ATTEMPT TO FJND EFFECT OF ORGANIC PERoxnxs.-In all the after-effect experiments the rate of inactivation was considerably greater than that found as a result of adding to phage suspensions the amount of H202 which would be produced by the radiation. Similarly it was found that phage still surviving at the end of radiation was subsequently inactivated at a much greater rate than phage put into buffer which had been exposed to the same dose of radiation.It was thought that organic material other than phage present in the suspensions might react with the active radicals to form toxic organic per- oxides and that this would explain the difference in the results. In order to test this explanation a suspension of phage was inactivated by heating for about 30 min at 60" C and then irradiated simultaneously with clean buffer and with a suspension of active phage. At the end of irradiation active phage was introduced into the irradiated buffer and the irradiated killed phage suspensions and sampling of these suspensions together with the irradiated suspension of active phage was continued for 140 min. The survival curves presented in fig. 16 show that the inactivation rate was the same for active phage intro- duced into the irradiated buffer or the irradiated killed phage suspension and much less ENHANCED SUSCEPTIBILITY TO ACTION OF IRRADIATED BUFFER SOLUTION AND H202.- 240 than the inactivation rate of the phage which had been present during irradiation and had survived the immediate effects thereof.Subsequent work has established the fact that the greater part of the delayed inactivation of phage after the end of irradiation is due to a change occurring as a result of the action of radicals which makes it much more susceptible thereafter to the action either of added H202 or of the H202 formed by irradiation of the suspending medium. Fig. 17 illus- trates the fraction of surviving phage particles at various times after the end of irradiation in the following suspensions of S13 (A) A dilute suspension (about 0.8 pg/ml total solids) in 10-3 M phosphate buffer exposed to 15,000 r of 200 kV X-rays.(B) 0-1 ml of suspension A introduced after irradiation into 1.9 ml buffer which had FIG. 18.4urvival curves phage INACTIVATION OF BACTERIOPHAGE been irradiated simultaneously with A. FIG. 17.4urvival curves of various suspensions after 16000 roentgens of X-rays. in 3 x 10-5 M H202. (C) 0.1 ml of non-irradiated suspension equal in concentration to A introduced into 1.9 ml of buffer which had been irradiated simultaneously with A. (D) 0.1 ml of suspension A afier irradiation introduced into 1-9 ml of non-irradiated buffer. (E) 0.1 ml of non-irradiated suspension equal in concentration to A introduced into 1.9 ml of non-irradiated buffer.(Control.) The curves illustrate the fact that the inactivation rate was identical for suspensions A and B and that to produce this rapid rate of inactivation it was necessary (i) that the phage suspension be exposed to radiation (ii) that it should be in contact thereafter with irradiated buffer solution. The additional curve F is the ratio between curves A (or B) and C and therefore expresses the number of survivors in the irradiated suspension as a fraction of the number of phage particles not affected by radiation which would survive the H202 produced. Curve F therefore depicts the inactivation due only to the interaction between phage damaged by the radiation and H202. This inactivation apparently pro- ceeded until a constant fraction remained presumably those particles not affected by the radiation.TIKVAH ALPER 241 In further experiments phage was exposed to a I3202 solution of concentration 3 X 10-5 My this being roughly the concentration which was produced in aerated water by 15,000 r of 100 kV X-rays (fig. 1). As can be seen from fig. 18 the inactivation rate for irradiated phage exposed to this concentration of Hz02 was very much greater than for non-irradiated phage exposed to the same concentration. The results with phage S13 have been reproduced with T3 which has been found to be much more radiation sensitive. The delayed effect on S13 after 15,000 r could be reproduced with T3 after 2,000r. No systematic comparison has as yet been made of the radiosensitivity of the two phages but the figures quoted indicate that the doses necessary to produce the same after-effects are roughly inversely proportional to the surface area of the phage particles.AFTER-EFFECT ON OXYGEN-FREE SUSPENSIONS.-The enhanced susceptibility of irradiated S13 to the action of H202 was observed in suspensions from which dissolved oxygen had been removed as well as in fully aerated suspensions. In order to demonstrate the effect it was necessary to introduce aliquots after irradiation into irradiated aerated buffer or into H202 solutions. A certain delayed effect was demonstrable with phage irradiated in oxygen free conditions as with irradiated phage introduced into non-irradiated buffer but this effect was small when compared with the inactivation resulting from the exposure 1 15 / O 2800 r/min.20 of the irradiated phage to irradiated aerated buffer. Fig. 19 presents the survival curves from an experiment in which the irradiated phage suspension had been rendered oxygen- free by bubbling nitrogen through for about 40 min before irradiation commenced. SURVIVAL CURVES AERATED AND OXYGEN-FREE SUSPENSIONS.-It Was found that during the X-irradiation of oxygen-free suspensions of S13 inactivation proceeded at a rate which was certainly no slower than that observed in aerated suspensions. The in- activation rate became greater in the latter only when reaction with H202 had begun to contribute to the inactivation (fig. 20). M/hotex offer end of /rrodblion duced into irradiated aerated buffer.FIG. 19.-Survival curves irradiated oxygen free suspension after about 10,000 r ; and aliquot intro- FIG. 20.-Survival curves aerated and oxygen-free suspensions of S13 X-rays DISCUSSION As bacterial viruses have been shown to consist to a large extent of DNA,12 it is interesting to speculate on whether enhanced susceptibility to H202 after exposure to active radicals is in fact a property of DNA. Butler and Conway 13 found that DNA continued to undergo degradation after the cessation of radiation only when dissolved oxygen was present during irradiation. They also found that while DNA was affected by H202 the concentration required to produce the same effect as a given dose of radiation was too high to have been formed by the radiation.On the other hand they found that the immediate effects of radiation INACTIVATION OF BACTERIOPHAGE 242 were not dependent on oxygen concentration. These results might well be ex- plained on the basis of DNA acquiring an enhanced sensitivity to H202 during irradiation. If the phenomenon which has been described is in fact a property of DNA and therefore of living cells it is clear that ionic yields based on observations on susceptible in vitro material and made immediately after cessation of radiation would appear much lower than if such damage as has here been reported for bacteriophage were given time to express itself. It is perhaps significant that damage to tissue cells well known to occur in general at much lower radiation levels than damage to in vitro material is commonly assessed by observations made some time after the end of radiation.The results of the experiment illustrated by fig. 17 make it possible to compare very roughly the ionic yield assessed in terms of immediate inactivation with that assessed in terms of damage to phage particles which makes them susceptible to Hz02. Curve F of fig. 17 shows that inactivation due only to reaction between damaged phage and H202 proceeded until after about 80 min the survivors remained at about 15 % of the number still active at the end of irradiation which was 50 % of the number of particles at the beginning of irradiation. Thus the fraction remaining completely un- damaged by the actual irradiation was 7.5 %.If the action of the radicals in damaging the phage was monotopic the dose required to damage 92.5 % would be about four times the dose required to damage 50 % (since 0.5 = e-0.7 and 0.075 = e-2*6) and an assessment of ionic yield based only on immediate inactiva- tion would be smaller by a factor 4 than an assessment based on the immediate inactivation plus the after-effect. As has been stated the results obtained with 15,000 r for S13 were roughly duplicated for T3 with a dose of 2,000 r ; which was sufficient to bring about inactivation of 93 % of the particles within 80 min. after irradiation ceased. It would seem that the order of magnitude of irradiation dose required to affect this type of in vitro material therefore does approach the doses which are commonly used in in vitro experiments on radiation effects.Some of the results described may be of interest in throwinglight on the behaviour of active radicals 14 in irradiated aqueous solutions. At the lowest concentrations of S13 the inactivation dose was constant so that ionic yield was directly pro- portional to phage concentration up to 10-6 glml. This accords with the theory set out by Dainton,ls and shows that the recombination of radicals plays the greatest part in their elimination in suspensions which contain fewer than about 1011 particles/ml of diameter whose order of magnitude is 15mp. The experiments in which the immediate effects of radiation on aerated and oxygen-free phage suspensions were compared demonstrate that for bacteriophage at least the presence of dissolved oxygen does not give rise to an indirect effect of greater efficiency except in so far as the oxygen is necessary for the formation of H202 which plays a part in a secondary reaction with phage particles already damaged by the radicals.An apparently enhanced inactivation in the presence of oxygen would however arise from the reaction between affected phage partides and Hz02 which can be formed by X- or y-rays only when oxygen is present. There is no evidence from this work therefore of the action of radicals of the H02 type which it has been thought might account for some of the increased effects of radiation on oxygenated material. In this connection it is interesting to note that Butler and Conway 13 in their studies on the X-irradiation of DNA found no dependence of immediate effects on the oxygen concentration of their solutions.These investigations have been carried out in several laboratories namely the Strangeways Research Laboratory Cambridge ; Onderstepoort Laboratories Pretoria South Africa ; the National Physical Laboratory Council for Scientific and Industrial Research Pretoria and the Radiotherapeutic Research Unit of the Medical Research Council Hammersmith Hospital. T am very grateful to the authorities at all these institutions for the generous facilities granted me. I TIKVAH ALPER am indebted to Mr. D. J. Savage and Miss Ilmary Reeler of the National Physical Laboratory South African Council for Scientific and Industrial Research for technical assistance.I should like to acknowledge my gratitude to Dr. W. Hayes of the Postgraduate Medical School for his assistance helpful advice and also for preparing and giving me a stock of the T3 phage. Dr. M. Ebert has been very kind in assisting me with many of the chemical problems. I have had the great privilege of Dr. L. H. Gray’s interest in this work from the time it was initiated and owe to him many of the ideas which have been followed 243 UP- 6 Savage Analyst 1951 76 224. 1 Dale Biochem. J. 1940 34 1367. 2 Elford and Andrews Brit. J. Expt. Path. 1932 13 446. 3 Lea Actions of Radiations on Living Cells (C.U.P. Cambridge 1946) chap. 3. 4 Elford personal communication. 5 Anderson paper in Symposium The Nature of the Bacterial Surface (Blackwell Oxford 1949) p.87. 7 Alper Stein and Wakley to be published. 8 Gray Brit. J. Radiol. (in press). 9 Ebert personal communication ; to be reported at this Discussion. 10 Friedewald and Anderson J. Expt. Med. 1940,45 713. 11 Lea Smith Holmes and Markham Parasitology 1944 36 110. 12 Cohen and Anderson J. Expt. Med. 1946 84 511. 13 Butler and Conway J. Chem. Soc. 1950 670 3418. 14 Weiss Nature 1944 153 748. 15 Dainton Ann. Reports 1949 45 5. 234 INACTIVATION OF BACTERIOPHAGE INDIRECT INACTIVATION OF BACTERIOPHAGE DURING AND AFTER EXPOSURE TO IONIZING RADIATION BY TIKVAH ALPER Radiotherapeutic Research Unit Hammersmith Hospital Ducane Road W. 12 Received 3 1st January 1952 Bacteriophages S13 and T3 have been irradiated with X-rays y-rays and U.V.light while suspended in varying concentrations in buffer solution. The problems studied included (i) variation of inactivation dose with concentration (ii) shape of survival curves (iii) inactivating effect of H202 (iv) after effect of radiation. Phage particles surviving irradiation (with ionizing rays) in dilute solution were found to be much more susceptible thereafter to the inactivating effect of H202. This change in the phage particles is an indirect effect of radiation but could not be attributed to the action of OH or HO2 radicals. Where material is capable of biological assay radiation effects can be studied at very low concentrations as was pointed out by Dale1 in the classical work in which he discovered the indirect effect of ionizing radiations on various enzymes.Bacterial viruses are commonly assayed by the plaque-counting technique single particles penetrate the host bacteria multiply and produce in a confluent growth of the bacteria areas of lysis which are visible to the naked eye. It is possible therefore to estimate the number of viable particles in very dilute suspensions. Studies on the effects of radiation on such suspensions can yield information both on the mechanism of radical action and on the behaviour during and after ir-radiation of the smallest living organisms which may in some cases be regarded as single macromolecules. EXPERIMENTAL (a) BACTERIOPHAGE PREPARATION.-In most of the investigations to be described bacteriophage S13 was used. This is a dysentery phage active against S.flexneri strain Y6R but for all assays a sensitive E. coli strain was used. Some work has also been done with the coliphage T3 TIKVAH ALPER 235 The diameter of S13 was determined by Elford 2 as being 13-18 mp and was estimated at 16 mp by Lea,3 who used radiations of different ion densities to obtain data which he interpreted in the light of the “one-hit” hypothesis. This good agreement between the size determination by Lea’s methods and others lends S13 a particular interest in radiobiology. Recent electron microscope studies by Elford 4 have revealed that S13 is a spherical virus and have confirmed that the diameter is about 15 mp The coliphage T3 is estimated as being about 45 mp in diameter and is a round phage with no tail.5 S13 stock suspensions were prepared by a method which was intended to reduce as far as possible organic material other than phage.The best stock preparation contained about 1.4 x 10-3 g/ml total solids of which about one part in 5 x 104 consisted of viable phage particles 1010/m1 in number. The lowest concentration of this stock used was a dilution of about 5 x 10-7 containing about 5000 phage particles per ml. In all the experiments suspensions of phage were in 10-3 M phosphate buffer pH 7 made up with Analar chemicals and glass distilled water. All glassware used was chemically clean. (b) IRRADIATION AND DosmmY.-(i) y-ruys.-In several sets of experiments the sources were 1 g radium 200 mg radium and 200 mc C060 the y-ray dose rates being respectively 140 r/min 30 r/min and 6 r/min.With the radium sources dose rates FIG. 1 .-H202 produced in aerated water by X-rays 100, from Maximar no filtration. were calculated from the known activity and the geometry of the arrangement. The Co60 was in 15 ml of solution in a thick glass bottle and calibrated small bakelite-graphite ionization chambers were used to measure the dose rate in the phage suspensions which were exposed in ampoules arranged round the C060 container. (ii) X-ruys.-Several sets of experiments were done some with X-ray machines running at about 200 kV with or without external filtration and some with a Maximar 100 beryllium window tube running at 100 kV. The doses received by the phage sus-pensions were measured by suitable dosimetry methods for the conditions of each experi-ment.Dose rates in the X-ray experiments varied from 800 r/mh to 3600 r/min. (iii) Ultra-violet light.-A low pressure discharge lamp running at 3 mA and 9 W, was used. No absolute determination was made of the energy absorbed; the quartz test tubes containing the phage suspensions were always exposed in the same position relative to the lamp and the results used for internal comparison. (c) H202 ESTIMATION.-when it became apparent that H202 played an important part in the indirect inactivation of bacteriophage it was necessary to measure the H2Oz produced at the rather low radiation doses which were inactivating the phage. As suggested by Savage,6 the oxidation of iodide to iodine was used to investigate the pro-duction in aerated water of H202 by X-rays emitted by a Maximar 100 beryllium window tube in the range 0-10,000 r total dose.The measurements made by Alper Stein and Wakley,7 are reproduced in fig. 1 236 INACTIVATION OF BACTERIOPHAGE I 0 L - FIG. 2.-Observations in 8 irradiations of S13 1.4 x 10-9 g/ml y-rays at 140 r/min. ._ 2: - 07 3 : > 7 - L . _ 4,' Dose in Rontycns X lo3 8 I1 12 (3 14 15 16 17 18 19 10, RESULTS SHAPE OF SURVIVAL CURVES.-The primary object of the first experiments (undertaken in collaboration with Dr. D. E. Lea) was to establish the relationship between inactivation dose and solid content of the phage suspension. It was at that time thought that indirect inactivation of phage particles resulted simply from single collisions with active radicals, the sort of mechanism which has since been called monotopic by Gray.* Where only this indirect effect of radiation is taking place survival curves should be exponential.However the survival curves for the lowest concentrations of phage exposed to y-rays at 140r/min showed a marked departure from the exponential. Fig. 2 presents the results of eight experiments on suspensions of 1.4 X 10-9 g solids/ml. The survival curves for the whole range of concentrations used up to 3.3 x 10-2 g/ml are presented in fig. 3 and 4. As the concentration was increased the curves apparently approached more nearly to the exponential. exponential nature of the curves might be explained by the presence of a protective sub-stance which on combining with active radicals lost its protective action.Such a protective substance could gradually be " cleaned up " by radiation and the survival curve would become exponential after the cleaning-up process was complete. Thus preliminary irradiation of the suspending buffer solution should dispose of such protec-tion Ampoules containing 0.38 ml buffer were exposed to large doses of y-rays (30,000 r or more) and the pre-irradiated buffer was used in the last dilution stage of the phage preparation the standard procedure being to add 0.02 ml of a phage suspension to the SURVIVAL CURVES PHAGE IN PRE-IRRADIATED BUFFER.-It Was thought that the non-irradiated ampoule. The highest phage concentration which could be prepared in this way was therefore 5 x 10-2 of the stock phage preparation. Parallel ampoules were always prepared one to be used as a control for the other which was to undergo further irradiation.Since the number of active phage particles in the controls decreased with time the " surviving fraction " in the irradiated ampoules was taken as the ratio between the number present in the irradiated ampoule and the number in the control at the time of each sampling. The survival curves presented in fig. 5 therefore represent the in-activation due only to the action of the y-rays on the phage in pre-irradiated buffer the action of the irradiated buffer on the phage being automatically allowed for. Comparison of fig. 5 with fig. 3 and 4 shows that the inactivation doses were con-siderably less when the phage was irradiated in pre-irradiated buffer. The survival curves are exponential for the lowest concentrations but depart from the exponential at higher concentrations showing that the cleaning-up hypothesis was not tenable.TEMPERATURE DEPENDENcE.-It was found that the slope of the survival curves was dependent on temperature in the y-ray experiments described the temperature dependence being particularly marked for the suspensions in pre-irradiated buffer (fig. 6 and 7). INACTIVATION BY H202.-An extensive series of experiments was undertaken to determine the effect of added H202 on S13 in order to assess the part played by the formed H202 in the irradiation experiments. The results were briefly, (i) inactivation of S13 by chemical H202 proceeded exponentially ; (ii) inactivation by H202 was dependent on temperature on H202 concentration, and on the solid content of the phage suspension (fig.8 9 and 10 illustrate these results) TIKVAH ALPER 237 IRRADIATION IN PRESENCE OF CATALASL-Since H202 inactivated the phage but re-quired time to do this it seemed possible that the curvature of the semi-logarithmically plotted survival curves might be due to the gradual building up in the phage suspensions of H202 or H202 together with some other persistent toxic product of irradiation. It was thought that some light might be thrown on the action of irradiation-formed H202 if catalase were present in a suspension undergoing irradiation. In case catalase could act as a competitor for radicals a control was used containing in the suspension an equal I / !06 1 iFTtL 2 4 - IO'STOCK 10' STOCK + 17- PEPTONE T A G € --I.[LEA AND SALAMAK] 'L J<.STOCK + 5% PEPTONE 3 ',STOCK PHAGE \ 5- \,37"C A , I 2 3 4 5 6 7 1 8 9 10 DOSE ROENTGENS x 103 I 20°C 0 25oc 3- TIME HOURS FIG. 3.-Survival curves S13 in various dilutions of stock preparation 'y-rays at 140 rlmin. FIG. 4.-Survival curves S13 in various dilutions y-rays at 140 r/min. FIG. 5.-SurvivaI curves S13 in various dilutions in pre-irradiated buffer : y-rays 140 r/min. FIG. 6.-Temperature dependence of survival curves y-rays 30 r/min. FIG. 7.-Temperature dependence of survival curves phage in pre-irradiated FIG. 8.-Survival curves phage exposed to H202 at two temperatures. amount of catalase which had been inactivated by heating for 15 min at 104" C. A phage suspension containing no catalase was irradiated simultaneously with the other two at 6 r/min.The survival curves (fig. 11) demonstrated that the presence of active catalase caused inactivation of the phage to proceed exponentially in contrast with the catalase free suspensions. It seemed probable therefore that where the time taken for irradiation was long enough the gradual build-up of peroxides was responsible for an ever increasing rate of inactivation. SURVIVAL CURVES AT A HIGHER DOSE RATE.-A series of experiments was then per-formed with X-rays at a much higher dose rate (3600 r/min) so that the total irradiation time was too short to allow of much action by the formed H202. It was found (fig. 12) buffer y-rays 140 r/min 238 INACTIVATION OF BACTERIOPHAGE that the survival curves were exponential at all concentrations.It should be noted, however that the 3600 r/min dose rate was obtained by the use of soft X-rays whereas y-rays were used in the lower dose rate experiments. It is possible that quality dependence played some part in the shape of the survival curves as M. Ebert 9 has found that H202 production in clean aerated water depends on the quality as well as the type of radiation. Recent irradiations of dilute phage suspensions at about 3000 r/min with 200 kV X-rays (9) I 4 HOUR5 1 2 3 4 5 m e + INACTIVATED CATALASE P H & E + C A T K * Y \ I 2 3 4 5 . 6 7 OOK ROENTGENS x 103 :.I;- (10) \ 2 3 5 x 10. STOU 2 X Id6 STOCK FIG. 9.-Survival curves phage of solid content 6 x 10-9 g/ml exposed to two different H202 concentrations.FIG. 10.-Survival curves two concentrations of stock S13 preparation in H202 solution 10-4 m. FIG. 11 .-Survival curves phage irradiated in presence and absence of catalase y-rays 6 r/min. FIG. 12.-All survival curves X-ray irradiation of phage at 3600 rlmin. FIG. 13.-" Dilution curve " X-irradiation at 3600 r/min. FIG. 14.-Survival curves phage irradiated with ultra-violet light. have yielded non-exponential survival curves although the curvature is not nearly so pronounced as in the curves at 140 r/min. EFFECT OF DILunoN.-The series of exponential curves at 3600 r/min was used to deter-mine the variation of inactivation dose with concentration of the suspension inactivation dose being defined as that dose needed to reduce the fraction of surviving phage particles to e-1.In fig. 13 inactivation dose has been plotted against total solid content. The pattern is the same as that found for rabbit papilloma virus by Friedewald and Anderson,*() and for tobacco mosaic virus by Lea Smith Holmes and Markham.11 The initial con-stancy of the inactivation dose is well defined up to a total solid content of about 10-6 g/ml TIKVAH ALPER 239 INACTIVATION BY ULTRA-VIOLET LIGHT.-It may be of interest to compare these results with those for inactivation by ultra-violet light. Inactivation was independent of con-centration in the range tested viz. 1.4 X 10-9 g/ml to 1.4 x 10-4 g/ml and proceeded exponentially at all concentrations (fig. 14). As the irradiation times were short no delayed effect would have shown up in the survival curves and such an effect was not looked for at the time these experiments were done.The fact that inactivation dose was independent of concentration was in contrast with the results for ionizing radiations or for added H202. AFTER-EFFECTS OF RADunoN.-Evidence from the experiments so far described led to the expectation that after the end of X- or y-irradiation phage would continue to be inactivated by the products of the irradiation formed in the suspending medium. This was in fact found to be the case as illustrated in fig. 15 by survival curves after various doses of X-rays. The curves were exponential as far as they went. In subsequent work, however greater initial concentrations of phage were used so that inactivation could FIG. 15.-Inactivation of phage -after various doses of X-rays 36CO R 4 5 0 0 R 1 6 0 0 0 R - - - L L I I 20 42 60 80 i00 !20 lo0 160 ;cO & MIPJUTES AFTER IRRADIATIOhI COb44EKE3 FIG.16.-Survival curves irradiated phage and non-irradiated phage in irradiated buffer and non-irradiated phage in heat-inactivated irradiated + Phaye A 6uJu /rruda/ed phage suspension. be followed for longer times and to smaller surviving fractions. It was then found that the curves departed from the exponential the inactivation rate decreasing with time. ATTEMPT TO FJND EFFECT OF ORGANIC PERoxnxs.-In all the after-effect experiments the rate of inactivation was considerably greater than that found as a result of adding to phage suspensions the amount of H202 which would be produced by the radiation.Similarly it was found that phage still surviving at the end of radiation was subsequently inactivated at a much greater rate than phage put into buffer which had been exposed to the same dose of radiation. It was thought that organic material other than phage, present in the suspensions might react with the active radicals to form toxic organic per-oxides and that this would explain the difference in the results. In order to test this explanation a suspension of phage was inactivated by heating for about 30 min at 60" C, and then irradiated simultaneously with clean buffer and with a suspension of active phage. At the end of irradiation active phage was introduced into the irradiated buffer and the irradiated killed phage suspensions and sampling of these suspensions together with the irradiated suspension of active phage was continued for 140 min.The survival curves, presented in fig. 16 show that the inactivation rate was the same for active phage intro-duced into the irradiated buffer or the irradiated killed phage suspension and much les 240 INACTIVATION OF BACTERIOPHAGE than the inactivation rate of the phage which had been present during irradiation and had survived the immediate effects thereof. ENHANCED SUSCEPTIBILITY TO ACTION OF IRRADIATED BUFFER SOLUTION AND H202.-Subsequent work has established the fact that the greater part of the delayed inactivation of phage after the end of irradiation is due to a change occurring as a result of the action of radicals which makes it much more susceptible thereafter to the action either of added H202 or of the H202 formed by irradiation of the suspending medium.Fig. 17 illus-trates the fraction of surviving phage particles at various times after the end of irradiation, in the following suspensions of S13 : (A) A dilute suspension (about 0.8 pg/ml total solids) in 10-3 M phosphate buffer, (B) 0-1 ml of suspension A introduced after irradiation into 1.9 ml buffer which had exposed to 15,000 r of 200 kV X-rays. been irradiated simultaneously with A. FIG. 17.4urvival curves of various suspensions after 16000 roentgens of X-rays. FIG. 18.4urvival curves phage in 3 x 10-5 M H202. (C) 0.1 ml of non-irradiated suspension equal in concentration to A introduced (D) 0.1 ml of suspension A afier irradiation introduced into 1-9 ml of non-irradiated (E) 0.1 ml of non-irradiated suspension equal in concentration to A introduced The curves illustrate the fact that the inactivation rate was identical for suspensions A and B and that to produce this rapid rate of inactivation it was necessary (i) that the phage suspension be exposed to radiation (ii) that it should be in contact thereafter with irradiated buffer solution.The additional curve F is the ratio between curves A (or B) and C and therefore expresses the number of survivors in the irradiated suspension as a fraction of the number of phage particles not affected by radiation which would survive the H202 produced. Curve F therefore depicts the inactivation due only to the interaction between phage damaged by the radiation and H202.This inactivation apparently pro-ceeded until a constant fraction remained presumably those particles not affected by the radiation. into 1.9 ml of buffer which had been irradiated simultaneously with A. buffer. into 1.9 ml of non-irradiated buffer. (Control. TIKVAH ALPER 241 In further experiments phage was exposed to a I3202 solution of concentration 3 X 10-5 My this being roughly the concentration which was produced in aerated water by 15,000 r of 100 kV X-rays (fig. 1). As can be seen from fig. 18 the inactivation rate for irradiated phage exposed to this concentration of Hz02 was very much greater than for non-irradiated phage exposed to the same concentration. The results with phage S13 have been reproduced with T3 which has been found to be much more radiation sensitive.The delayed effect on S13 after 15,000 r could be reproduced with T3 after 2,000r. No systematic comparison has as yet been made of the radiosensitivity of the two phages but the figures quoted indicate that the doses necessary to produce the same after-effects are roughly inversely proportional to the surface area of the phage particles. S13 to the action of H202 was observed in suspensions from which dissolved oxygen had been removed as well as in fully aerated suspensions. In order to demonstrate the effect it was necessary to introduce aliquots after irradiation into irradiated aerated buffer or into H202 solutions. A certain delayed effect was demonstrable with phage irradiated in oxygen free conditions as with irradiated phage introduced into non-irradiated buffer, but this effect was small when compared with the inactivation resulting from the exposure AFTER-EFFECT ON OXYGEN-FREE SUSPENSIONS.-The enhanced susceptibility of irradiated 1 M/hotex offer end of /rrodblion FIG.19.-Survival curves irradiated oxygen free suspension after about 10,000 r ; and aliquot intro-duced into irradiated aerated buffer. / O 15 20 FIG. 20.-Survival curves aerated and oxygen-free suspensions of S13 X-rays, 2800 r/min. of the irradiated phage to irradiated aerated buffer. Fig. 19 presents the survival curves from an experiment in which the irradiated phage suspension had been rendered oxygen-free by bubbling nitrogen through for about 40 min before irradiation commenced. the X-irradiation of oxygen-free suspensions of S13 inactivation proceeded at a rate which was certainly no slower than that observed in aerated suspensions.The in-activation rate became greater in the latter only when reaction with H202 had begun to contribute to the inactivation (fig. 20). SURVIVAL CURVES AERATED AND OXYGEN-FREE SUSPENSIONS.-It Was found that during DISCUSSION As bacterial viruses have been shown to consist to a large extent of DNA,12 it is interesting to speculate on whether enhanced susceptibility to H202 after exposure to active radicals is in fact a property of DNA. Butler and Conway 13 found that DNA continued to undergo degradation after the cessation of radiation only when dissolved oxygen was present during irradiation. They also found that while DNA was affected by H202 the concentration required to produce the same effect as a given dose of radiation was too high to have been formed by the radiation.On the other hand they found that the immediate effects of radiatio 242 INACTIVATION OF BACTERIOPHAGE were not dependent on oxygen concentration. These results might well be ex-plained on the basis of DNA acquiring an enhanced sensitivity to H202 during irradiation. If the phenomenon which has been described is in fact a property of DNA, and therefore of living cells it is clear that ionic yields based on observations on susceptible in vitro material and made immediately after cessation of radiation, would appear much lower than if such damage as has here been reported for bacteriophage were given time to express itself.It is perhaps significant that damage to tissue cells well known to occur in general at much lower radiation levels than damage to in vitro material is commonly assessed by observations made some time after the end of radiation. The results of the experiment illustrated by fig. 17 make it possible to compare very roughly the ionic yield assessed in terms of immediate inactivation with that assessed in terms of damage to phage particles which makes them susceptible to Hz02. Curve F of fig. 17 shows that inactivation due only to reaction between damaged phage and H202 proceeded until after about 80 min the survivors remained at about 15 % of the number still active at the end of irradiation which was 50 % of the number of particles at the beginning of irradiation.Thus the fraction remaining completely un-damaged by the actual irradiation was 7.5 %. If the action of the radicals in damaging the phage was monotopic the dose required to damage 92.5 % would be about four times the dose required to damage 50 % (since 0.5 = e-0.7 and 0.075 = e-2*6) and an assessment of ionic yield based only on immediate inactiva-tion would be smaller by a factor 4 than an assessment based on the immediate inactivation plus the after-effect. As has been stated the results obtained with 15,000 r for S13 were roughly duplicated for T3 with a dose of 2,000 r ; which was sufficient to bring about inactivation of 93 % of the particles within 80 min. after irradiation ceased. It would seem that the order of magnitude of irradiation dose required to affect this type of in vitro material therefore does approach the doses which are commonly used in in vitro experiments on radiation effects.Some of the results described may be of interest in throwinglight on the behaviour of active radicals 14 in irradiated aqueous solutions. At the lowest concentrations of S13 the inactivation dose was constant so that ionic yield was directly pro-portional to phage concentration up to 10-6 glml. This accords with the theory set out by Dainton,ls and shows that the recombination of radicals plays the greatest part in their elimination in suspensions which contain fewer than about 1011 particles/ml of diameter whose order of magnitude is 15mp. The experiments in which the immediate effects of radiation on aerated and oxygen-free phage suspensions were compared demonstrate that for bacteriophage at least the presence of dissolved oxygen does not give rise to an indirect effect of greater efficiency except in so far as the oxygen is necessary for the formation of H202 which plays a part in a secondary reaction with phage particles already damaged by the radicals.An apparently enhanced inactivation in the presence of oxygen would however arise from the reaction between affected phage partides and Hz02 which can be formed by X- or y-rays only when oxygen is present. There is no evidence from this work therefore of the action of radicals of the H02 type which it has been thought might account for some of the increased effects of radiation on oxygenated material. In this connection it is interesting to note that Butler and Conway 13 in their studies on the X-irradiation of DNA, found no dependence of immediate effects on the oxygen concentration of their solutions.These investigations have been carried out in several laboratories namely, the Strangeways Research Laboratory Cambridge ; Onderstepoort Laboratories, Pretoria South Africa ; the National Physical Laboratory Council for Scientific and Industrial Research Pretoria and the Radiotherapeutic Research Unit of the Medical Research Council Hammersmith Hospital. T am very grateful to the authorities at all these institutions for the generous facilities granted me. TIKVAH ALPER 243 am indebted to Mr. D. J. Savage and Miss Ilmary Reeler of the National Physical Laboratory South African Council for Scientific and Industrial Research for technical assistance. I should like to acknowledge my gratitude to Dr. W. Hayes, of the Postgraduate Medical School for his assistance helpful advice and also for preparing and giving me a stock of the T3 phage. Dr. M. Ebert has been very kind in assisting me with many of the chemical problems. I have had the great privilege of Dr. L. H. Gray’s interest in this work from the time it was initiated and owe to him many of the ideas which have been followed UP-1 Dale Biochem. J. 1940 34 1367. 2 Elford and Andrews Brit. J. Expt. Path. 1932 13 446. 3 Lea Actions of Radiations on Living Cells (C.U.P. Cambridge 1946) chap. 3. 4 Elford personal communication. 5 Anderson paper in Symposium The Nature of the Bacterial Surface (Blackwell, 6 Savage Analyst 1951 76 224. 7 Alper Stein and Wakley to be published. 8 Gray Brit. J. Radiol. (in press). 9 Ebert personal communication ; to be reported at this Discussion. 10 Friedewald and Anderson J. Expt. Med. 1940,45 713. 11 Lea Smith Holmes and Markham Parasitology 1944 36 110. 12 Cohen and Anderson J. Expt. Med. 1946 84 511. 13 Butler and Conway J. Chem. Soc. 1950 670 3418. 14 Weiss Nature 1944 153 748. 15 Dainton Ann. Reports 1949 45 5. Oxford 1949) p. 87

ISSN:0366-9033    

DOI:10.1039/DF9521200234

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

TIKVAH ALPER 243 GENERAL DISCUSSION Dr. N. Uri (University of Chicago) (communicated) One of the fundamental quantities in the energetics of the radiation chemistry of aqueous solutions is the bond dissociation energy H . . . OH. An accurate determination has been achieved by the fine experimental work of Dwyer and Oldenberg.1 It is however over- looked that in the evaluation of this bond dissociation energy and that of 0 . . . H in the OH radical they have not used the now generally accepted values for the bond dissociation energy of H . . . H and of the heat of formation of water. good approximation is however (decimals are hardly justified) 120 kcal for H . . . The value of 118.2 kcal can therefore not be accepted as absolutely correct. A OH and 100 kcal for 0 .. . H. Dr. G. Stein (Jerusalem) said There are a number of reactions in radiation chemistry which indicate that effects due to H atoms i.e. reduction processes do proceed readily under suitable conditions. As the reduction of methylene blue which is reversible on the admission of molecular oxygen shows these re- duction processes are often due to €3 atoms formed primarily rather than in a secondary interaction between H2 molecules and OH radicals. As found the addition of benzoic acid to methylene blue solutions in the absence of 0 2 results in a considerable increase in the decoloration. Benzoic acid is known to react readily with OH radicals. Were the OH radicals the source of the H atoms which reduce the methylene blue the addition of benzoic acid would result in a decrease and not an increase of the yield as observed.Hardwick 2 has shown recently that ceric sulphate solutions are reduced readily the reduction yield being the same in aerated and in evacuated solutions and higher in solutions containing H2. Milling Stein and Weiss (to be published) have recently also studied this reaction as well as the reduction of ceric per- chlorate. In this work using 200 kvp X-rays results in good agreement with 1 Dwyer and Oldenberg J . Chern. Phys. 1944 12 351. ZHardwick Can. J. Chem. 1952,30 23. GENERAL DISCUSSION H + 0 2 + H02 244 those of Hardwick were obtained. The results could be interpreted in terms of a reaction mechanism which did not necessitate the assumption of a reduction of ceric salts by OH radicals.It is perhaps possible that the difference between the results of these workers and those of Haissinsky is to some extent due to the very qoft radiation employed by the latter. Reduction processes may become masked under conditions where a1 terna tive acceptors are available for the H atoms. Thus in the presence of molecular oxygen the reaction will effectively remove the H atoms and prevent reduction processes from occurring unless the acceptor is capable of reacting with the HO2 radical or its ion 02- and of being reduced by these. This seems to be the case with ceric ions and such reactions have been previously assumed by Weiss and by Baxendale for the re- duction of the ferric ion. On the whole it appears that when due allowance has been made for the particular experimental conditions the bulk of the experimental evidence now available is capable of explanation by assuming the formation of OH radicals and H atoms followed by the appropriate reactions of these.Dr. M. Magat (Paris) said In the short note in which Dr. Haissinsky and myself suggested that the reaction would explain a large amount of experimental evidence we stated that this reaction is thermodynamically possible but did not indicate the argument for i t ; this I would like to present here. We can calculate the exothermicity of this and of the Weiss reaction + OH-, f Q2 (H20 + e)aq by the following cycle Q + A + D - E - S where A the heat of evaporation is 10 kcal in both cases D is the dissociation energy E the electron affinity and S the solvation energy.According to Gaydon 3 the best values for D1 and D2 are 11 5.6 and 117.8 kcal. For El and E2 we took the " best values " quoted by Massey,4 50.7 and 48.5 kcal. The solvation energy is more difficult to evaluate but one can show that S(O-) > S(OH-1 independently of the absolute values. Indeed in both cases all the 4 solvating water molecules have a hydrogen pointing towards the negative ion. This result is independent of the particular charge distribution that is assumed for the H20 molecule such as the one recently proposed by Lennard-Jones and Pople 5 or the one used earlier by myself6 and others. The a pviori possible configuration H/O H\ . . . H - 6 is less favourable by at least 30 kcal which is a value much higher than the errors usually made in electrostatic solvation calculations.This means that there exists for the OH- solvation a repulsion between the hydrogen atoms of the OH ion and the hydrogen of one of the four solvating water molecules. This repulsion is difficult to calculate but if we assume that it is 4 kcal the error will probably not exceed 2 kcal. 3 Gaydon Dissociation Energies. 4 Massey Negative ions (Cambridge) ; see also this Discussion. 5 Lennard-Jones and Popple Proc. Roy. Soc. A. 1950. 6 Magat Ann. Physique 1936 5 108. GENERAL DISCUSSION 3320 + e -+ H2 -1- OG and that this reaction is followed by 0; + H20 -+ OH.& + OH. H20 + e -+ H + OH,,. and putting 245 7f one introduces the two sets of values in the eqn.(3) one finds that Ql - Q2 = 8 4 2 kcal reaction (1) being more exothermic than reaction (2). (This last conclusion would remain even if S(0-) = S(OH-).) From the fact that the reaction which we have suggested is more exothermic than the reaction (2) usually assumed it cannot be deduced that reaction (1) will predominate. But it shows that there is no a priori impossibility for the reaction to occur. A knowledge of the activation energies for the two reactions would help in making a decision. But this is difficult to obtain and although we are attempting an evaluation any statement at the moment would be premature. Prof. F. S. Dainton (Leeds University) said Dr. Haissinsky has referred to the interesting suggestion which he and Dr.Magat 7 have made that the molecular hydrogen formed from water and aqueous solutions originates chiefly from the electron capture reaction (1 5 ) (1 5 ) (1 6) In this connection the following two considerations are of importance. Firstly we may compare on energetic grounds reaction (15) with reaction (14) which is the more commonly accepted process (14) Using the values Eo = 2.2 eV EOH = 2.1 eV DOH = 99.4 kcal D H ~ = 103-4 kcal we obtain (a) (15 + 16) SOH- - So- = 6 kcal - (AE14 - A&) ; D,o+ 213 + 0 = 219 kcal AE1 = 16 kcal + AE14 - AE15 = 22 kcal - (SOH- - So-). Thus for reaction (14) to be preferred to reaction (15) on energetic grounds AE14 > AEls i.e. SOH- - So- < 6 kcal and hence the energy of activation of reaction (16) must exceed 16 kcal.Hence the factors which made H2 + 0- more probable than H + OH- as products of electron capture by water are those which made reaction (16) less likely. This is evident from inspection of the equations concerned ; in fact if reaction (15) occurs readily Oiq is to be regarded as a more stable entity than OH;. We therefore conclude that if (15) occurs lo the exclusion of (14) it is most improbable that it will be succeeded by (16). The second consideration arises from the fact that electron transfer to water from a donor which may be a reducing cation MZf or anion Ag- to a water mole- cule of its hydration shell can readily be achieved by thermal or photochemical means.8 In the absence of any radical acceptor these electron transfer reactions always result in the evolution of hydrogen gas and the oxidation of the reducing ion e.g.the divalent ions V2+ Fez+ Cr2+ E u ~ + are converted to the corresponding trivalent ions. According to reaction (14) the molecular hydrogen results from the dimerization of the H atoms formed in this process and we may write e.g. M2+ H2O -+ M3+. H20- -+ M3f OH- + H(+ ?- ; H2). According to the Magat-Haissinsky reactions for the formation of the hydrogen gas we should write M2+ 2H20 3 M3f OH- + H2 I- OH 7 Haissinsky and Magat Compt. rend. 1951 233 954. 8 Dainton and James J. Chim. Phys. 1951,48 1 . GENERAL DISCUSSION Mz+ + OH -+ M3' OH- 246 which would presumably be followed by leading to a net process 2M2+. 2H20 + 2M3+ OH- + HZ OH + H2 + H20 + H HOI+LH- X-+HzO,+ X (4 and in passing we note that when the initial electron transfer is stimulated by light +a = 0.5$6.Quantum yields greater than unity would therefore afford some support to the Magat-Haissinsky scheme. These have not been observed but this fact alone is not of great consequence. A much stronger argument against reaction (15) is provided by the observation that radical acceptors such as water- soluble vinyl compounds suppress the gas evolution without affecting the oxidation of the reducing ion and are at the same time themselves polymerized. This is to be expected if the electron transfer reaction is represented by (a) whereas according to the Magat-Haissinsky scheme for the breakdown of HzO- the hydrogen gas evolution should be unaffected by the presence of the monomer.For these two reasons I think that although reactions (1 5) and (1 6 ) may occur they play a minor role compared to that of reaction (14) and cannot take place to an extent adequate to account for the observed radiation yields of molecular hydrogen. Dr. Haissinsky has also suggested that methylene blue and other dyestuffs may be preferentially reduced because the dye molecules compete unsuccessfully with molecular hydrogen for hydroxyl radicals. However the reaction probably has a much lower velocity constant than any other reaction involving OH radicals. It is known 9 that in the gas phase the energy of activation is about 14 kcal ; and in so far as the energy of activation of the H atom abstraction reaction follows the general rule of proportionality the dissociation energy of the bond broken i.e.DH . . . x the energy of activation for the case when X = H will be greater than for any Y-H bond in the dye molecule. Other possible reactions between dye and OH are probably additive involving even smaller energies of activation-see Dr. Waters' comment. An important point often neglected in considering the decoloration of the dyes is the fact that even in de-aerated solution the net effect on prolonged irradiation may be oxidution and not reduction; (Dr. Collinson has shown this to be true for rnethylene blue and Dr. Dale has men- tioned that thiourea protects MB against radiation bleaching) ; and to this extent the observations are in keeping with the known redox potential of the dye and the presumed e.r.p.of water. Dr. G. W. R. Bartindale (College of Technology Munchester) said In spite of the objections of M. Haissinsky I should like to support the following mechanism for the decomposition of the ion H20- (1) followed by H20- + H- + OH H- + H20 -+ H2 + OH-. H20 + e -+ H2 + 0- two bonds are broken and a fresh one formed. (2) The products of reaction (1) are both known to exist ; H- is the anion of the salts LiH CaH2 etc. Reaction (2) is also well known as exemplified by the action of water on lithium or calcium hydrides. Further no drastic rearrangement of bonds is involved in either reaction (1) or (2) whereas in the reaction proposed by Haissinsky viz. 9 e.g. Frankenburger and Klinkhardt Trans.Firradiry SOC. 1931 27 231 ; and von Elbe and Lewis J. Amer. Chem. SOC. 1932 552 821. GENERAL DISCUSSION 10 Smith and Goetz Ind. Eng. Chem. (Anal.) 1938 10 191. 11 Evans and Uri Nature 1950 166 602. 12 Hardwick and Robertson Can. J. Chem. 1951 29 828. 247 Mr. J. Wright (A.E.R.E. Harwell) said The observations concerning hydrogen peroxide production in boric acid solutions in the pile are of interest in connection with our own work on ferrous sulphate solutions containing boric acid. It seems quite probable that deviation from additivity of chemical efl'ects will vary according to the system studied. The lower G value for H202 production by the B(n a) reaction in the pile compared with that for H202 production by radon a-particles reported by Dr.M. Lefort earlier in this Discussion may be a measure of the neglected in discussing the results. In our work with ferrous sulphate + H3B03 deviation from additivity for this system which should clearly therefore not be solutions we have found a linear relation between ferrous oxidation and dose varying the dose both by increasing the time of irradiation and (in the centre of the pile) by increasing boron concentration. Deviation from linearity with boron concentration in the thermal column irradiations was in the opposite sense from that observed by Dr. Haissinsky. At the maximum H3B03 concentration used in our work (0-08 M) the absorption of thermal neutrons was only 3 % per cm and could be neglected but this might become important for higher concentrations and for different geometrical arrangements of the irradiation vessel.The increased H and OH radical production due to the presence of pile y-radiation may lead to the establishment of lower equilibrium concentrations of H202 which could explain the deviation from linearity with neutron flux observed by Dr. Haissinsky. In connection with the scheme proposed in eqn. (25)-(29) for the effect of boric acid we have found no effect of this substance on the rate of oxidation of ferrous sulphate in 0.8 N H2SO4 and find it difficult to believe that the chain reaction represented by eqn. (26) and (27) is effective in removing OH radicals under these conditions. Tests have been carried out with fission-product gamma radiation and by bombardment with 1-2 MeV electrons.Dr. C. B. Amphlett (A.E.R.E. Harwell) (communicated) The statement that complex formation in sulphuric acid solutions of ceric ion will affect only the rate of reduction and not its mechanism is likely to be misleading since (i) there is no evidence that this is so with ceric solutions and (ii) it may be extended by inference to other systems where it is clearly incorrect. The difference in Ei for the cerous- ceric couple as a function of the acid anion is very great e.g. in 1N solutions the values are - 1-70V (HC104) - 1.61 (HNO3) - 1.44 (H2SO4) - 1-28 (HCl),lo all potentials being expressed in the U.S. convention. The overall change of 0.42 V in going from HCI04 to HCl solutions is as great as that between the systems FeI1/Fe1Ir and Fe phy/Fe ph;' (ph = o-phenanthroline) so that with respect to a suitable system with an Ei value midway between - 1-28 and - 1.70 V the course of a cerous-ceric reaction could be reversed it is not inconceivable therefore that the mechanism of the reaction may be altered by complexing.In this connection it seems worthwhile emphasizing that by neglecting the true state of the ions in solution we may be obscuring the correct interpretation of the results. In the photochemical reduction of perchloric acid solutions of ceric ion the reduc- tion step is usually written as involving the CeOH3+ ion thus :I1 CeOH3+ + OH -+ Ce3+ -i- H202 etc. this providing a pH dependence similar to that given by step (2) in Haissinsky's paper. In sulphate solutions however spectrophotometric data 12 show that the sulphate complexes contain neither hydrogen nor hydroxyl ion so that the analogous step must be differently written.Although evidence is gradually ac- cumulating on the effects of complexing upon the rates and mechanism of ion -+ ion reactions in solution (in which sulphate complexing sometimes plays a notable part,l3) there is as yet little systematic data upon similar effects in radical + ion 13 e.g. Sykes J. Chem. Soc. 1952 124. GENERAL DISCUSSION 14 This Discussion. 248 reactions which are of fundamental importance in the interpretation of the radia- tion chemistry of aqueous solutions. Such evidence is likely to come from three sources viz. (i) a study of the pH effects in redox systems under irradiation in presence and absence of complexing agents (ii) study of the ionic species in the solutions concerned and (iii) efTect of pH and complexing upon suitable isotopic exchange systems.Such evidence as exists concerning the ferrous-ferric system is summarized in my paper,14 where it leads to the concept of increased ease of electron transfer when the hydration shell or other co-ordination shell is broken. Dr. N. Uri (University of Chicago) (cornnzunicatecl) The fundamental difference in the mechanism of the reduction of ceric ion by the OH radical as suggested by Haissinsky on the one hand and later by Evans and Uri on the other is to be sought in the participation of the ion pair Ce4+0H-. According to Evans and Uri it is the ion pair which reacts with the OH radical.This is essential as otherwise the reaction involves a termolecular collision. At the time when the original Haber-Weiss mechanism was published the significance of ion pairs was not yet recognized and this would have made the assumption of a back reaction in the oxidation of ferrous ion by hydrogen peroxide unlikely. From our knowledge today I consider that this back reaction is an essential part of the Haber-Weiss mechanism. It would be interesting to learn something of the intermediate stages in the reduction of dichromate ion by OH radicals as the mere formulation of overall reactions involving a large number of entities is extremely unsatisfactory in present day kinetics. In those cases where the mechanism has not been estab- lished such as in the chromate ion reduction the oxidation-reduction potential is of little value in the judgment or prediction of reactions occurring.Dr. M. Haissinsky (Paris) (parfly comnzutzicated) In answer to Amphlett and Uri's criticism about the real form of the ion of Cerv in sulphuric acid solution I may note that the aim of eqn. ( 2 ) of my paper where this ion was formulated as Ce4+ was only to show that the reduction is performed by OH radicals and not by H atoms as it was generally considered previously. The precise form of CeKv can hardly be established by radiochemical methods alone and the question must be studied by other physico-chemical techniques. But it is necessary to note that the potentials of Ce47/Ce3~ in various strong acid solutions are always sufficiently high for assuming the same fundamental mechanism as expressed by eqn.(2) for all these media. It is not the same for the Fe3+/FeZt quoted by Amphlett. The potential of this system is much lower probably at the limit of a possible reduction by OH. Even a small variation of the conditions can then not only modify the reaction mechanism but reverse the direction of the reaction (oxidation or reduction). At the same time I should like to indicate that 1 am not opposed to the e.r.p. concept of Collinson and Dainton which is an interesting development of the ideas of Lefort and myself on the role of the redox potential in radiation chemistry of aqueous solutions. I wanted only to point out that a constant value which would be independent of the conditions of the medium and the nature of the radi- ation cannot be attributed to the e.r.p.Thus I was glad to notice that during this Discussion Collinson and Dainton expressed a similar opinion namely that " the e.r.p. is a function of the type of radiation of the pH of the 0 2 content etc." The interesting observations of Dr. Wright on the radiochemical reactions taking place in the pile in the presence of boric acid do not seem to be in contra- diction with our suggestion of a competition of this acid for OH radicals and H202 at least with a-particles. Dr. Wright also found that the oxidation rate of ferrous sulphate in the pile diminishes in the presence of H3B03. If the decrease is more marked in the centre of the pile than in the thermal column it is probably due to the superposition of the action of the y-rays which are more intense in the GENERAL DISCUSSION 249 centre on the effect of boric acid.We have shown indeed with Dr. Lefort that the action of a-rays on various redox systems is due in larger part to H202 acting as an intermediate. Now this reagent is decomposed by y-rays in such a way that the total action of the radiations on the solutes becomes smaller. I think that it is in this sense that one should understand the non-additivity of the a- and y-actions postulated by Wright. The boric acid is not the only " indifferent " compound which modifies the radiochemical yields in aqueous solutions. In addition to the phosphoric acid mentioned by Dr. Conway we observed such effects with Li2SO4 LiC104 MgS04 etc.15 In any case the experimental data on the reactions in the pile are as yet too rudimentary to obtain a satisfactory full understanding.The suggestion of Dr. Bartindale of the formation of H- by decomposition of H20- has already been taken into consideration by Magat and myself in our note quoted in my paper. For energetical considerations we prefer eqn. (2) leading to the formation of 0-. I must however acknowledge that some aspects of the present Discussion seem to pose the question is the activation energy correctly treated in radiation chemistry ? One has the impression that something fundamental concerning perhaps the excited state of the radicals ions and mole- cules in the radiation field still escapes us and this " something " could account at least partly for the numerous disagreements which have become obvious at this Discussion.OH + OH = H202 which Dr. Weiss insists on the majority of radiochemists For example in spite of the activation energy necessary for the combination consider that the formation of hydrogen peroxide in solution takes place in this way. On the other hand even after the instructive experiments of Stein and Day and of Collinson on dyestuffs the energetic conditions and the mechanism of the radiochemical transformations of these substances are as yet far from clear. It is therefore also difficult to understand the role of benzoic acid ip Stein's experi- ments. I have already noted that the interpretation of the actions of " protectors " or " accelerators " is very delicate since the final and intermediate products can also participate in the competition for radicals.Finally in spite of the relatively high activation energy of the process H2 + OH pointed out by Dainton many authors agree that this reaction plays an important role in radiation chemistry of aqueous solutions especially in all the cases where the addition of hydrogen modifies the yields. The high activation energy calculated by Dr. Dainton for the reaction (16)' which is admitted by Magat and myself as following the reaction (15) would be another difficulty of this type. On the con- trary our hypothesis of the direct formation of Hz does not imply that a thermal photochemical ekctron transfer between one ion and a water molecule follows the same mechanism as the decomposition of a free H20- ion.One can conceive that the decomposition of a complex configuration 16 such as Me2+ . H20- gives MeOH2+ 4 H even if the free ion H20- is decomposed into 0- and H2. Only new and probably extensive experiments will allow a choice between the various possible primary reactions. Our suggestion was made principally on the basis of two essential arguments (i) high radiochemical yields of hydrogen ; (ii) predominance of oxidation reactions and the apparent inertia of the hydrogen. While the high hydrogen yield measured by Lefort and myself for ferrous sulphate was confirmed recently by Rigg Stein and Weiss much lower Hz yields are indicated by various authors for the Ce'" reduction.We shall re-investigate these experiments in order to discover the origin of the disagreement. We shall also examine again the effect of the initial presence of H2 and 0 2 on the rate of reduction of CeIV since our results are in this case also contrary to those of Hardwick. On the other hand the apparent inertia 15 Haissinsky and Pucheault J. Chim. Physique 1952 49 294. 16 See Farkas and Farkas Trans. Faraday Soc. 1938,37 11 13. GENERAL DISCUSSION H202 + OH -+ H20 + H02 H02 + 02- + H+ H202 + 02- -+ 0 2 + OH- + OH; 250 of the hydrogen has so far not found any other satisfactory explanation although it is one of the fundamental characteristics of the radiation chemistry of aqueous solutions. Dr. B. E. Conway (Chester Beatty Res.Inst. S. W.3) said In connection with Dr. Haissinsky’s observation that apparently indifferent substances such as H3B03 diminish the H202 yield in the irradiation of aqueous solutions of a-par- ticles it may be of interest to record a possibly related phenomenon. In some studies with Dr. J. A. V. Butler on radiation chemistry of sodium deoxyribonucleic acid we have investigated the effect of photochemically produced OH (and HO2) radicals from dilute aqueous H202 on mono- di- and tri-ethylphosphate esters as model substances. The U.V. irradiation itself causes no degradation of the esters but the photochemically produced radicals cause fission of the ester bond giving free inorganic Po&. At the same time the efficiency of the photochemical decomposition of H202 is increased for the mono- and di-ethyl esters which are present as anions in the aqueous solution whilst for the neutral tri-ethylphosphate the H202 is less readily decomposed.The effect is magnified if higher concen- trations of the mono- and di-esters are used and may be due to an interference with the secondary chain processes which can occur after the primary photochemical fission viz. these reactions can also occur in the radiochemical production of H202. In view of the facility with which PO$ can be accurately determined the use of a simple phosphate ester in radiation dosimetry might be suggested. Dr. J. Weiss (Durham University) (communicated) I agree with Dr. Matheson that more experiments are required to settle the dependence on the hydrogen peroxide concentration in the photochemical decomposition.However I cannot agree with him on the interpretation of some points in my paper. In the latter I have made an attempt to treat the general case of the non-stationary reaction by introducing certain simplifications and approximations as stated in my paper. It is obvious that while a linear term could not appear under stationary state con- ditions (cf. the treatment of the photochemical decomposition) it will always appear in a general treatment of the non-stationary state. Therefore Dr. Matheson’s criticism on this point is not justified. On the other hand as I have shown in my paper the dependence on [H202]* does not follow as an “ of course ” matter but holds only under certain specific assumptions the nature of which has been clearly stated.My introductory remarks are therefore not in any way a modification of the views expressed in the paper but rather a restatement of the physical conditions corresponding to the more mathematical treatment given in the paper. I think one would certainly agree with Dr. Matheson that the ‘‘ track ” con- cept in a narrow sense cannot be applied to cobalt 60 y-rays. In fact all that matters is that the formation of radicals occurs in discreet “ clusters ”. On the other hand it is doubtful whether any conclusions can be drawn from any very detailed picture about the distribution and size of clusters etc. as presented by Dr. Matheson. All I think that can be decided is whether the interaction of the radicals (i.e.chain-breaking) occurs (i) in or among the clusters created by the same particle (ii) between the radicals in clusters created by diferent ionizing particles or (iii) whether both is the case. In this way the experimental facts give a perfectly clear decision because in as much as the overall decomposition depends on the square root of the dose rate it is clear there must be an appreciable inter- action of radicals from clusters generated by different ionizing particles. How- ever in order to explain the [H202]* dependence one has to assume that one has GENERAL DISCUSSION 251 not only interaction between radicals created by different electrons but also inter- action between radicals created by the same electron. This assumption may not be justified but all that I intended to demonstrate was that this assumption is necessary to explain the experimental facts on the basis of this theory.However the very detailed picture regarding the clusters such as presented by Dr. Matheson does not seem to me to have any real physical basis apart from the fact that efficiency factors of 10-3 or even very much lower are quite common in chemical kinetics. Dr. E. Collinson and Prof. F. S. Dainton (Leeds University) said Dr. Amphlett's paper illustrates very clearly the way in which the reduction potential of a couple is important in determining the final composition of the mixture obtained when an aqueous solution of the simple reducing or oxidizing solute comprising the couple is subjected to prolonged irradiation.A short time ago 17 we suggested that it might be possible to assign to irradiated water a value of a reduction poten- tial designated the " equivalent reduction potential = e.r.p.," which was a function of the type of radiation used the pH and the oxygen content of the water etc. The reducing partner of a couple of reduction potential more positive (U.S. con- vention) than the e.r.p. by at least 0-2V would within the limits of analytical measurement always be completely oxidized whereas the oxidizing partner of couples of E" value more negative by at least 0-2 V would be reduced and system lying within this range e.r.p. f 0.2 V might attain a radiation " equilibrium " capable of approach from either side. The utility of such a concept if it proves to be valid is twofold firstly as a convenient shorthand representation of the net oxidizing power of irradiated water and secondly in providing a criterion by which any theory of the action of ionizing radiation on water may be tested.As an example of the former we would cite the prediction that Tl3+ solutions would be completely reduced to T1+ inert to radiation since Eo at pHo in perchloric acid for the thallous-hallic couple is - 1-25 V. Dr. T. J. Hardwick has in fact told us that his preliminary experiments show this to be the case. The purpose of the following paragraphs is to clarify certain points which could not be discussed within the limited scope of a review article. Imagine a de-aerated aqueous system in which H atoms and OH radicals are continuously generated at equal rates containing a solute in two states of oxidation represented for convenience by the cationic forms M+ and M2+.If the only reactions taking place are (1) and (2) (3) where the subscript s denotes the concentration of the species at the stationary state. This will be the true thermodynamic equilibrium if in which E& is the standard reversible reduction potential of the M+/M2+ couple and no is the standard reversible reduction potential of the system (5) OH + M+ -f OH- + M++ H + M++ + H+ + Mf a stationary state will ultimately be reached given by kl [OHls[M+ls = k2[HIs[M2+1 H(aq) + OHG~) + H Z ~ + OH(,@ + 2. From (3) and (4) the condition for attainment of true equilibrium is 17 Dainton and Collinson Ann.Rev. Physic. Chem. 1952 2 99. GENERAL DISCUSSION 252 and this condition will be satisfied when the M+/Mz+ couple is thermodynamically reversible to H and OH. This situation is most likely to occur when the processes concerned are simple electron transfers of the type instanced but in other oxida- tion systems which involve addition and/or subtraction of atoms or groups of atoms e.g. NO2-/NO3- and Mn2+/Mn04- eqn. (6) may not hold. Therefore even on the basis of the simplest model we would be surprised if all solutes gave the same value of the e.r.p. The species H and OH may enter into other reactions besides (1) and (2) above. The recombination reaction (7) H + OH -+ H20 (7) will not influence the position of the stationary state because it involves the removal of H and OH in a symmetrical way.The effect of reaction (7) competing with (1) and (2) to different extents as the nature of the solute is changed will merely alter the G values for the approach to the stationary state from either side. A correlation between the reduction potential of the solute and the yield is therefore very unlikely. The combination reactions of the radicals (8) 2H0 3 H202 2H + H2 kl COHI,[M+I B ~ 8 ~ ~ ~ 1 s z k 2 ~ ~ i s ~ ~ 2 + i . + k9w]s2 (9) may have an important effect on the position of the stationary state. Only if In solutions where the total solute concentration ([M+Is + [M2+]J is very low the will the stationary state be identical with the state obtained from (1) and (2) alone.final values of [MfIs/[M2+ls as well as of the yields may therefore differ appreci- ably from those in which the concentration is high. For the same reason some systems might manifest a dependence of the stationary state on the dose rate. A further complication in dilute solutions is that the hydrogen peroxide produced in reaction (8) may cause some slight oxidation (e.g. of Fez+) or reduction (e.g. of CeXv or MnO4-). Assuming that the conditions discussed above are sufficiently closely approached by a range of solutes for the e.r.p. to be independent of the precise chemical nature of the solute we may now consider the effect of other experimental variables such as pH radiation quality and aeration. Owing to the appearance of different chemical species at different pH's a change of pH will generally modify the reduc- tion potential of the solute couple (E").Assuming the effect of pH on no to be inappreciable the effect of pH on 7~ will be given by the term containing [OH-] and [H+] in eqn. (4). If E" and 7~ are altered unequally by pH the stationary state will also be sensitive to pH. The values of the reduction potential for the reaction calculated from thermodynamic data are - 0.37 3- 0.05 and $. 0.46 V at pH 0 7 and 14 respectively and it would be expected that the values of n- whilst not identical with these would vary with pH in a similar way. Generally E" also becomes more positive with increase in pH but the magnitude of the effect varies considerably from solute to solute. The effects of change of pH on the radiation equilibrium of different solutes may therefore differ not only in magnitude but also potential changes from - 1.44 V at pH 0 to + 0.77 V at pH 14 which is much in direction.Two examples will suffice to illustrate this. The cerous-ceric larger than the expected change of n-. Accordingly reduction is observed in acid media and oxidation in alkaline media.18 On the other hand Amphlett's data indicate that in the pH range 0 to 3 the variation of T is greater than the variation in E" and increase of pH therefore favours reduction. 18 Haissinsky Lefort and Le Bail J. Chim. Phys. 1951,48,208. GENERAL DISCUSSION produced by X- or y-rays. In aerated solutions reaction (1 1) may occur H + 0 2 e H O 2 observations. The back reaction Fe3+ + 0 2 - + Fe2+ + 0 2 0 2 - + 02- (or H02) -+ 0 2 + 022- (or H023 02- + OH -+ 0 2 + OH-.253 In so far as the type of radiation controls the spatial distribution of the radicals and therefore the [OH]/[H] ratio it will be a factor in determining the radiation equilibrium. On these grounds kr.p. a might be appreciably more negative than e.r.p. x or ,, and the limited data available show this to be true. However it should also be pointed out that this shift in e.r.p. could equally well be attributed to pro- duction by a-rays of a higher percentage of a more oxidizing entity than would be (1 1) and if this competes favourably with reactions (2) and (7) the initial yield of oxida- tion will be increased and the position of equilibrium shifted to the oxidation side.Thus the stationary state finally reached will also depend on the competition between reactions (2) and (1 l) and since k2 is likely to differ from solute to solute it is probable that the displacement of the e.r.p. brought about by aeration will vary from solute to solute. It is also possible that k2 is so large that the displace- ment is quite small and reduction of the same solute may occur in both aerated and de-aerated solutions. Dr. J. H. BaxendaIe (Manchester University) said Dr. Amphett’s paper draws attention to an important feature of the oxidation of aerated ferrous ion solutions by ionizing radiation which still requires clarification. This is the decrease in yield as the acidity is decreased below pH 2. If as is commonly supposed H02 to occur in the Fez+ + Fe3+ + H202 system 19 should be sufficient to explain the and OH are the only reactive species then it is probable that the reactions shown has been invoked by Rigg et al.but as Amphlett points out the decrease is apparent in the initial yields i.e. when ferric ion is absent a point which would not show up when working at constant dose as was done by Rigg et al. Amphlett suggests that in the higher alkalinities HO2 exists as 02- which is not able to oxidize Fe2f but if this is the case alternative reactions for 0 2 - must be sought. Possible ones are However these would lead to a dependence of the yield on [Fe2+] and dose rate which is not observed. There is one aspect in which the radiation system may differ from the H202 system viz.the possible existence of a non-homogeneous distribution of radicals tration then in order to estimate the importance of the Fe3+ + 02- reaction the in the former. If radicals are produced in isolated regions of high local concen- relative concentrations of Fe2f and Fe3+ in each of these regions must be con- sidered. Thus if the H02 concentration is sufficiently high it may arise that in at least part of each of such regions the local Fe3+/Fe2+ ratio is high enough to make this back reaction important although when the ratio is averaged over the whole solution it would appear negligible. On this picture one would expect a decreased yield even initially and a more detailed treatment of this aspect of the ferrous ion oxidation might lead to useful information on the distribution of radicals in aqueous systems which is an outstanding problem at present.and reported by anomalous The others also emphasi7e results given the difference perchlorate between acid the solutions radiation and Fe2+ by + Amphlett Fe3+ + H 2 0 2 systems. In our extensive work on the latter 19 no analogous observations were made although the rate of OH radical production was just as high as in the 19 Barb Baxendale George and Hargrave Trans. Faraday Soc. 1951 47 608. GENERAL DISCUSSION 254 radiation system. This difference may arise either from the presence of H atoms in the latter or again from high local concentrations of radicals assuming of course that the reactive entities are the same in both cases.with some caution. We have found 20 in an investigation of the Fez+ + Fe3+ + Observations using complex ions with organic ligands should be interpreted H202 system in the presence of ax'-dipyridyl that extensive oxidation of dipyridyl occurs even when it is combined as Fe(dipy)y. Dr. C. B. Amphlett (A.E.R.E. Harwell) (communicated) Dr. Collinson and Prof. Dainton have given an admirable summary of the ideas leading up to the concept of the '' equivalent reduction potential " together with some of the qualitative trends which might be expected under varying conditions. The general scheme outlined by them requires some modification if we have to allow for the introduction of species other than H and OH e.g. H02 H2+ OH+ etc.as they have already indicated; it will also require modification if the primary radicals are generated at unequal rates as in the primary steps proposed by Haissinsky and Magat,21 and by Lefort.22 The participation of species such as H02 and H2+ would lead to additional sources of pH-dependence with respect to suitable systems. In cases where H202 is able to react with one of the oxidation states of the couple its perturbing influence may be quite large because of its high potential oxidizing or reducing power; in particular it may partake to a very important extent in ferrous solutions at low concentrations (< 10-4 M) in the absence of oxygen where the OH radicals are not efficiently utilized. All these factors will help to make the precise location of e.r.p.H20 difficult to achieve even for simple couples of the type Mn+/M(n + I)+ Apart from these factors the effect of pH upon the final steady-state may vary for a given couple as a function of the anion present if 7~ varies in the same manner irrespective of the anion then the steady-state will be determined by the value of EM (or E'o as we have expressed it in our paper). Measurements of the system Fe2+/Fe3+ in HClO4 and in H2SO4 solutions as a function of pH have shown23 that apart from the difference in the standard potential in these two solutions there is also a qualitative difference in the pH-dependence in the two cases. Sulphate - 0.691 V solutions in 0-5 M (1.5 acid x to 10-4 - M - in 0.68 total V Fe) at pH show 1 followed an increase by in a potential decrease from to a minimum of - 0.708 V at pH 2.5 after which the potential becomes more positive aq again.1 M acid and pH 2 (E'o = - 0.732 to - 0.735 V) after which of independent rises steeply Perchlorate and at solutions pH 3 is - on - the 0.635 other V. hand The are behaviour in HC104 the pH solutions potential between is considered to be representative of hydrolysis of Fe3+ to FeOH2+ while the more complex behaviour in H2SO4 solutions presumably reflects the competition for FeI" between hydrolysis and sulphate complexing. It is hoped to proceed further with this work when a reliable value for the sulphate complexing constant is available.Since the steady-state ratio is dependent upon the difference (E - n) we might expect different values for sulphate and perchlorate solutions if the analytical methods available could be made sufficiently sensitive ; it should be noted however that T may vary if H2SO4 is replaced by HC104 since the measure- ments of initial yield indicate an increased oxidizing power in HC104 solutions below pH 2. Although it is true that in general we would not expect any correlation between reduction potential of the solute and the initial yield a correlation may arise in related systems where the kinetics are very similar e.g. in the ferrous-ferric system whereas if the pH is varied we obtain a parallel effect upon initial ferrous oxidation 20 Barb Baxendale George and Hargrave (unpublished).21 Haissinsky and Magat Compt. rend. 1951 233 954. 22 Lefort Compt. rend. 1951 233 1194. 23 Amphlett and (Mrs.) Davidge unpublished work. rate initial ferric reduction rate (in the reverse sense) and a final steady-state ferrous/ferric ratio. In conclusion we see the need for a systematic study of redox systems to cover all these aspects. Dr. J. Weiss (Durham University Newcastle) (communicated) As we have stated elsewhere,24 the action of X-rays on perchloric acid in aqueous solution leads in general to the formation of chlorate and oxygen while in the presence of ferrous salt instead of the former an equivalent amount of chloride is formed. It is how- ever not surprising that no interference by perchlorate is observed under ordinary chemical conditions because we have found that the decomposition of the per- chlorate is proportional to its concentration,25 which strongly suggests there is a direct action of the radiation on the perchlorate.In the experiments 24 regarding the oxidation of ferrous sulphate by X-rays in the presence of air pH N 2.5 was the highest pH used. We have ascertained that up to this pH and under the conditions of our experiments there is no initial pH effect. Dr. H. A. Dewhurst (Edinburgh University) (communicated) I would like to describe a few experimental results on the effect of sulphuric acid concentration on the oxidation of ferrous sulphate by C060 y-rays in aerated and air-free solution which are relevant to Dr.Amphlett’s paper. The main results are summarized in the accompanying table. 255 aerated -__ H2S04 mole/l. 1.0 0.4 GENERAL DISCUSSION [Fez+] = 5 x 10-4 M dose rate = 1-1 x 1017 eV/ml h HzS04 mole/l. 1.0 5 x 10-2 Gi Fez+ oxid/100 eV 20-2 20.2 20.0 18-4 15.0 14.0 5 x 10-1 5 x 10-2 8 x 10-4 5 x 10-4 26 Hart private communication. It is evident from a comparison of these results that the aerated and air-free systems exhibit a somewhat similar dependence on acid concentration. In both systems in the region of acid dependence the oxidation was found to be first order with respect to the ferrous ion concentration. Contrary to Dr. Amphlett’s results 1 have found neither an equilibrium concentration of Fe3f ions nor reduc- tion of Fe3f ions in either aerated or air-free solutions over the range of acid con- centration studied.In the region of acid dependence however the addition of Fe3+ ions to the ferrous solution decreased the initial yield. It was also found that the oxidation curve from any time t could be reproduced only with solutions of the same ferrous and ferric concentrations as at time t. We have repeatedly found that low concentrations of chloride ion have no effect on the oxidation yield in aerated 0-8 N N2SO4 solution and that even at 10-2 M chloride ion the initial yield decreases by only 5 % ; while we have also found that 10-3 M chloride ion has no effect on the oxidation yield in air-free 0.8N H$O4 solution. The results in aerated solution have been confirmed by Dr.E. J. H a r t 3 We have recently had an opportunity to test the effect of chloride ions on aerated ferrous sulphate solutions prepared by Dr. T. J. Hardwick and by Dr. A. 0. Allen and again found no difference in yield. Dr. C. B. Amphlett (A.E.R.E. Harwell) said The initial yields in aerated solu- tions quoted by Dr. Dewhurst are in quite good agreement with those given in my paper. It is interesting to note that the difference in yield between aerated 24 Rigg Stein and Weiss Proc. Roy. SOC. A 1952 211 375. 25 Milling Stein and Weiss to be published. 2.0 6.1 3-2 air-free ‘i Fez+ oxid/100 eV 8-2 8-2 7.6 0-4 5 x 10-3 8 x 10-4 5 x 10-4 (9 (ii) GENERAL DISCUSSION Fe3+ + H02 -2 k Fez+ + H+ + 0 2 Fe2+ + HO2 -2 k Fe3+ + H02- 256 and air-free solutions increases as the pH is increased a fact which is also evident from the earlier work of Fricke and Hart; 27 this may be significant in an inter- pretation of the results since it implies a more effectively pH-dependent mechan- ism in air-free solutions.It would be preferable to obtain air-free yields at rather higher concentrations where the yield is a maximum at high acidities,28 but I am well aware of the difficulties involved at high pH under such conditions. It is difficult to understand why no steady state has been observed at high pH nor reduction of ferric ion The effect produced by addition of ferric ion which has also been observed by Rigg Stein and Weiss,28 implies competition between Feat and FelIr for the species formed on irradiation and it is difficult to see what can happen to Fell1 other than reduction.The necessity for ferric ions as well as ferrous ions being present in order to reproduce an oxidation curie from any given Fe2f + H202 system.29 Since my paper was submitted we have obtained further time is also a consequence of this competition and was noted earlier for the evidence concerning reduction of ferric ion which will be presented elsewhere. The discrepancy between our results with chloride ion and those of other workers may be due to the presence of bromide in our chloride samples which were un- treated A.R. material; the effect of this upon the radicals produced in irradiated water has already been noted.30 I cannot agree with Dr.Uri that the experimental data are insufficient to show a fundamental inadequacy in the earlier kinetic mechanism. It is insufficient to enable us to decide what the inadequacy is but the fact that we are now considering HO2 dissociation and the participation of FeOH2- (and as Dr. Uri rightly points out even higher species) indicates the insufficiency of the simpler treatment. As Dr. Lefort has also pointed out in a private communication the pH dependence in air-free solution must also be explained and this cannot be done either on the basis of the old mechanism or with the aid of HO2 dissociation. Dr. J. H. Baxendale (Manchester University) (communicated) I would like to enlarge upon my suggestion that if the radicals are produced in localized regions of high concentration then the reactions can account for the decreased yield of Fez+ oxidation even initially.To illustrate this I will take a somewhat idealized case. Let us assume that the HO- and HOz reactions do not interfere so that we can treat each separately. Suppose further that the rate constants of reactions (i) and (ii) above (k4 and k3) are so large that (a) all the HO2 present initially in one of these regions reacts by (i) and (ii) before the H202 produced in (ii) can react appreciably with Fe2+; and (b) no diffusion occurs within the time taken for all the H02 to react. Let the pH be about 3 where k4/k3 is approximately unity.31 Then if the initial [Fez+] is a (Fe3+ = 0) and the initial concentration of HO2 produced in the localized volumes is b it can be shown that when all the HO2 has reacted by (i) and (ii) the total amount of Fe2+ oxidized by H02 (and hence H202 formed) is x where x is given by 2bJa = log (a/(a - 2x)).Examination of this expression shows that if the initial H02 concentration b is say 1 % of the initial ferrous ion concentration a then x will be within 1 % of b i.e. all the H02 will react with ferrous ion. If however b approaches a in magni- tude or exceeds it only 40-50 % of the H02 reacts with ferrous ion 50 % being 27 Fricke and Hart J. Chent. Physics 1935 3 60. 28 Rigg Stein and Weiss Proc. Roy. SOC. A 1952 211 375. 29 Barb Baxendale George and Hargrave Trans. Faraday Soc. 195 I 47 469. 30 Allen Ghormley Hochanadel and Davis J.Physic. Chem. 1952 56 575. 31 Barb Baxendale George and Hargrave Trans. Faraday Soc. 1951,47,608. GENERAL DISCUSSION 32 Barb Baxendale George and Hargrave Trans. Faraday Soc. 195 1 47 59 1. 33 Collinson and Dainton this Discussion. 257 the maximum attainable amount in these conditions. This would lead to a fall of about 40 % in the overall yield (taking into account HO* and H202) compared with that given in acid solution. Moreover the yield will be independent of ferrous ion concentration up to concentrations of the order of the initial HO2 concentration. It would also be independent of dose rates until the localizeci regions were so numerous as to overlap. This picture is undoubtedly oversimplified but it has the merit of using only established reactions and since if it is applicable it may give an indication of radical concentrations in irradiated aqueous solutions it would appear to justify further examination.Dr. C. B. Amphlett (A.E.R.E. HarweEZ) (communicated) The suggestion made by Dr. Baxendale to explain the dependence of the initial yield of ferrous oxidation upon pH merits further examination. As he has pointed out the mechanism proposed in my paper requires an additional step in order to dispose of the 0 2 - ions and the experimental results appear to rule out the two possibilities which he has considered; however this does not rule out the possibility of another alternative so that in the absence of further evidence this remains an open question. The mechanism proposed by Baxendale involves a non-uniform production of free radicals which could lead to a high ZocaE ferric ion concentration (and hence a back-reaction involving step (5)) under conditions where the overalZ ferric ion concentration was so low as to make it seem impossible for this reaction to contribute appreciably.On the basis of this mechanism as he has shown we can explain a decrease in the initial yield with increasing pH. If then the initial linear portion of the oxidation curve is due to local competition between ferrous explain the subsequent deviation from linearity; this may be due to the Fe3+ + and ferric ions for HO2 radicals then we must invoke another back-reaction to H02- reaction which will only become appreciable at much higher ratios of Fe3+/Fe2+ and at high pH.32 However we would still not expect the system to attain a steady state at reasonably low values of [Fe3+]/[Fe2+] on the basis of these kinetics unless the rate of depletion of ferrous ion within the volume ele- ments was considerably greater than its rate of replenishment (which may be con- sidered as proceeding both by diffusion of ferrous ion into the elements and also effectively by diffusion of some radicals out of the latter).Otherwise ferrous ions will continue to diffuse into the elements and be oxidized leading to a steady- state characteristic of the balance between ferrous oxidation and the Fe3+ + H02- reaction which would lead to a greater degree of oxidation than that observed. The general problem of the effects of non-uniform radical distribution is sufficiently important to require further expansion in connection with this system.The effects likely to be observed with X- and with y-radiation have only recently received attention chiefly as a result of the complex variation in dose-rate exponent in polymerization reactions.33 It has also been suggested by Magee,34 on theor- etical grounds that the radical distribution is non-uniform in water and aqueous systems at moderate dose-rates although his theory contains several simplifying assumptions. There are no quantitative data on either the fraction of the total volume which is occupied by the elements in which the radicals are formed or as to the magnitudes of the diffusion rates and distances involved.As a result we must be cautious in attempting to explain too much on the basis of this model. For example if we consider the oxidation of aerated solutions of ferrous ion in 0.8 N HzSO4 up to the oxygen break-point we can on this basis envisage the local depletion of 0 2 in the volume elements before it is depleted in the bulk of the solution provided that the rate of 0 2 consumption IocaIIy (which is dependent on rate of radical production) is greater than the rate of diffusion of 0 2 into the 34 Magee J. Amer. Chem. SOC. 1951 73 3270. GENERAL DISCUSSION 258 A[Fe2+] = 4[02] at the oxygen break-point for dose-rates where elements do not elements from the solution. This should result in a decrease in the stoichiometry is observed from overlap known - 30 the the r/min oxygen above to > concentration stoichiometry 3000 r/min,35 being is the rate that of in over oxidation the a whole wide being solution.range linear of dose-rates So up to far the as break-point within experimental accuracy. This suggests either that the volume elements occupy most of the bulk volume in all these cases or that the diffusion rates (of 0 2 into the elements and of H atoms out of them) are comparable with the local rate of 0 2 depletion. A solution to this problem might be obtained from further work over a much wider range of dose-rates than has hitherto been possible. with that both ph and its conjugate acid phH+ are destroyed solution aqueous Feph33-t of Concerning much lower the G reduction values than those for in Feph? in a concentration upon we have irradiation of observed - 10-4 but M.36 The reduction curve for Fephr upon irradiation is consistent with competition between Feph:' and some other species for the radicals formed (since Fephy is unaffected by prolonged irradiation this cannot be the competitor). However a tenfold excess of phenanthroline has no detectable effect upon the initial yield of reduction. Thus while agreeing that the results of experiments on such systems should be interpreted with caution we feel that our yields are representative of the reduction of Feph? ions by species formed on irradiation of the solvent.It is hoped to report this work in more detail at a later date. Mr. N . W. Luft (Waltham Abbey) said Although one might agree that under the conditions of the experiments reported to this Discussion the HO+ ion plays no important role views concerning its existence are rather conflicting. The reaction HO+ + HO- -+ H202 need not necessarily impair the oxidizing power of alkaline solutions since on the other hand HO- ions would promote decom- position of H202. In aqueous solutions of various hydroxy compounds the enthalpy of dissocia- tion HOX -+ HO+ + X- is calculated as AH (kcal) = b + 24 b + 8 b - 9 b - 10 b - 12 b - 40 for X = NO2 C103 OH C10 CI where b denotes the enthalpy of formation referred to AHf(H+ aq.) = 0 of HO+ in aqueous solution.This is related to the enthalpy change c in the reaction F viz. by b = c + 27 (kcal). Here use is made of the known value 37 of AH = 4.2 kcal for the reaction (1) (2) From the bond energies and ionization potentials 38 of individual charge clouds one would expect c to be small. A crude electrostatic estimate gives c = 18 kcal i.e. b = 45 kcal but the true value might be smaller. This shows that HO+ H202 . . . H+ -+ HO+ . . . OH2 AH = C H202 . . . H+ + H2O -+ H202 + H20 . . . H+. HOF or H20 + F2 which seems to have exceptional kinetic and oxidation is probably not important for H202 solutions but may be characteristic of aqueous properties and might be used as a means of studying the hydrated HO+ ion. Dr. J. Weiss (Durham University Newcastle) (cornrnmicated) A recent calculation of Prof.C. A. Coulson 39 has shown that the OH+ ion is unlikely to be of any importance in aqueous systems because it should be very unstable with regard to the dissociation according to OH&&. + 0 + Htydr. 35 Fricke and Morse PJiil. Mag. 1929 7 129. Miller J. Chem. Physics 1950,18,79. Rigg Stein and Weiss Pror. Roy. Soc. A 1952 211 375. Wright this Discussion. 36 Amphlett unpublished work. 37 Evans and Uri Trans. Faraday SOC. 1945 45 224. 39 private communication. 38 Mulliken J. Chem. Physics 1935 3 506. GENERAL DISCUSSION 259 one of the reasons being that the heat of hydration of the OH+ is presumably very much smaller than that of the proton. This is due to the relatively very small residual positive charge in OH+ caused by the shielding of the proton by the electronic cloud of the oxygen.The situation is quite different with H2+ where one has a relatively large residual charge because there is practically no electronic charge on the remote side of the protons as pointed out by Prof. Coulson elsewhere.40 Furthermore the binding energy of Hz+ which is about 61 kcal/mole also acts in a way as to oppose the dissociation into H f Hi. Dr. N. Uri (University of Chicago) (communicated) I would like to make a few suggestions concerning the mechanism put forward by Garrison and Rollefson The formation of hydrogen is postulated to occur via the reaction H + H +H2. in the radiation induced oxidation of ferrous ion in the presence of carbon dioxide. I consider that the reaction Fe2+(HOH) + H -+ Fe3+OH- + H2 is more likely to occur than the recombination of H atoms present at small concentrations.The reaction which I quoted is exothermic to an extent of 25 kcal. Furthermore formation of formic acid is more likely to occur via the electron transfer reaction HC02 + Fez+ + Fe3f + HCOO- than by a combination of H atoms and HC02 of radicals. formic acid Is it is not suppressed feasible that by the at reaction larger ferric Fe3f ion + HC02 concentrations +- Fe2+ + the H+ formation + C02 or possibly Fe3+ + C02- -+ Fe2t + C02 if an ionic dissociation of HC02 can be assumed.;! I wonder whether Garrison and Rollefson have any information on the effect of changes in the pH and whether they have tried to evaluate the electron affinity of carbon dioxide or the heat of formation of the HC02 radical.I feel that Amphlett is to be congratulated on having introduced energetic con- siderations into radiation chemistry which previously were insufficiently applied. In a reviewing article I have also pointed out the fact that the ionic dissociation of the H02 radical in aqueous solution has been hitherto completely neglected in radiation chemistry. The same applies to the significance of ion pairs in the reaction kinetics and on this point I would like to remark at a pH above 2-3 not only Fe3+OH- but also higher ion pairs such as Fe3+(OH)22- have been taken into consideration. As our knowledge on these higher ion pairs is still limited I would be reluctant to base any mechanism on experimental data obtained on the basis of experiments carried out at a pH of say 3.8.The published results do not appear to be sufficient for the claim that a fundamental inadequacy of the kinetics has been exposed. Dr. M. Haissinsky (Labordoire Curie Paris) said It seems difficult to under- stand why the reduction of C02 by H atoms is brought about in aqueous solution more easily than in a gaseous phase and that no activation energy is necessary. One would think that the reaction is produced by OH radicals which react primarily with HCO3- ions to give percarbonic free radicals similar to the perboric radicals HC03- + OH = C/- 0. \;HO The percarbonic radical reacts then with another OH + OH- C020H + OH = HCOOH + 0 2 .In presence of H2 formation of H atoms occurs (H2 + OH = H + H20) and This hypothesis implies that 0 2 in a quantity equivalent to HCOOH is formed. leads to the production of oxalic acid or followed by HCOOH + H = COOH + H2 C020H + H = COOH + OH COOH t COOH COOH - COOH. 40 cf. Rigg Stein and Weiss Proc. Roy. SOC. A 1952 211 375. GENERAL DISCUSSION 260 A similar mechanism for the decomposition of HCOOH by OH radicals (work of Hart) can be formulated. Dr. W. M. Garrison and Prof. G. K. Rollefson (University of Culijbniu) (cum- municated) The fact that carbon dioxide and hydrogen atoms do not react readily in the gas phase does not mean that hydrogen atoms are not involved in the formation of formic acid from carbon dioxide in aqueous solution.As we suggested in our paper the reactive species of dissolved carbon dioxide could be carbonic acid. If the probable assumption is made that reactions (2) (3) occur without appreciable or about the same activation energy then in order to reach a stationary concentration of formic acid as low as that observed in our experiments the reactive form of carbon dioxide must be at a concentration comparable to that of the formic acid. Inspection of our data will show that this is consistent with the assumption that carbonic acid is the reactive species since it is known thak about 1 % of dissolved carbon dioxide is present as the hydrate. The hypothesis that the reduction of carbon dioxide observed in our experiments occurs via a reaction involving hydroxyl radicals implies that hydroxyl radicals produced in the 1 M ferrous sulphate solutions are not reduced by Fez+ but instead react preferentially with bicarbonate which is at a concentration of less than 10-5 M.It seems more logical to us to assume that all the hydroxyl radicals are reduced by Fe2+. Dr. J. Weiss (Durham University Newcastle) said Although it has been possible only to give a very rough calculation in my paper nevertheless under certain assumptions a clear and definite physical picture emerges. The fact that the rate of the decomposition is proportional to the square root of the dose rate shows clearly that the only important part of the decomposition reaction is the chain reaction which takes place after the overlapping of the tracks.The question is now-what determines the concentration of the active radicals in this chain re- action which proceeds after the overlapping of the tracks? Clearly this must depend on the processes within the tracks before overlapping of the tracks has taken place. In the tracks (or clusters) the OH radicals are generated according to the eqn. (9) H20 -+ H + OH (and (lo) H202 + H-t H20 - OH). The OH radicals thus formed can now attack the hydrogen peroxide according to the re- action (2) H202 + OH -+ H20 + H02 which is known to be a fast reaction. However at this stage the H02 radicals thus formed apparently do not enter into the second (chain) reaction (reaction (3)) which in fact is known to be a relatively slow process.This is also borne out by the experimental facts which show that under these conditions i.e. within the tracks reaction (3) evidently does not occur to any appreciable extent. In the tracks the H02 radical concentration is appar- ently relatively high and the second chain reaction may not be able to compete with the fast reaction (7) 2 H02 -+ H 2 0 2 + 0 2 . Thus reactions (2) and (7) may lead in the first instance to a nun-chain decomposition of the hydrogen per- oxide within the tracks which may contribute only very little towards the total decomposition. The concentration of the radicals (n) for this process can be calculated to a first approximation by eqn. (IV) given in mq paper where the linear term corresponds to reaction (3) and the quadratic term corresponds to reaction (7).In this simplified equation n represents both the OH radicals and the H02 radicals because as was pointed out in the paper the concentration of the OH radicals is assumed to be proportional to the concentration of the H02 radicals. Now after the overlapping of the tracks the actual chain decomposition sets in and for this reaction the (stationary) concentration of the radicals (n,) is given by the usual type of equation i.e. eqn. (XVIIu) in the paper which can be written simply as (XVIlIa') I N - k'ns2 = 0 i.e. for this chain decomposition the linear term now disappears and one is left only with the bimolecular chain-breaking. In this equation IN is the number of radicals produced per unit of time and volume GENERAL DISCUSSION rate cc [H202] .ns K (dose rate)* [H202]*. - d[HzOz]/dt = 21 + 2(k2k3KH02/k5)’ (L/[HTI)’W2021* In fact the stationary state treatment applied to Dr. Weiss’ scheme gives - d[H202]/dt = la + 2(k2k3K~0~/k5)~ (la/[H+I)* [HzOZI. 261 which is relevant for the intertrack chain reaction. As has been shown in greater detail in the paper as a result of the non-chain process (before the overlap) IN may be given by a relation of the form IN K (dose rate) [H202]-*. Thus from eqn. (XVIIla’) it follows that n K (dose rate)* [H202]-* and the rate of the chain decomposition is then given by Dr. W. G. Barb (Cuurtaulds Ltrt. Maidenhead) said In table 1 of his paper Dr. Weiss has listed the kinetic equations to be expected for different termination steps.However there appears to be an error in the rate expression for termina- tion by OH + H02. Dr. Weiss gives this as (i - d[HzOz]/dt = la + (I2 + 4(k2k&02/k5) (lu/[Hfl)[H202l2)* (ii> If we are dealing with fairly long chains then by making the approximation (x2 + l)$ [N (x + 1 / 2 x ) ] ~ x for large values of x we obtain from (ii) (iii) Ths differs in the first term from eqn. (i). Eqn. (i) could in fact only be obtained from (ii) by the unjustifiable approximation (x2 4- l)* si (x + 1). accept termination involving a hydrogen-bonded H02 + H202 complex because With reference to Hart and Matheson’s paper it is somewhat difficult to (i) it is not clear why such a complex should show enhanced reactivity over HO2 hydrogen-bonded to water ; (ii) in dilute H202 only a small fraction of the total H02 would be hydrogen- bonded to H202 unless hydrogen-bonding between HO2 + H202 is taken to be considerably more probable than that between H02 + H20.On the other hand it must be admitted that a dependence of rate of reaction on [H202]* seems very difficult to obtain with other mechanisms. I would there- fore ask Dr. Hart whether he thinks the [Hz02]* dependence could possibly be in fact some other more complex dependence intermediate between ~ e r o and first order which might simulate a square-root dependence over a limited range of [H202]. Such a relation could perhaps be obtained with the more usual second- order termination reactions. In connection with Ebert and Boag’s paper it seems worth while pointing out that conductivity is not a reliable measure of water purity where traces of organic matter which might react with radicals are concerned ; even reproducibility of results after further purification is not always a reliable guide in this matter.41 I would propose that where doubt exists as to the interference of organic impurities (and this is not to imply that such is necessarily the case in Dr.Ebert’s work) the only reliable measure of these impurities is that employed by Fricke and Hart,42. 43 i.e. irradiating a purified evacuated sample of water and determining the carbon dioxide formed. It may also be that pre-irradiation followed by heating or by treatment with ultra-violet light of a suitable wavelength to destroy any H202 would prove to be a satisfactory method of water purification.Dr. J. Weiss (Durham University Newcastle) said Although the expression now given by Dr. Barb only differs by a factor of 2 in the first term from the equa- tion given in my paper in table l (line l) nevertheless I must insist that my expres- sion as given there is correct under the approximations which I have made which are concerned entirely with the physical nature of the process and do not involve 41 Barb Baxendale George and Hargrave Trans. Faraday Soc. 1951 47,462. 42 Fricke and Hart J. Chem. Physics. 1936 4 418. 43 Fricke Hart and Smith J. Chem. Physics 1938 6 229. GENERAL DISCUSSION 262 any such mathematical approximations as Dr. Barb suggests. This can be shown very simply as follows for the stationary state one obtains d(OH)/dt = 21a - k2[H202][OH] + k3[H202][02-] - k~[H021[OH] = 0 (i) (ii) which leads to d(H02)W = k21H2021 [OH1 - k3 [H2021 [%-I - kdH021 [OH1 (iii) and by introducing this into eqn.(ii) and assuming that k2[H202] > k5 [H02] l a = kdH02I[OHl (i.e. approximation for long chains) one obtains and thus finally with eqn. (iii) and (iv) which is identical with the equation given in my paper. The reason why I have given this particular approximation is that it leads to the correct approximation for long chains and also to the correct minimum value for the quantum yield for the non-chain process (ymin = 2). Dr. W. G. Barb (Courtaulds Ltd. Maidenhead) (communicated) Dr. Weiss apparently accepts my exact solution (eqn.(ii) in my note) and now seeks to justify his original expression (eqn. (i) in my note) as an approximation. I must therefore point out (i) No mention of any approximation was made in Dr. Weiss' paper. (ii) I have already shown that eqn. (i) is not a suitable approximation. (iii) Dr. Weiss states that he gave this particular approximation because it gives correct values for the two limiting conditions. In fact the number of otherwise unsatisfactory approximations which can be written solely to satisfy this limiting criterion is easily shown to be infinite. (iv) The fallacy in Dr. Weiss' new derivation is manifest. It involves the successive stages (c) k5[OH][HO2] is not neghgible with respect to k3[02-][H202]. (a) k~[OH][H021 is negligible with respect to k2[OH][H202] (6) k2[OHl[H2021= k3[02-"2021 Prof.W. Mund (Louvain University Belgium) said Dr. J. Weiss has developed theoretically the possible consequences of the heterogeneous distribution of the chain-initiating elementary processes along the path of a particle or a photon. Considering both cases of reactions confined to what may be called an isolated track and of a reaction site expanding from the initial track so as to bring about a general overlapping he seems to assume that in the second case the reactions between radicals or activated molecules originating from different tracks will modify the kinetics of the overall reaction. These theoretical expectations should clearly apply to the irradiation by x-rays where the initial heterogeneity is very pronounced indeed.Now I wish to point out that at least in one typical instance there do exist experimental features of the chain termination which rule out any effect of the overlapping of broadening tracks. The experiments to which I refer could not have been known to Dr. Weiss as they are only about to be published. They are described in a paper by Dr. van Meerssche M. Monigny and myself which is now ready but was not sent in time to be submitted to this Discussion. In brief the polymerization rate GENERAL DISCUSSION 263 of vinyl chloride under the action of the x-rays from a small amount of radon incorporated in the gas phase has been investigated at constant pressure. At four different pressures ranging from 20 to 70 cm Hg the ionic yield was found to be strictly independent of the intensity of irradiation which after one week was reduced to a quarter of its initial value.This points to a purely monomolecular mechanism of chain termination. On the other hand there is strong evidence that at the lower pressures and in smaller vessels an important fraction of the chains is only broken at the walls. The complementary fraction predominant at higher pressures or in larger vessels terminates in the gas phase but only by some monomolecular destruction of the growing radicals. Otherwise an effect proportional to the square root of the amount of radon left in the bulb should manifest itself. Dr. J. Weiss (Durham University Newcastle) said I was very interested in Prof.Mund's results although they refer to a gaseous system and I have only dealt with systems in solutions. I think it follows also from my paper that with first order termination reactions the yield will in every case be independent of the dose rate. This in fact corresponds exactly to one of the cases which is given also in table 1 of my paper where in the corresponding case for the photochemical reaction (table 1 reaction (6)) the quantum yield is independent of the intensity. Dr. M. Lefort (Institut du Radium Paris) said It has not yet been definitely settled which of the two reactions (1) or OH + OH = H202 OH + OH = H20 + 0 O + O = 0 2 is the more probable after irradiation of liquid water by a- X- or y-rays. How- ever there is both theoretical 44 and experimental 45 evidence that in most cases OH radicals have enough activation energy to react through (l) since this reaction is very exothermic.The results of Gunther and Holzapfel in 1939 46 mentioned by Dr. Allen seem to indicate again that (1) is the important reaction. Irradiating carefully de-aerated pure water these authors found a continuous evolution of hydrogen. From their results a G-value for H2 of 2.3 can be calculated. It is incidentally six times higher than the constant molecular hydrogen yield assumed by Dr. Allen. They did not find any oxygen. The formation of H2 may be due to the escape of this gas into the large volume above the irradiation cell a process which excludes any back reaction in the solution. But at the same trme OH radicals must be available.They will then react according to (1) or (2). If they produce H202 it is possible that under these conditions either the H202 is not decomposed or more probably the decomposition occurs without evolution of oxygen according to a chain reaction involving both OH and H radicals H202 + H = H20 + OH. OH + H202 = H02 + H20 HO2 + H = H202. Whatever the mechanism may be the combination OH + OH = H20 + 0 followed by 0 + 0 = 0 2 cannot be taken into consideration because the oxygen formed in such a way should escape as well as hydrogen into the large volume above the irradiated water. Dr. J. Weiss (Durham University Newcastle) said I cannot see any clear experimental evidence for the recombination of the OH radicals to form hydrogen peroxide.As I have already pointed out in my paper in the gaseous state it has 44 Dainton Ann. Reports 1949 45 27. 45 Lefort J . Chim. Phys. 1950 47 785. 46 Gunther and Holzapfel 2. physik. Chem. B 1939,44,374. 264 been shown by Bonhoeffer and Pearson that this recombination does not occur and the photochemical work of Lea on hydrogen peroxide also does not support this view. However I think that we have recently been able to obtain some further evidence which again shows that there can be no appreciable recombination of the OH radicals to form hydrogen peroxide. In these experiments which were carried out by Mr. Milling we have measured the yield of the hydrogen peroxide formed in the irradiation of water in the presence of (i) air (ii) oxygen (1 atm) and (iii) in the presence of mixtures of oxygen and hydrogen.In the experiments in the presence of air or oxygen we found that hydrogen peroxide was formed with an initial yield of G(H202) N 2-5. For this we have suggested the following reaction scheme (1) (2) (3) (4) If in this case hydrogen peroxide was also formed by the recombination of OH radicals and the same experiment is now carried out in the presence of a mixture of oxygen and hydrogen one should certainly not expect an increase in the yield of hydrogen peroxide because when the OH radicals are transformed to a greater or lesser extent into hydrogen atoms according to H2 + OH -+ H2O + H GENERAL DISCUSSION H20-+H + OH H + 0 2 -+ HO2 2H02 -+ H202 + 0 2 20H + H2O + 0 2 0 3 0 2 HO2 + 0 2 - + H02- + 0 2 H202 + OH = H20 + HO2; HO2 + OH = H2O + 0 2 .( 5 ) there should be a decrease in the initial yield of hydrogen peroxide. However the experiments with mixtures of hydrogen and oxygen give the seemingly para- doxical result that there is a considerable increase in the initial yield of hydrogen peroxide in the presence of hydrogen. In fact we have found that the maximum initial yield of hydrogen peroxide in mixtures of hydrogen and oxygen can under suitable coiiditions be nearly twice as much as that obtained in the presence of oxygen alone. This shows quite clearly that OH radicals themselves do not recombine to form hydrogen peroxide but when transformed into hydrogen atoms (according to eqn.(5)) they can again form H02 with molecular oxygen which then leads to an increase in the yield of hydrogen peroxide. In this connection I should like to mention that we have come to the con- clusion that the formation of hydrogen peroxide from H02 radicals very likely proceeds by the reaction between HO2 and 02- according to (6) rather than by the interaction of two H02 radicals (reaction (3)). Dr. M. Haissinsky (Paris) said The increase of H202 formation in the experi- ments of Weiss and Milling by adding hydrogen to the oxygen is not a convincing argument for rejecting the generally admitted mechanism OH+ OH= H202. There is good evidence that the measured overall rates of H202 formation are differences between the amounts really formed and those which are destroyed by the action of free radicals namely Consequently the addition of H2 protects H202 against decomposition by OH ac- cording to the reaction (5) and contributes to its formation according to (2).The last process is of course a second mode of formation of hydrogen peroxide when oxygen is present. Prof. F. S. Dainton and Dr. J. Rowbottom (Leeds University) said Anyone who has worked with solutions of " pure " hydrogen peroxide cannot fail to admire the skill with which Dr. Hart and Dr. Matheson have conducted their investigation. Nevertheless there is one important feature of their results which seems to us to be anomalous when considered in relation to the results of other workers which propagation 265 is in conflict with our own observations 47 and which obliges the authors to depart from the commonly accepted chain termination step and to propose one in which H202 molecules acting as third bodies must be at least lo4 times as efficient as water molecules.This feature is the proportionality of the decomposition yield to the reciprocal of the square root of the hydrogen peroxide concentration. They claim that this is also a characteristic of the photochemical reaction. In our submission the balance of the photochemical evidence is overwhelmingly in favour of this chain reaction being first order with respect to hydrogen peroxide. Lea's work 48 (quoted by the authors) supports this as also does that of Tian,@ of Henri and Wurmser,50 of Jeu and Alyea,sl of Kornfeld,s' of Taylor and Anderson.53 The only evidence for dependence of the rate on [H202]0.5 is that provided by Dain and Shvartz,54 and to a less degree by Allmand and Style.55 We have ourselves redetermined the kinetics of the decomposition of unbuffered carefully purified aqueous solutions caused by the mercury 3650A triplet using a sector technique and find that over the concentration range 1-22 M the rate is proportional to 10.5 [H202]1-0.As the Concentration is reduced the quantum yield decreases until the reaction ceases to be a chain reaction when as both Heidt 56 and Lea 48 have demonstrated the rate is proportional to 1:; and independent of the peroxide concentration. Thus with the exception of Dain and Shvartz' results and those of Allmand and Style (which have other anomalous aspects) the published data on the photolysis can be accounted for by the reaction scheme (1) ( 2 ) (3) termination (4) Using the same solution we find the y-radiolysis to have the same temperature coefficient and to follow the same kinetic laws as the photolysis.We therefore regard the radiolysis as initiated by reactions (5) and (6) ( 5 ) (6) H + H202 + H20 -k OH instead of (l) and followed by reactions (2) (3) and (4). We have not investigated the reaction at very low reactant concentrations because (i) we were anxious to ensure that all the radicals formed in reaction (5) reacted with hydrogen peroxide molecuIes in reactions (6) and (2) and (ii) we anticipated that as the peroxide concentration was reduced the order with respect to H202 would fall and the intensity exponent rise until the ultimate kinetic law was rate cc (R)1.0.It is of considerable interest that Risse 57 observed this changein the kinetics of the X-radio- (rate K (R)O*5 . [H202]0*5) are due to the fact that their concentration ranges and lysis as long ago as 1929. It is possible that the results of Johnson58 and of Fricke59 GENERAL DISCUSSION h V initiation H202 -+ 20H HO f H202 + H20 + HO2 HO2 (or 0 2 3 + H202 -f 0 2 + OH + H20 (or OH-) 2 H02 -+ H202 -j- 0 2 . H2O -L.+ H + OH 47 Dainton and Rowbottom Nature 1952 169 370. 48 Lea Trans. Faraday SOC. 1949 45 81. 49 Tian Compt. rend. 1910 151 1040. 50 Henri and Wurmser Compt. rend. 1913 157 126. 51 Jeu and Alyea J. Amer. Chem. Soc.1933 55 575. 52 Kornfeld 2. Wiss. Phot. 1921 21 66. 53 Anderson and Taylor J. Amer. Chem. SOC. 1923 45 650. 54 Dain and Shvartz Acta physicochim 1935,3 291. 55 Allmand and Style J. Chem. Soc. 1930 596. 56 Heidt J. Amer. Chem. Soc. 1932 54 2840. 57 Risse 2-physik. Chern. 1929 140 133. 58 Johnson J. Chem. Physics 1951 19 1204. 59 Fricke J. Chem. Physics 1935 3 364. I 266 GENERAL DISCUSSION dose rates were appropriate to these “ transition ” conditions. This effect would not however account for the difference between Hart and Matheson’s results and our own both of which correspond to conditions in which the kinetic chain length is considerable. This major discrepancy is in our view only to be resolved by further experimentation. Dr. J. Weiss (Durhnni University Newcastle) (coninirinicated) I was very interested in Prof.Dainton and Dr. Rowbottom’s remarks. In fact the mechanism represented by their eqn. (I) to (4) is identical with one of the mechanisms which is given in my paper.60 I cannot see however how at the present state of the experimental evidence all other chain-breaking processes can be excluded in favour of 2H02 -r H202 -I- 0 2 . Dr. E. J. Hart (Argonne Natioizal Lab. Chicago) said In reply to Dr. Barb experimentally we find that the y-ray induced decomposition follows the (H202)* dependence in the concentration range from 0.008 to 1.0 M. Under our experi- mental conditions the one-half order provides the best fit for the data in this 125-fold change in concentration. The transition stages to lower and higher orders occur outside this range of concentration.Below 0.002 M the behaviour is complex being approximately zero order down to a concentration of 0.00002 M. This is in the range of concentration where chain lengths of the order of unity are obtained. We also find complex behaviour above a concentration of 1.0 M hydrogen peroxide although this concentration range has not been extensively investigated. Mr. N. M. Luft (Waltharn Abbey) said The six-membered ring complexes of hydroperoxide seem to be quite an interesting proposition. If they can be formed by the participation of an HO;! radical then one would conclude they might also arise from H202 molecules alone. The binding energy of such rings (N 25 kcal) would be probably sufficient to ensure stability.There is an unexplained low Raman shift in concentrated H202 at 135 cm-1 which might correspond to ring puckering and the recently reported system61 between 500 and 630 cm-1 could involve ring deformation besides torsion of the monomer. Dr. M. Ebert (Hamn.tersmith Hmpitul W. 12) said Since sending in our paper we have carried out some further experiments with a beam of 500 kV electrons. In this work the water depth was 0.95 mm and the initial ionic yield was found to be about 0.4 at a dose rate of 1-4 x 105 ergs/g sec. The equilibrium H202 con- centration reached was 180 pmole H202/1. The initial yield was thus similar to that found in the work with 1 MeV electrons reported in the paper and the equi- librium concentration was intermediate between those for 1.2 MeV X-rays and for 1 MeV electrons.Miss T. Alper Dr. M. Ebert and Dr. L. H. Gray (Hamniersmith Hospital) Dr. M. Lefort (Paris) Dr. H. C. Sutton and Prof. F. S. Dainton (Leeds University) (comnmnicated) In connection with Dr. Ebert’s paper it seems desirable to publish here the results of some measurements of H202 yields in aerated water which are of special interest in that most of them were obtained in a joint investigation by several workers whose various techniques could be correlated and applied under the same conditions to samples taken from the same batch of water. The out- standing conclusion to be drawn from the results is that there is a definite difference between the effects of X-rays of 30 and 220 keV on the one hand and of 1 MeV electrons on the other despite the comparatively small difference in ion density of these radiations.The joint work was carried out with 1 MeV electrons from the Van de Graaf generator at the Radiotherapeutic Research Centre at Hammersmith Hospital by Miss T. Alper M. Ebert M. Lefort and H. C . Sutton. Samples of water freshly redistilIed from KMn04 and aerated of inverse conductivity 300 to 600 kQ were irradiated in open vessels in the manner described in Dr. Ebert’s paper; 40 this DiscuFcion. 61 Giguere Can. J . Res. B 1950 28 485. 267 irradiation and all dosimetric calculaiions are due to J. Boag. Low yields of H202 (< 3 x 10-5 M) in the irradiated samples were measured by T.A. using the method described by Savage 62 in which potassium iodide in acidified aqueous solution is oxidized by the H202 and the iodine liberated is measured colormetric- ally with a starch reagent and by H.C.S.using a technique in which the same reaction is carried out with K1 solutions strictly buffered to pH 5 and the re- sultant iodine is determined as 13- from its absorption at 3520 A (cf. Ghormley 63). Higher yields were estimated by M. L. and M. E. independently using the colori- metric titanium method. All these methods were calibrated from the same standard H202 solution. The results with 30 keV X-rays were obtained by M. L. at the Institute du Radium in Paris they are taken in part from previous pubkations and in part from recent measurements at low doses using Ghormley’s methcd of H202 estimation.The 220 keV X-ray results are taken from an invcstigation of 11. C. S. in the University of Leeds. t + M.L. and M.E. GENERAL DISCUSSION FIG. 1~.-H202 yields in aerated water. 1 MeV electrons. 0 1,000 r/sec T 60,000 , H20 -+ b”’ ‘ 10 r/sec A -O- b 300 loo 30 ” , ” 1 ‘,MI-. and M.E. The results are set out in graphical form in fig. 1 and 2. For convenience in presentation the results with small doses are shown on a larger scale in fig. 2. These results demonstrate that the inirial value of GHzO2 is independent of primary act H 2 0 2 formation with radical destruction ( 1 ) (2) (3) (4) i ‘> lo’ooo ” I dose rate but not of radiation quality; in fact it is nearly doubled in going from 1 MeV electrons (Go - 1.10) to X-rays (Go = 2-28).An explanation for this may be envisaged along the following lines. In aerated water the principal reactions involved are probably the following provided we consider only those radicals which escape recombination and assume that the free H atoms are quantitatively oxidized to HO;! W t OH H + 0 2 -f 3 4 0 2 OH + OH + H202 NO2 f H02 -+ H202 f 0 2 62 Savage Aiialyst 1951 76 224. 63 Ghormley O.R.N.L. 130 Oct. 1 1 . 1949. 268 (A) 1 MeV electrons. v 0 1,000 y 0 10,ooo , J I GENERAL DISCUSSION FIG. 1~.-H202 yields in aerated water. 30 keV X-rays. A 152 r/sec 'II 76 ) }M.L. 0 304 ) 152 r/sec } M.L. (R) X-rays. A 30 MeV V 30 MeV 220 MeV 76 , 10 r/sec H.C.S.GENERAL DISCUSSION 269 HO2 4 OH -f €320 + 0 2 radical removal H202 destruction without radical removal OH + H202 -+ H2O + H02 and/or (5) (6) (71 In the initial stages of irradiation where [H202] is less than 10-5 M the linearity of the yield against dose curve indicates that reactions (6) and (7) are unimportant. The initial GH202 will therefore be determined largely by a competition between reactions (4) and (3) on the one hand (with (4) probably predominating though this treatment is independent of their relative importance) and (5) on the other. The relative importance of these reactions would be expected to depend on the distribution of the OH and H02 radicals in such a way that high local concentra- tions of one radical in regions comparatively free of the other would favour (3) and (4) and lead to enhanced H202 formation.This asymmetry is probably at a maximum in regions of maximum ion density so that we might expect the initial GH202 to increase with the average ion density of the radiation used as is observed. One would not expect a large difference between 30 keV and 220 keV X-rays since the mean energy of the secondary electrons from these sources are about the same but the mean ion density of initially 1 MeV electrons is ap- preciably lower (see Prof. Spiers’ paper). At high doses the H202 yield builds up to a limiting stationary concentration which our results show to be insensitive to changes of dose rate with 1 MeV electrons. Whilst the stationary state has not been attained with X-rays the appearance of the figures shows that the equilibrium concentration of H202 is in this case dependent on dose rate and is much higher in proportion to the initial G than with 1 MeV electrons.We have been unable to correlate this difference with any external features of the irradiations such as enhanced ozone formation from 1 MeV electrons since the H202 yield for a given dose of X-rays was un- influenced by the addition of large amounts of ozone to the surrounding air. It is possible that these differences between the effects of the two radiations are due to differences in the kinetics of H202 decomposition in the two cases; and preliminary investigations of the radiolysis conform with this view. The station- ary state yield is attained when the rates of the formation and radiolysis of H202 are equal.As we have discussed above the formation is largely ascribed to radical interactions within tracks in regions of greatest ion density and as such is greater for more densely ionizing radiations but for a given type of radiation is independent of the dose rate i.e. G formation is constant. The radiolysis on the contrary is probably initiated by radicals formed in or diffused to the more uniform regions where the chances of radical-radical interaction are lowered relative to those of reaction with H202. To account for a stationary state concentration independent of dose rate (eg. 1 MeV electrons) the rate of the back (radiolysis) reaction must be proportional to the first power of the dose rate.Lea’s photochemical results 64 show that in a homogeneous system this state of affairs is only possible at high rates of formation of initiating radicals and low H202 concentrations. We must therefore presume that at the dose rates used with 1 MeV electrons this con- dition is fulfilled-a view which is in harmony with very approximate quanti- tative comparisons with Lea’s data. When lower energy radiation is used a smaller proportion of the radicals formed is in the homogeneous regions and available for the initiation of the back reaction and the H202 yield is higher. If the radical concentration in the homogeneous region is sufficiently low the back reaction will proceed in part by a chain mechanism; and again using Lea’s photochemical experiments as a model we expect the rate to become dependent on a power of 64 Lea Trans.Faraday SOC. 1949 45 81. 270 the dose rate less than unity. An increase of the dose rate should then displace the stationary concentration of H202 to higher values as is observed with X-rays. The arbitrary distinction drawn between track-like and homogeneous reaction regions is clearly to some extent unrealistic. Nevertheless it provides a useful basis for comparison of different types of radiation. It is of interest that the results of Fricke 45 on radiolysis of dilute H202 in air free solutions with X-rays of effective wavelength of 0-35 A and of Johnson 66 on the same system but using 2 MeV X-rays show differences in dose rate dependence which are also com- patible with such a model.Dr. M. S. Matheson (Argorzne National Labs. Chicago) said In connection with the H202 radiolysis and photolysis we agree with Rowbottom and Dainton that more experimental work is necessary to establish whether the rates in the two cases have a different dependence on H202 concentration. Our own extensive experiments plus those of Fricke 67 and Johnson 68 lead us to believe that in X- and y-radiolysis the over-all rate indeed depends on (H202)i. However the literature results are in disagreement as to the dependence on (H202) in the photochemical decomposition. Of the references subsequent to 1920 cited by Rowbottom and Dainton in their comment as showing that the photolytic chain reaction is first order with respect to hydrogen peroxide few are valid Kornfeld’s 68a data when analyzed fit a square root dependence on H202 as well or better than first order ; Jeu and Alyea69 use the same type of termination with and without added in- hibitor ; Anderson and Taylor 70 give only one sample of data to illustrate the first order law and these data were obtained with all wavelengths between 2000 and 4000A so that much of the light must have been strongly absorbed.Therefore only the work of Lca 71 (not very extensive on H202 dependence) and that of Dainton and Rowbottom72 (at higher concentrations than those used by us) appears to us at the present time to support fjrst order dependence on H202. If both radiolysis and photolysis rates are proportional to (H202)3 then only a termination step such as we have proposed seems likely to account for the dependence.If the radiolysis rate is proportional to (H202)+ and the photo- chemical rate is proportional to (H202) then one can account for the difference only in the mechanisms of initiation since it is difficult to conceive how the two reactions can differ in the later stages of the reaction. One way of accounting for such a differing dependence is as follows. The radiolysis mechanism is as we have given it but in photolysis the quantum yield of initiation is low and is pro- portional to (H202). However Lea 71 finds a high quantum yield of initialion which seems to rule this out. A second way would be the approach used by Weiss,S wherein in the radiolysis the fraction of the free radicals in a given track which survive to escape from the track and induce the chain reaction is propor- tional to (H202)-1.This second case would account for a radiolytic rate cc(H202)a without a third order termination step. In his paper Weiss73 considers decomposition in a single track using (I&) and (IIIb) i.e. both reactions ( 2 ) and (3) occurring and destruction of radicals by interaction of 20H. By this mechanism the concentration of radicals will be changed by 20H interacting and by diffusion but not by the reaction of OH or H02 with H202 since in these reactions one radical is formed for each radical GENERAL DISCUSSION 65 Fricke J. CAem. Physics 1935 3 364. 66 Johnson J . Ciienz. Physics 1951 19 1204. 67 Fricke J. Clzern. Physics 1935 3 364.68 Johnson J. Clzem. Physics 1951 19 1204. 68a Kornfeld 2. wiss. Pilot. 1921 21 66. 69 Jeu and Alyea J. Amer. Chenz. SOC. 133 55 575. 70 Anderson and Taylor J. Anzer. C h n . Suc. 1923 45 650. 71 Lea Trans. Faraday SOC. 1949 45 81. 72 Dainton and Rowbottom Nature 1952 169 370. 73 Weiss this Discussion. GENERAL DISCUSSION 27 1 consumed. Therefore in eqn. (IV) the term ka(S)n should be replaced by a dif- fusion term. Using (IIIa) and (IIIb) the assumption that (H02) cc (OH) leads to the result that (OH) and (H02) are constant with time within a track since d(OH)/d(HOz) is also constant. In the “reaction due to the interaction of the tracks” again in eqn. (VIII) the ka(S)n term should be zero because reaction with the solute does not destroy radicals.The important item to note here is that in eqn. (XVITI) the term A arises because ka(S)n is erroneously included in eqn. (VIII). If A is made iero then it is no longer possible to obtain a rate proportional to (H202)* from eqn. (XVIII). Nevertheless thc modifications Weiss included in his introductory remarks under certain assumptions may lead to the proper inclusion of the ka(S)n term. Certain considerations however suggest that the “ track ” concept should not be applied to the C060 y-ray induced radiolysis of H202 and that the radio- lysis and photolysis should not differ appreciably in kinetics. (Dainton and Rowbottom7 find similar H202 dependence for the two cases but not however the same H202 dependence we obtain,) Thus Spiers,74 in his table 1 estimates 10 ion pairs per micron for the mean ion density along the path of the recoil electron from a C060 y-ray.Further as Spiers has noted in the average cluster of ionization along the primary electron path there will be only three ion pairs. These clusters will be spaced about 3000A apart whereas from the radical con- centration in our sector experiments the average distance between radicals through- out the solution is only about 16,OOOA. Even if four of the six radicals in each cluster recombine it is difficult to see why the remaining radical pair should re- combine as readily as a pair generated photochemically since the photochemical pair is generated in a solvent cage. Also it should be noted that if the (H202)* rate dependence in radiolysis is explained by bimolecular termination of two HO2 radicals and initiation cc (H202)-1 then since the (H202) range variation covered in our experiments is over 100-fold the lowest initiation efficiency in our experiments must be < 0.001.To account for such a low efficiency in the cluster picture given above one must assume that only one radical pair escapes from 300 clusters by three radical pairs each and an ion pair escapes only when separated by 0.1 mm along the track from its neighbour. Dr. A. G. Maddock (Cambridge University) said I should like to ask Dr. Ebert and Dr. J. W. Boag if tests were made for traces of the oxyacids of nitrogen after the irradiations in nitrogen-saturated solutions ? Dr. M. Ebert (Hammersmith Hospital W.12) said We agree with Dr.Barb that the tests for impurities employed by Fricke and Hart are possibly the best which can be made. They are of course only possible when using evacuated and sealed irradiation vessels. All our work was done with solutions open to the atmosphere and we found that aeration of the solution with unwashed air did introduce impurities which led to irreproducibIe results. When carefully washed air was used for aeration however reproducible results were obtained which were not affected by the several different techniques of further purification employed. We still think that this is fairly strong evidence that any remaining impurities were not important. In reply to Dr. Maddock we did not make routine tests for the presence of the oxyacids of nitrogen as they would not in any case interfere with the titanium test for H202.We do know hohever. from prior experiments that traces of these oxyacids are formed by the irradiation of nitrogenated water. views on the reaction HO + HO -+ Hz02 have been greatly influenced by Weiss’s 75 Mr. N. W. Luft (Waltham Abbey) (cammrmicated) It appears that current investigations. This author derived an activation energy for the gas phase 74 Spiers this Diccuwion. 75 Weiss Ti-mu. Fnrrrchy SOC. 1940 36 856. recombination 272 process of E - GENERAL p2/r3 - 4.6 DISCUSSION kcal by assuming dipole repulsion with p = 1.66 D and Y ~ l 2 A. Apart from possible differences in the dipole moments of the OH radical and OH bond this estimate overlooks the predominant influence of the quadrupoIe potentials of bonds and lone electrons.If all these factors are taken into consideration by an electrostatic method using dipole and quad- rupole moments and polarizabilities then for a range of values and distances equal to or larger than the normal 0-0 separation no potential maximum is obtained for the configuration of minimum energy (dihedral angle $I 110'). The poten- tial maximum in the cis-position decreases rapidly with r from its value of ca. 12 kcal at r = 1.48 A. Since in the critical range the two bonding electrons interact strongly approximately according to Morse's law with DO =; 52 kcal it is clear that the true activation energy of HO recombination is zero. The same holds for recombination of NH2 and CH3 radicals although quadrupole repulsions are appreciable in the latter case.The theoretical results for CH3 are supported by recent experiments.76 In view of the known instability of aqueous H202 solutions there js no need to postulate a finite activation energy for HO recombination in order to explain poor H202 yields. Moreover the extent to which OH recombination occurs is limited by competing reactions e.g. the other chain-breaking process HO -i- HO2 which seems to be important in thermal decomposition at high H202 concentrations. Dr. J. Weiss (Durhnnz University Netrnntle) (communicated) My objection to the assumption of a recornbination of OH radicals to give hydrogen peroxide was never based primarily on theoretical grounds but originates from the fact that the experimental evidence suggests strongly that this reaction does not occur.77 My earlier suggestion that this reaction might have a considerable heat of activ- ation due to the repulsion of the dipoles of the OH radicals was mainly to furnish a qualitative theoretical explanation for the experimentally observed facts and was not meant as a calculation of the heat of activation of this reaction which to give with any degree of accuracy would be in any case practically impossible.I still think that experimental evidence for the recombination of OH radicals is lacking. On the contrary some new evidence which I have presented here78 definitely favours the other alternatives. Dr. C. B. Amphlett (A.E.R.E. Harwell) said In connection with the resurts of Dr.Hardwick obtained with the ferrous sulphate system I wish to present the following preliminary yields of ferrous ion oxidation in 0-8N H2SO4 using 0.92 mV electrons. The dose-rates expressed in e.u./min are overall values calculated from the total energy dissipation within the absorbing volume i.e. from the product of beam voltage and cell current the latter being measured via a high resistance and sensitive microammeter connected to earth. Owing to the variations in dose-rate along electron tracks the overall dose-rate will in fact be lower than the maximum dose-rate within the cell. Apart from the value at 0.001 pA the oxidation curves obtained (which were initially linear) all showed characteristics of oxidation in de-aerated solutions i.e.beyond the oxygen break points; oxidation was in all cases practically complete and was then followed by a build up of peroxide the latter attaining a steady-state concentration of ca. 3 x 10-4 M in 0.1 M Fe3+ sohtions in 0.8 N H2S04 at 0.92 mV and 1 pA. The results obtained are given below 1.03 x 10-2 0.1 0.0 1 5 x 10-4 0.001 N 103 1.03 X 10-1 1.0 - 106 - 104 - 10s - [Fez+], moles/l. cell current pA overall dose-rate e.u./min Go (aerated) - 6.0 - 7.3 4.2 (de-aerated) - 17 76 Gomer J. Chem. Physics 1951 19 85. 77 cf. Bonhoeffer and Pearson 2. physik. Chem. By 1931 14 1 ; Lea Trans. Furuday Soc. 1949 45 81. 78 this Discussion. GENERAL DISCUSSION 273 Compared with the maximum 79 de-aerated yield of about 10 the yield is seen to decrease with increasing dose-rate at high dose-rates; this is probably due to increasing importance of recombination reactions both of radicals and of ions.It is interesting to note that at the highest dose-rates studied the yield approaches the value of 3.05 for or-particle irradiations quoted by Hardwick.80 Dr. M. Lefort (Institut dzi Radium Paris) said Dr. Hardwick's suggestion that the reduction of ceric ions would occur in two different ways when irradiated by soft X-rays (reduction by H202) or by y-rays (reduction by H atoms) may be an explanation of the different results obtained with these two kinds of radiation. Dr. Haissinsky and myself had already reached the conclusion that the reduc- tion was mainly occurring through the hydrogen peroxide formed from water with x-rays.Indeed for this radiation we know that even pure water gives fairly large amounts of H202 and H2 in the absence of air. With soft X-rays on the contrary it does not appear from experiments on pure de-aerated water that there is more H202 produced than with y-rays or electrons of high energy. According to the differences in ion density soft X-rays should give effects more similar to y-ray than to x-ray effects. The ion density with or-rays of 5 MeV is roughly 100 times greater than with electrons of 10 kV (soft X-rays) and only about 7 times greater than with 1 MeV electrons. We agree that we could expect slight quantitative variations in the yield of reduction of ceric ions. We observed variations of this kind for the formation of hydrogen peroxide in aerated water as one changes from soft X-rays to 1 MeV electrons.But we do not think that a complete change in the mechanism could occur. Therefore the important differences in the results obtained with X-rays and y-rays for example for the evolution and the influence of hydrogen are still not explained. Before drawing any conclusion we think we must repeat the experiments of irradiation at the same time and under conditions as similar as possible with soft X-rays hard X-rays and y-rays. Dr. W. G. Barb (Courtaulds Ltd. Muidenhertd) (partly co~?unrmic.aterl) Collinson and Dainton have described certain kinetic features in which their system deviates from the usual vinyl polymerization kinetics and have suggested some of the differences may be due to a non-uniform radical distribution when ionizing radia- tion is used for initiation.It must however be pointed out that phenomena qualitatively similar to the present case are found in a number of polymerizations where the polymer is insoluble in the reaction mixture even though other means than ionizing radiation are used to initiate the reactions. This does not imply that Collinson and Dainton's suggestion about spatially inhomogeneous distribu- tion is necessarily incorrect even though the authors themselves point out a serious objection i.e. the long radical life-time. What does seem imperative is to take the phenomena due to polymer insolubility into account first and then to examine whether any features remain which require the stipulation of phenomena peculiar to radiative initiation.I have had the opportunity of discussing this point with Dr. Magat and Dr. Chapiro as well as with Dr. Bamford and Dr. Jenkins of the Courtauld (Maidenhead) laboratories and find they share my view very cIosely. Essentially the peculiarity in the kinetics of insoluble polymer formation corre- sponds to the absence of stationary state conditions ; a stationary state is not established because the radical termination velocity coefficient is low and prob- ably also time-dependent. The cause appears to be the coiling of the insoluble polymer radicals and their coalescence with polymer molecules both of which considerably reduce the ease of mutual termination while chain propagation is less affected (more detailed discussions of the factors involved are in course of publication 81'82).In such systems one observes some or all of the following 79 Rigg Stein and Weiss Proc. Roy. SOC. A 1952 211 375. 80 Allen Hochanadel Ghormley and Davis AECU-1413. 81 Bamford Barb and Jenkins Nature 1952 169 1044. 82 Bamford and Barb Dircr4ssion.s Farads-v Suc. 1952 in course of publication. I* 274 phenomena prolonged acceleration periods,81-84 abnormally large after-effects,sl unusual dependences on monomer concentration and rate of initiation,gL 82,85 ap- preciable temperature effect on termination.86 (References quoted above are pur- posely confined to work in which ionizing radiation was not used.) All of these phenomena have also been found by Collinson and Dainton in the present system yet they have not been related to the insolubility of the reaction product.It is true that mention is made of coagulation phenomena in connection with abnormal effects at monomer concentrations greater than 0.75 M but the authors consider that coagulation promotes the termination step; in fact such appears to be the case only under rather special conditions,87 and where the polymer forms a definite precipitate the effect is generally one of retarded termination. Again if the authors admit that such physical factors affect the reaction rate at monomer con- centrations above 0.75 M there is no reason to suppose they are necessarily negli- gible below that concentration and that an interpretation of the results should be sought which does not allow for them.Collinson and Dainton draw attention to the fact that very different kinetics are observed in the polymerization of aqueous acrylonitrile solutions when Fe3+OH- is used as a photo-initiator. At first this seems to emphasize the peculiarity of the y-ray and X-ray initiation systems. However closer considera- tion reveals various possible reasons why the radiochemical and photochemical kinetics may differ. Thus the light-scattering by precipitated polymcr produces a type of " skin-effect "88 in the latter case which is probably very different for ionizing radiations. (This light-scattering effect also leads to different kinetics for thermally and photochemically initiated insoluble-polymer systems 1 and throws doubt on the claim of Collinson and Dainton that in their system photochemical initiation leads to a uniform generation of radicals throughout the reaction mixture).Further points to be borne in mind are (i) certain entities are present in one system and not the other e.g. iron ions in the photochemical case and H atoms in the radio- chemical case which could affect the kinetics (ii) the effects of precipitation discussed earlier are also operative in the photochemical case but their quantitative role can vary with several factors which may be different in the two sets of experi- ments e.g. the rate of reaction and the molecular weight of the polymer formed (iii) a stationary state treatment is not strictly applicable to either system so that the significance of a quantitative comparison is doubtful.Again one cannot claim that these points necessarily explain the observed differences completely ; but they do draw attention to the difficulties in comparing the two sets of results and obtaining evidence of any spatial effects specifically associated with initiation by ionizing radiation. Dr. M. Magat (Paris) said We were very interested in the experimental results of Collinson and Dainton. However we are not quite convinced by their theoretical arguments. We agree with Dr. Barb that caution must be exercised when dealing with precipitating polymers. We have also some objections to the interpretation of the band appearing at 2200cm-1 as being due to CD vibration.Mrs. Prevost-Bernas Miss Fiquet and Dr. Chapiro havee repated the experiments of Collinson and Dainton using X-rays of 37 kV Mo target 2400 r/min on the surface and 0.12 M solutions of acryloni- trile. They found that the band in question was about equally strong when H20 and D20 were used as solvents and appeared moreover when the polymerization was initiated by NaNH2 in liquid ammonia in absence of water. A very strong absorption band appears at the same wavelength when propionitrile is treated GENERAL DISCUSSION 83 Bengough and Norrish Proc. Roy. Soc. A 1950 200 301. 8 4 Prat Mem. Sevv. chim. Z ' h t (Paris) 1946 32 319. 85 Abere Goldfinger Naidus and Mark J . Physic. Chem. 1945 49 211. 86 Burnett and Melville Trans. Faraday SOC. 1950 46 976.87 Chapiro J . Chim. Phys. 1950 47 747 and 764. 58 Bateman and Gee Pror. Rov. Soc. A 1950 195 376. 275 GENERAL DISCUSSION with NaNH2.89 In the latter case the C r N band disappears almost completely. This leads us to believe that the 2200 cm-1 band is due to a conjugated N=C-C-N vibration although the second harmonic of the CH2 rocking frequency is located in the same region.90 Hence the argument for the CD band becomes a quantitative one i.e. one has to show quantitatively that the ratio of absorption intensity at 2200cm-1 to the absorption of the C r N band is larger in the case of polymerization in D2O than in HzO. Thrs is difficult to decide because the apparent ratio of the % absorption by the 2200cm-1 bond and by the C r N band depends quite strongly on the thickness of the film as was found by Mrs.Prevost-Bernas Miss Fiquet and Dr. Chapiro. If we assume that Beer’s law holds and we compare the curves IA and IIA reproduced in fig. 9 of the paper by Collinson and Dainton then with the irradiation conditions given one would expect the chain length of the polymer for ~ I A to be about 3 to 9 times shorter than for IA. In other words there are at least 3 times as many CD bands for each C-N band in case IIA than in case IA. Assuming that the 2200 cm-1 band is due to CD and using Beer’s law one can calculate the CD/CH ratio from the two curves. One finds then that there is about 7 % less CD for IIA than for IA (the figure of 7 % is probably within the experimental error). These considerations in addition to the fact that the 2200cm 1 band does not appear in the spectrum of the polymethacrylonitrile polymerized by X-rays in H20 and in DzO lead us to believe that these experiments do not prove the primary production of H and D atoms by ionizing radiations.Dr. A. D. Jenkins (Courtaulds Ltd. Maidenhead) (communicated) Among the phenomena reported by Collinson and Dainton to occur in the radiation-induced polymerization of acrylonitrile are a number of features which have also been observed during reactions initiated by free radical catalysts or by ultra-violet light. Thus inhibition periods were found by Koningsberger and Salomon,91 although these workers failed to observe a subsequent period of acceleration. Recent work in these laboratories,92 however has shown that the periods of ac- celeration are inherent features of the polymerization of the undiluted monomer and that in the complete absence of oxygen there is no inhibition period.This has been proved by carrying out the reaction in dilatometers (filled with monomer and catalyst in vacuo) capable of detecting less than 0.01 % of reaction. Periods of acceleration were found in all polymerizations of carefully purified acrylonitrile and these remained unchanged if monomer recovered from a 60 % polymerization was used. In fact an acceleration with time has been shown to be a common feature of all polymerization reactions which produce a polymer insoluble in the reaction medium.93 Photo-initiated reactions were found to commence immediately the light was switched on both by observation of the level of the meniscus and by visual ob- servation.(Scattering of light by the polymer particles enabled the latter to be detected much earlier in a photo-reaction than was otherwise possible.) In the presence of oxygen reaction was completely inhibited for long periods. It can therefore be safely assumed that the periods of inhibition observed by Collinson and Dainton were either artefacts arising from the method of direct visual observation which they employed or if genuine due to the incomplete removal of oxygen. Prat 94,95 has found that vinyl chloride similarly exhibits an inhibition period in the presence of oxygen. 89 Lander unpublished results. 90 Barchevitz private communication. 91 Koningsberger apd Saloinon J .Polymer Sci 1946 1 200. 92 Bamford and Jenkins. in course of publication. 93 Bamford Barb and Jenkins Natrrre 1952 169 1044. 94 Prat ilfeni. Serv. Chim. I’Etat (Paris) 1940 32 319. 95 Prat Con7pt. rend. J . Znt. Plastiques (Paris) 58 (Jan. 1949). 276 With regard to the molecular weights of the polymers it should be noted that Lanzl 96 has published values for the Houwink constants for polyacrylonitrile these being K = 1.75 x 10-3 100 ml/g and ,8 = 0.66. The extreme values of the function used by Collinson and Dainton are not dependent on /3 but it is clearly not satisfactory to assume direct propor- tionality between molecular weight and intrinsic viscosity. Dr. J. H. Baxendale (Munchester University) said The work of Baxendale Evans Kilham and Bywater 57,98 on the polymerization of methyl methacrylate initiated by Fez- $- H202 in aqueous systems showed that the polymerization kinetics are consistent with the assumption that monomer polymer and aqueous phase form an homogeneous solution in the presence of emulsifying agent.In such systems polymerization rates were found proportional to the square root of the rate of production of the initiating radicals. However in view of the insolu- bility of polymethyl methacrylate it is clear that the detailed picture is more complex than this and that polymerization does not occur in true aqueous solution. It would seem that the ideas put forward95 to explain emulsion polymerizations of the same system must also apply to the solution case viz.that polymerization occurs in a swollen polymer-monomer phase which is held dispersed by the emulsifying agent. In view of this there is some doubt as to the exact significance of the rate constants used but the observed conformity to the kinetic scheme seems to show the absence of any specific effects due to polymer insolubility. However experiments done in collaboration with Dr. G. W. Madaras show that such is not the case for the analogous polymeriiation of acrylonitrile. Here the results cannot be fitted to the same kinetic scheme or any of the usual modi- fications of it. The rate and extent of polymerization are much more dependent on monomer concentration and in addition the polymerization continues slowly for a much longer time than is the case for methyl methacrylate.Data obtained in collaboration with Dr. G. L. McLeavy show that vinylidene chloride behaves similarly to acrylonitrile. We had attributed these differences to the fact that acrylonitrile and vinylidene chloride are both insoluble in their polymers so that although propagation and termination may occur in true solution while the polymer radical is small enough to be soluble new features due to the immobility of the growing radicals are introduced when the radicals are coagulated. In view of the complications observed with these systems it is doubtful whether the usual kinetics can be applied as has been done by Collinson and Dainton. Dr. A. Chapiro (Paris) said Collinson and Dainton have mentioned two of our previous results (i) that the effect of benzene on y-ray initiated polymerization is not appreciable and (ii) that the polymerization rate of pure monomer is pro- portional to the square root of the intensity.I must point out that these results were obtained with styrene in homogeneous polymerization and that without further experiments they cannot be extended to the polymerization of acrylonitrile where the polymer precipj tates during the reaction. This precipitation of the polymer makes the kinetic interpretation of the reaction more difficult as has already been shown by Dr. Barb in this Discussion. I must add that in my own experiments 100 the y-ray initiated polymerization of styrene and methyl methacrylate could be interpreted by normal kinetics assuming CjENERAL DISCUSSION II__-- d (log N) - d (log 4) d(log M ) d (log R) d (log R) d (log R)' 96 Lanzl see Houtz Textile Res.J. 1950 20 786. 97 Baxendale Evans and Kilham Trans. Farariay SOC. 1946 42 668. 98 Baxendale Bywater and Evans Trans. Faradaj? SOC. 1946 42 675. 99 Baxendale Evans and Kilham J. Polymer Sci. 1946 1 466. 100 Chapiro J. Chirn. Phys. 1950 47 747 and 764. GENERAL DISCUSSION 277 a homogeneous distribution of free radicals as long as the reaction took place in solution. In very dilute solutions of the monomer in presence of polymer precipitants phenomena similar to those observed by Collinson and Dainton were found. Dr. N. Uri (University of Chicago IZZ.) (cornmunicated) Collinson and Dainton suggest that the assumption of oxygen as the inhibitor of polymerization would imply that very little 0 2 is removed in the process of H T 0 2 .1 wonder whether the authors had considered the possibility of HOz being itself an inhibitor e.g. by effective termination of growing polymer chain. On the whole it appears from the results reported in the literature that larger atoms or radicals such as Br or SCN are very effective chain terminators. Prof. F. S. Dainton (Leeds University) (cornnzunicated) Dr. P. Smith’s results,lOl concerned with the dependence of the rate of polymerization in aqueous media on catalyst concentration where H202 is the catalyst indicate quite clearly that in accordance with Uri’s suggestion HO2 radicals are effective chain terminating agents and we did consider this possibility in relation to the present work.How- ever for the reasons given in our paper we regard this reaction as not playing a major role in radiation induced polymerization. Dr. E. Collinson and Prof. F. S. Dainton (Leeds University) said Criticisms of our suggested interpretation of the results for the radiation polymerization of acrylonitrile have been levelled by Dr. Magat Barb Baxendale and Jenkins. These fall under three main headings (i) that similar kinetic effects can be obtained in non-radiation systems in which the polymer is insoluble in either the monomer or the solvent ; (ii) that it is not permissible to compare radiation and photochemical results ; (iii) that polymerizations of acrylonitrile in non-radiation systems cannot be explained on the basis of stationary state homogeneous kinetics as can those of say methyl methacrylate for which the polymer is soluble in the monomer.Though we acknowledge the possibility that polymer insolubility may play some part in the radiation polymerizations we think that the differences between the results of the photo and radiation polymerizations are so marked and yet each set of results so clear cut that we cannot attribute the principal features to this cause alone. We also find no reason to believe that such considerations play a major-part except in the case of high monomer concentrations. The effects which can be obtained for systems of insoluble polymers in which polymerization is not initiated by radiation and which appear in our results have been instanced by Dr.Barb. The crux of the matter would seem to be not whether these effects have each or all been found in non-radiation work generally but whether they can be reproduced for this particular monomer in aqueous solution together with the dose rate and monomer dependence found by us using non- radiation methods. In our paper we referred to the work of Mr. James on the polymerization of acrylonitrile initiated by ultra-violet light irradiation of the (FeOH)2r ion in aqueous solution. In view of the criticism of this comparison we propose to give some of the facts in more detail. Mr. James’ work shows every indication of being explicable on the basis of stationary state kinetics. The results are so self-consistent and reproducible and differ so clearly from those of the radiation polymerizations that an explanation of the difference on the basis of a skin effect would seem to be strained in the extreme.As was pointed out in our paper the dependence of polymerization rate on intensity and monomer concentration in this case is given by rate cc [rn~]Z.$,~ contrasting markedly with the reIation rate cc (R)x [m# for the radiation work. The I:bs relation was established by the use of (i) a high speed sector of variable angle (ii) by varying the initial concentration of FelI1 with a constant 10. 101 P. Smith Dim. (Cambridge 1952). GENERAL DISCUSSION 102 Evans Santappa and Uri J. Polymer Sci. 1951 7 243. 103 Prkvot Compt. rend. 1950 230 288. 104 Chapiro Cousin Lander and Magat Rec.truv. chirn. 1950 68 1056. 278 The overall rate of radical generation giving the I:bs dependence was about 10- 3 times the overall rate of radical generation giving a polymerization rate propor- tional to (R)* in the radiation case. Thus the chance of the photochemical rate of radical generation being just that required to give the relation was very small. The dependence of the photochemical polymerization rate on [m# was very clear cut in all the work over a concentration range covered by at least 0.3 to 1-2 M. In this connection it is important to note that the only published results (other than our own) concerned with the photo-initiated polymerization of acrylo- nitrile in aqueous solution are those of Evans et a1.102 and are in accord with Mr.James' findings. Not only do the principal kinetic results for the two sets of experiments differ so markedly but also there are other differences of detail. In the photochemical work inhibition periods assessed visually were never greater than 30 sec. A short acceleration period was followed by a period of maximum rate covering about 7 % of the total polymerization before the rate began to fall. Moreover the acceleration period was not due to the production of a continuously increasing number of long-lived growing polymer chains but to an increasing light absorption in the reaction vessel caused by the light scattering of the precipitated polymer. (A similar effect was observed in the polymerization of methyl methacrylate.) The evidence for this was twofold.In the first place the acceleration period invariably continued up to the time when a certain weight of polymer (which was independent of either [ml] or Jabs) was formed. Secondly the rate of formation of FeIr was linear and somewhat larger in the case of a monomer concentration sufficient to give turbidity than in the case of a monomer concentration high enough to prevent back reaction of Fexl and OH radicals but too low to give rise to turbidity. More- over for the hydrogen peroxide photosensitized polymerization of acrylonitrile Mr. P. Smith has observed a similar acceleration period. By measurement of the intensity of light transmitted he has shown the acceleration period to end at the same time as the amount of light transmitted falls to zero.The percentage of light transmitted can in fact be used instead of dilatometry for following the initial stages of the reaction. A further indication of the origin of the acceleration period was the fact that on stopping a radiation polymerization allowing the post- irradiation effect to go almost to completion and then restarting the new rate was equal to that obtaining immediately prior to cessation of irradiation even though the polymer was coagulated. In the photochemical case this was only true if the polymer was not coagulated or filtered from the solution. In both of the last cases an acceleration period was found before the steady rate was re-attained. The post-irradiation polymerization mentioned for the radiation results also occurred but to a much less degree in the photopolymerization work.However with regard to Dr. Baxendale's comment our experience is that in both radiation and photopolymerizations methyl methacrylate gives much inore marked after- effects than does acrylonitrile. In one radiation case the rate of the post poly- merization of methyl methacrylate was as high as 34 % of the irradiation rate and the total polymer was trebled during 16 days following the cessation of radiation. If post-polymerization is any criterion of the degree to which polymer insolubility controls the kinetic behaviour of a polymerization it would appear from this that the kinetics of the acrylonitrile polymerization should be more amenable to treat- ment on the stationary state method than those of methyl methacrylate.We may also note that both pure acrylonitrile 103,104 and methyl methacrylate or solutions in methanol up to 50 % monomer104 show linear percentage contraction against time curves over the initial stages of the polymerization (i.e. up to 10 %) and that methyl methacrylate shows an increasing rate with time at some degree of poly- merization above this in spite of the solubility of the polymer in the monomer. 279 GENERAL DISCUSSION by the reaction It is of course true that there are certain entities present in the photopolymeriza- tion case which are absent in the radiation case but it is difficult to believe that these entities can so radically alter the kinetics. Change of pH had a negligible effect on the rate of the radiation polymerizations in the range pH 7 to 0.8 N sulphuric acid.Change of ionic strength had a negligible effect on the photo- polymerizations suggesting that the rate of coagulation of colloidal polymer particles was not a determining factor. An important effect which was observed however was that both reducing ions hydrogen peroxide can act as chain terminating agents. The former where mj" represents a growing polymer radical and Pi a dead polymer chain and the latter by an unknown reaction which is probably mj" + H202 -+ Pj f- H02. followed by the production of the chain-terminating agents 0 2 and HO2 from H202. The effect of adding reducing ions e.g. FezL to the photoinitiated polymer- ization of acrylonitrile is to raise the intensity exponent and decrease the rate.Throughout all such variations of intensity exponent due to Fe3+ however the rate remained proportional to [ml]'. It may well be that this effect of ferrous ion was a contributory factor to the anomalous results with acrylonitrile found by Dr. Baxendalc but which were not observed by Evans et a1.102 In reply to Dr. Barb's criticism of our statement that coagulation above 0.75 M may promote the termination step this is undoubtedly what occurs with styrene polymeri7ed in akohol as the monomer concentration is increased beyond a certain poi n t .lo7 The acrylonitrile case differs from the latter inasmuch as the polymer is in- soluble in the monomer but as pointed out in the paper the fall in intrinsic viscosity of the polymers prepared in solutions of greater concentration than 0-75 M is in agreement with an increased termination rate.We did not invoke a consideration of coagulation effects at concentrations below 0.75 M because the considerable irreproducibility of the results above 1 M which one might expect to accompany such effects contrasted markedly with the consistent [m1]2 relation found for all systems in which the monomer concentration was less than 0.75 M. This indicated that coagulation effects played Iittle part at the lower concentrations. It is also noteworthy that the difficulty of termination envisaged for two mutually interacting insoluble polymer radicals disappears if termination is by H or OH. We now know 105 that the lifetime of the chains is likely to be about 0.1 sec so that one of our objections to a radical termination mechanism loses much of its force.Dr. Chapiro suggests that similar results to ours were obtained for very dilute solutions of styrene in precipitants for the polymer. However apart from super- ficial resemblances it is not possible to make a comparison of the two sets of results since the results for the styrene system do not extend to a determination of the dependence of rate on dose rate and monomer concentration. We apologize for misquoting Dr. Chapiro's work ; the statement that the rate of polymerization of pure acrylonitrile is dependent on the square root of the dose rate actually occurs in a paper by another member of the same laboratory.103 With regard to our use of benzene to give an indication of the importance of direct action in the polymerizations we consider this to be permissible.The important question is whether benzene acts as an inhibitor of vinyl polymerizations. As judged by the results of Dr. Chapiro for styrene it does not and there is therefore no obvious reason why it should be an inhibitor for the acrylonitrile polymerization. If this is so then in the light of the fact that benzene is known to be one of the 105 D. G. L. James unpublished results. GENERAL DISCUSSION 280 most radiation resistant substances (as is shown by the work of Burton 106 and by the results of Chapiro 107 for styrene) a polymerization carried out in benzene should give a fair indication of the polymerization initiated by direct action on the mono- mer.The polymer is equally insoluble in benzene and water so that no hypothe- tical interfering effects can arise from differing solubilities of the polymer in these two media. We agree with Dr. Jenkins’ conclusion regarding the inhibition period; we have in fact reached the same conclusion in the paper. We hope to obtain further clarification of this difficult problem of polymeriza- tion in aqueous solution from a comparison of the photo thermal and radiation induced polymerization of methacrylic acid. The data quoted by Dr. Jenkins on the relation between intrinsic viscosity and molecular weight were unknown to us at the time of writing the paper. If the same value of p = 0.66 applies to unfractionated samples as to fractionated samples then the assumption that [TI = KF is certainly unienable but there is reason to believe that the value of /3 is nearer unity for unfractionated samples such as ours.The relation 7lSp/c = KniM for this polymer in dimethyl formamide is quoted on p. 55 The Clzeniistry QfAcrylonitriZe (American Cyanamide Co. 1951). Dr. Magat’s results indicate that the band at 2200 cm-1 which is well known to be typical of the C-D frequency may also arise from the attack of sodamide in liquid ammonia on the cyanide group. Though it is not yet certain just what grouping gives rise to the absorption in these polymers we are grateful to him for pointing out the ambiguity. Dr. G. Stein (Jerusalenz) said Interest in radiation chemistry tended to con- centrate on aqueous solutions in recent years.For the interpretation of phe- nomena in biological systems in particular it might be of advantage to consider first of all the similarities between solutions and solids on the one hand and the information obtained in the radiation chemistry of solids on the other. Biological systems might in particular behave in some respects rather like solids. Concepts such as electron trapping centres conductance bands etc. well known from the study of electronic phenomena in ionic crystals can be introduced into the study of the biological action of radiations. These would emphasize the possibility of direct electron-capture processes in these systems as well as their likelihood even in aqueous solutions containing suitable acceptors. One may perhaps use these concepts based on the thermal or photochemical release of trapped electrons from shallow traps to interpret some recent work of Swanson and Yost 108 on the effect of temperature on the induction of activated stable states in the chromosomes of Tradescanfia.Another aspect of interest is the evident similarity between the reactions of radicals produced in aqueous solution by ionizing radiations and some reactions due to enzyme systems. The examples quoted in the paper from our work in- dicate that by this means further evidence can be obtained regarding the mechanism of free radical reactions. Such free radical reactions may then be of importance in biological systems not necessarily through the formation of free OH radicals there as well but rather through the fact that semiquinone or other organic radicals (which can be obtained under specific conditions through the action of OH radicals on complex organic molecules in aqueous solution) may play a role in biological systems where they are formed under the influence of enzymes.Dr. M. J. Day (Royal Victaria Infirmary Newcastle) said Some further observations made in collaboration with Dr. Stein on the radiation chemistry of aqueous solutions of methylene blue may be of interest. X-rays generated at 200 kV with an average dose rate of about 3,000 r/min gave the following results. 106Burton J. Physic. Chem. 1948 52 564. 107 Chapiro J . Chim. Phys. 1950 47 747. 10s Swanson and Yost Proc. Nat. Acnd. Sci. 1951 37 796. 28 I (i) Presence of oxygen in agreement with other workers we find that irre- versible destruction of the dye occurs and eventually a dark coloured precipitate forms.Presumably the effects are due to OH and H02 radicals which bring about irreversible changes in the conjugated ring system of the dye. (ii) Absence of oxygen further experiments show that the irreversible effect mentioned above still occurs (though with reduced yield) but reduction of the dye also takes place. re-oxidation with partial recovery of the colour occurring when oxygen is admitted to the system. We assume that the OH radical is responsible for the irreversible change while the H atom (or possibly electron capture) brings about the reduction of the dye to its leuco-base. (iii) Presence of 0.5 % sodium benzoate and oxygen (air) up to about 60,000 r there is no detectable effect on the dye.If irradi- ation is continued beyond this point however the colour is discharged but there is a fairly rapid recovery process which is thought to be due to re-oxidation of leuco-methylene blue by hydrogen peroxide formed in the initial period. The admission of air accelerates this process and the solution recovers at least 98 "/o of its original photometric extinction coefficient. If the system be re-irradiated the same cycle of reversible reduction and re-oxidation can be repeated. (iv) Presence of 0.5 % sodium benzoate ; absence of oxygen reduction of methylene blue to its leuco-base occurs in high yield and subsequent admission of oxygen results in virtually complete restoration of the colour.The irreversible decolora- tion observed without sodium benzoate is not observed. GENERAL DISCUSSION The simplest interpretation of (iv) would seem to be that the benzoate reacts with OH radicals leaving H atoms free to reduce methylene blue. In the presence of air (iii) H atoms react preferentially with oxygen forming H02 which enters into reaction with benzoate leaving the dye unaffected. The saturation concentra- tion of oxygen is depleted by this process after about 60,000 r and the reaction subsequently proceeds almost as in the absence of oxygen. The fact that the addition of an OH acceptor protects the dye from the irre- versible effect but enhances the reduction effect is contrary to Haissinsky's theory of indirect reduction by OH radicals and is strongly in favour of a direct effect of primary H atoms.The radiation effect is seen to be very dependent on experi- mental conditions which may explain some of the confusion referred to by Dr. Dale in his paper. Dr. H. C. Sutton (Leeds University) said Dr. Stein has mentioned the work of Forssberglog on catalase deactivation as a specific instance of a biological system in which irradiation effects have been attributed to H atoms. Since this work has been referred to a number of times in this Discussion it may be of interest to mention here some preliminary experiments on deactivation of catalase the results of which are opposed to this view. These experiments were carried out by Miss L. Mee and myself in the Department of Radiotherapeutics Cambridge University.Firstly an attempt was made to deactivate air free solutions of crystalline catalase by H atoms produced chemically. The methods used were ultra-violet photolysis of NaI solutions in the presence of catalase thereby producing H and I atoms and the diffusion of cathodically produced H atoms through a palladium foil into an air-free catalase solution after the method of Parravano.110 Both methods failed to effect any significant deactivation other than that observed in control experiments. Nevertheless such methods are genuine sources of H atoms ; in particular the second method caused complete reduction of methylene blue to its leuco base at an appreciable rate. Secondly protection effects in the X-irradiation of aqueous catalase solutions were found to lead to the same conclusion.It was confirmed that 0-2 % potassium iodide solution completely inhibits the X-ray effect a result which can scarcely be 109 Forssberg Nature 1947 159 309. 110 Parravano J . Amer. Chem. Soc. 1951 73 628. GENERAL DISCUSSION 282 interpreted in terms of a reaction between H atoms and iodide ions but which is understandable if the OH radical is the effective agent. Finally it was shown that saturating the catalase solution with 0 2 or N2 prior to X-irradiation caused only a slight protective effect whilst pre-saturation with H2 caused a marked protection such that the amount of deactivation for a given dose was reduced about sevenfold compared to that observed in oxygen saturated solutions.This result also suggests that OH is the effective agent and that H2 protects the catalase by converting OH into H. It should be emphasized that this work is only preliminary and takes account of only certain aspects of the very comprehensive work of Frossberg but it is nevertheless clear that the results are opposed to the view that the H atom is the effective agent in this system. This result is of peculiar interest since this is one of the very few biological systems in which a primary role has been claimed for H atoms. Dr. M. Haissinsky (Laboratoiue Curie Paris) said Dr. Stein in his paper and Rigg Stain and Weiss in a recent paper advance arguments based on pH dependence in favour of or against primary reactions leading to the formation of free radicals or molecules.I do not think that such arguments can be decisive if they are taken separately in each particular case. I have quoted in my paper several examples showing that oxidation can either increase or decrease with change of pH. It seems that it is rather the chemical nature of the whole system under conditions of radiochemical competition which determines the influence of pH. For example the increase of H2 evolution with acidity mentioned by Stein as an indicabion of H2+ ions can also be explained in the following manner. The preceding discussion has shown that many acids even if they are not apparently attacked can react with OH radicals leading to the formation and decomposition of unstable transitory compounds (boric phosphoric and even sulphuric acids) An equivalent amount of H2 would be evolved.The choice between the two interpretations could be made if the specific influence of the anion on gas evolution were more precisely known. The best example is given by the radiochemical behaviour of the two similar acids arsenious and phosphorus. Both are oxidized by X-rays but the oxidation increases with the pH for the former while it decreases for the latter. Now according to the general chemical proprieties and the free energy variations both would be more easily oxidized in basic solutions. The only notable difference between these acids from redox point of view is that arsenites are readily oxidized by H202 but that phosphites are inert towards this compound (in spite of the strongly negative (calculated) potential).One can then assume that Po$- is also more resistant than AsOa- to the action of OH radicals and when competing for these with another compound the former will have less chances to capture them. This would be the case in basic solution where the tendency of H202 to be de- composed by OH radicals is marked. It follows that arsenites can be oxidized in this medium either by Hz02 or by OH at a rate greater than in acid solution as can be expected from their general chemical behaviour. On the contrary phosphites which do not react with hydrogen peroxide are less oxidized even by the OH radicals as the latter react more strongly with H 2 0 2 . Dr. G. Stein (Jerusalem) said In connection with the dependence of H2 evolu- tion on the pH this phenomenon may serve to elucidate the mechanism of the process only if it is first ensured that the compounds added to the water in order to vary :he pH are themselves unaffected by the reactive intermediates.Unless this is so the reactions of the added substance with the active radicaIs formed may completely obscure the situation. Both arsenious and phosphorous acid un- doubtedly strongly react with some of the radicals formed and are thus unsuitable to serve as examples. Regarding boric acid a similar situation may exist as ap- parent also from the remarks of Dr. Wright in this Discussion. Tt seems however that sulphuric and phosphoric acids may serve a$ suitable additions under suitably GENERAL DISCUSSION 283 chosen conditions.Evidence exists which indicates that the addition of these anions does not influence the yield in the radiation chemistry of metal ions through reactions with the radicals formed by the radiation. Using these anions in pre- liminary experiments (Milling Stein and Weiss) no elidence was found to in- dicate the formation of products from these anions. It appears therefore that these anions may serve to elucidate the dependence of some of the primary processes on pH. Dr. N. Uri (University of Chicago Ill.) (conzmunicated) In the well-informed paper presented by Stein there is one statement which I think is inaccurate and 1 would like to draw attention to it as similar statements occur not infrequently in the literature. It is said that the molecule with the greatest electron affinity will retain in the end all those electrons or H atoms which have not undergone other irreversible processes.A similar impression on the parallelism of the electron affinity and the H atom abstraction arises also from other passages in this paper. 1 shallquote only a few examples in order to show that the assumption of such parallelism is not permissible the electron affinity in solution of the C1 atom is somewhat larger (by about 3 kcal) than that of the OH radical. On the other hand the abstraction of an H atom by an OH radical is considerably more exothermic (by at least 12 kcal) than that by a C1 atom. Thermodynamically this difference will express itself in the different heats of ionic dissociation of water and hydrochloric acid.The energy change in the abstraction of H atoms by C1 atoms or phenyl radicals will be about equal but a phenyl radical has a very much lower electron affinity in soluLion than a C1 atom. Some recent observations orr " promoter " and " sup- pressor " effects in oxidations by Fenton's reagent are based on this very pheno- menon-that oxidation by eleciron transfer of ferrous ion and abstraction of hydrogen from an organic substrate do not go hand in hand.1'1 C1 atoms would show much more preference in oxidation by electron transfer than OH radicals where this competes with oxidacion by hydrogen abstraction. The same applies to the acetate radical. The energy changes involved i n such reactions as are very different. It might appear preferable to avoid mentioning the two different modes of oxidation as if they were entirely parallel occurrences.Dr. G. Stein (Jerusalem) (conmzunicated) In the systems considered by Dr. Uri there certainly exists a considerable difference betweFn electron transfer and dehydrogenation processes. In those systems the electron or the H atom have to be abstracted from an existing molecule or ion by the attacking species and considerable differences will exist in such case between a process like the abstrac- tion of an electron from a hydrated ion with the formation of a higher oxidation state or on the orher hand the dehydrogenation of for example an aromatic molecule with the formation of a free radical. The situation is however quite different in the radiation chemistry of aqueous solutions.In this system the ejected electron or the hydrogen atom formed through the reaction of the electron with the aqueous medium are not bound to any existing molecule and arz free to be captured by a suitable acceptor. In fact the detachment process has been performed in the primary radiation chemical process so that onr has to consider not an oxidation or a coupled oxidation-reduction process but on the contrary an isolated reduction brocess alone. Moreover owing to the spatial separation between the electron or H atom and the original molecule from which it has been detached the back reaction is considerably less influenced by the nature of the original molecule than in photochemistry. As a result in the presence of some suitable substrates such as methylene blue or metal ions (ferric ceric) we have been unable to differentiate experimentally whether the reduction process occurring 111 Kolthoff and Medalia J .Amer. Clwn. Soc. 1949 71 3777. 3784. GENERAL DISCUSSION 0 2 + e + 0 2 - a97 where A and B are hetero-atoms 284 was due to direct electron capture or to reaction with an H atom already formed from the electron. Regarding systems in which the primary reduction will be followed by further reversible processes the possibility of differences in the sense pointed out by Dr. Uri is not excluded in aqueous systems. However in the multicomponent systems known to us such effects have not been observed and although we have considered this point we could not dekise a decisive experiment.In non-aqueous and especially solid systems the process of reversible electron trapping will certainly yield the results as discussed in the paper. Regarding electron or H atom capture by 02 it has been impossible to differ- entiate between the two processes where the electron is captured by the 0 2 molecule the ion thus formed becoming hydrated and 0 2 + H -+ H02 where the H atom has already been formed by the preceding hydration of the original electron. Accordingly one deals here again with two isolated reduction processes which in this particular case are formally parallel. Dr. W. A. Waters (Oxford University) said In connection with the use of dyestuffs as radical deteclors it should be noted that these are in general conjugated quinonoid systems A=C-C-C-C-B (0 0 in indigo; N N in methylene blue perhaps S).Though in general it is true that free radicals very easily add on to quinones the nature and facility of the addition depends on the chemical character of the radical concerned. Thus H* adds easily to 0 and N but less easily to C whilst *OH will not add to 0 probably not to N but very easily to C. Thus from radiochemical fission of water one would expect reduction in pre- ference to oxidation (as found). The *OH attack would be on Ihe inner C=C structure and would necessarily lead to the complete destruction of the whole conjugated system. This has been established for indigo sulphonic acid which breaks down to isatin sulphonic acid. Again the reaction must occur in stages e.g.D 4- *H -f DH* or DH2 +-:OH + DH. S(H20 giving semi-quinonoid (or similar) radicals. In kinetic work it is often forgotten that in general these semi-quinonoid radicals do not persist-they could be spotted immediately from their very intense colours-but react much more readily than either the dyes D or their stabler leuco compounds DH2. It seems that these radicals DH* resemble free metallic ions such as Fez+ or Fe3+ in being immediately reversible oxidation-reduction systems each individual radical having its own redox potential. Some radicals for instance can easily reduce Fe3+ whilst others easily oxidise Fez+. Reactions due to these intermediates can be expected to set in whenever organic radicals are produced by irradiations of solutions of organic compounds and consequently in the interpretation of radiochemical processes care should be Laken to see that one is not being confused by secondary reacLions of radicals produced subsequently to the primary chemical decomposition.With regard to the action of *OH radicals on benzene etc. Dr. Stein has used the hypothesis that direct attack on H occurs viz. HO* -/- C6H6 -+ H20 + *C6H5 but many atoms and radicals e.g. C1* CH2 (diradical) are known to act by primary addition to the 7~ electron system. An alternative interpretation of radical substitution via addition should therefore be borne in mind and until the primary 285 mechanism is made evident the theoretical interpretation of substitution laws for actions of radicals upon aromatic systems should be regarded as an open question.Dr. G. Stein (Jerusalem) (partly communicuted) The remarks of Dr. Waters . GENERAL DISCUSSION are most illuminating regarding the different possibilities of attack by the various radicals on dyes. They emphasize the observations of Dr. Collinson reported in this Discussion and the experiments of Mr. Day and myself. There can be little doubt that both modes of reaction can be demonstrated under suitably chosen conditions. Regarding the mechanism of aromatic substitution by radicals it is indeed reasonable to assume that the primary attack is by way of an addition of the radical to the r r electron system as pointed out by Dr. Waters. Our own interest has been in establishing experimentally whether this formation of a transition complex is followed by (i) a displacement reaction (ii) a dehydrogenation reaction or (iii) further addition of a second incoming radical to the addition complex forming e.g.a dihydro-dihydroxy compound. In these cases the final substitution would be caused (i) by the first radical in the first stage (ii) by a second radical in a distinct second stage or (iii) by two radicals acting jointly in the transition compound. From the experimental evidence obtained 112 it seems strongly indi- cated that the substitution reaction proceeds in two distinct stages as in (ii). In the first dehydrogenation occurs presumably via an addition complex with the formation of an organic radical derived from the aromatic molecule followed by a second step in which the organic radical thus formed interacts with a second molecule or free radical thus bringing about the final substitution step e.g.7 /OH .Dr. h. Collinson (Leeds University) (partly communicated) In connection with Stein’s paper I should like to mention some results obtained on the aqueous methylene blue system irradiated by 220 kV X-rays. Our interest in this system was stimulated by the fact that it was reported that this solute is reduced by irradiation in de-aerated solutions in spite of its reduction potential being well over to the reducing side a result which appeared anomalous from the point of view of the concept of the equivalent reduction potential.113 The justification for classing the decolorization as a reduction seemed to depend on the occurrence of a restora- tion of colour on admitting oxygen and although this seemed sound evidence it was thought that it would be useful to attempt a more direct check on the products of irradiation.Under the conditions employed the methylene blue dye exhibited absorption bands at 6575 6100 2910 and 2460A. By quantitative reduction of the dye solution and spectrophotometric measurement under vacuum of the resulting solution a spectrum of the leuco dye was obtained. This possessed only one absorption band in the wavelength region 2200 to 7000 A viz. a sharp band at 2560A. Irradiation by ultra-violet light of a solution of methylene blue con- taining hydrogen peroxide followed by spectrophotometric measurement yielded an absorption spectrum of the oxidation products due to the action of OH radicals upon the dye.This spectrum had no marked absorption from 7000 to 3500A 112 Stein and Weiss J. Clzem. Soc. 1949,3245 and following work quoted in the paper under discussion. 113 Collinson and Dainton Ann. Rev. Physic. Cliem. 1951 2 99. GENERAL DISCUSSION 286 the absorption rising slowly and continuously at lower wavelengths. Irradiation of aerated methylene blue solutions gave rise to the same spectrum as that obtained from the attack of OH radicals no trace of a band at 2560 A being present. Colour could not be restored to a solution decolorized in this way either by oxygec hydrogen peroxide zinc amalgam or sodium hydrosulphite. The progress of irradiation of de-aerated dye solutions could be followed by measuring the spectrum at relevant points after successive irradiations the solution being under vacuum throughout.Not only could the rate of removal of the dye colour be measured by this method but also the rate of development of the leuco dye. The rate of decolorization in de-aerated solution was much faster than that in aerated solution and it was found that the predominant action was undoubtedly one of reduction. However at no stage could the decolorization be attributed entirely to leuco dye formation. The proportion of decolorization due to the formation of leuco dye decreased as the irradiation continued and at complete decolorization in a neutral solution the leuco dye concentration was about half that of the original dye.In acid solutions the leuco dye concentration at complete decolorization was a higher proportion of the original dye concentration. On continuing irradia- tions beyond the point of complete decolorization the absorption band at 2560 A slowly disappeared the rate of destruction of the leuco dye being about 0.1 times the initial rate of decolorization of the dye and the rate of destruction in acid solution being lower than the rate of destruction in neutral solution. The ultimate spectrum was the same as that obtained from irradiation in aerated solution or by the action of OH radicals. Therefore the ultimate result of irradiation of an aqueous solution of methylene blue even when de-aerated is one of irreversible oxidaiion.However the fact that predominant reduction occurs in the early stages of an irradiation still seems anomalous from the point of view of the e.r.p. Several features of this rather complex system may combine to give this result but the most important factor is probably that ihe hydrogen atom or othcr reducing agent can react in only one specific way with the dye molecule namely to reduce it to the leiico dye whilst the OH radical or other oxidant may attack the molecule at several points some of which may cause an inappreciable change in the absorption spectrum. Moreover the hydrogen atom may havc no further point of ready attack once the leuco stage has been reached whilst the oxidized products and the leuco dye may still be open to attack by OH radicals even after the dye molecule has been broken down into smaller fragments.The fact that the leuco dye disappears as oxidation products apparently without passing through the dye stage supports the view that the attack of OH is mainly on other parts of the molecule than those respons- ible for the reversible colour change. Hence the increased decolorization yield in the presence of benzoate reported by Stein seems more likely to be due to prevention of radical recombination than to prevention of re-oxidation of the leuco dye by OH radicals. Dr. G. Stein (Jerusalem) (partly conmiunicated) Dr. Collinson’s interesting experiments show conclusively that the decoloration of methylene blue may take place by two mechanisms. One of these an irreversible decoloration is presumably an oxidation reaction.The other a reversible decoloration has now been shown to consist of a reduction process. Our aim in this respect was to show that under suitably chosen experimental conditions the reversible reductive decolor- ation may predominate and to devise experiments which enable one to elucidate the mechanism of this reaction. The fact that a reductive decoloration can be obtained with a high yield and can be rcversed almost completely as shown by Mr. Day’s results communicated at the meeting is not necessarily in contradiction with the useful concept of the equivalent reduction potential of Dainton and Collinson. The e.r.p. is the result of the establishment of an equilibrium in a system where the various reactive species (OH H02 H H2+) can mutually reverse each others actions.In our 287 GENERAL DISCUSSION system however this reversibility is upset by the occurrence of irreversible re- actions. Thus the introduction of benzoic acid reacting irreversibly with the OH radicals makes impossible the establishment of the usual reversible equilibrium necessary for the creation of an e.r.p. in the system. Dr. J. H. Baxendale and Dr. J. Magee (Manchester University) said We would like to emphasize that certain differences may occur between the oxidation of a substrate by Fenton’s reagent and oxidation by radiation in aqueous solution. In addition to the possible interference by H atoms in the latter the presence of oxidizable and reducible metal ions in the former could give rise to reactions which are not present in the radiation case.We have found evidence for such reactions in a detailed quantitative study of the oxidation of benzene by Fenton’s reagent. In vacuo the products of this oxidation have been shown previously to be diphenyl and phenol. Reactions which have been previously considered to give rise to hydroxylated products are Ph* + H202 -+ PhOH + OH (4 and Ph* + OH .+ PhOH (b) Our observations are that with 10-4 M Fez+ and H202 70 % diphenyl and 30 % phenol are produced and the oxidation balance precludes the presence of other products. The products are unaffected by change in hydrogen peroxide concen- tration so that reaction (a) can be excluded. However the initial presence of ferric ion alters the relative amounts of diphenyl and phenol appreciably in the sense that with increasing ferric ion concentration the proportion of phenol increases.Simultaneously there is a decrease in the net amount of ferrous ion oxidized. These observations are consistent with the oxidation of the phenyl radical to phenol by Fe3f or FeOH2f. It is possible that even in the absence of ferric ion initially the ferric ion formed during the reaction produces all the phenol. Hence phenol can be formed by entirely different mechanisms in the radiation and Fenton sys terns. The reduction of phenyl radicals by ferrous ion must also be assumed to account for our observations. It is clear that analogous reactions are possible when other substrates are used and these must be borne in mind when comparing the effects of the radiation and Fenton systems.Dr. J. Weiss (Durham University Newcastle) (commzmicated) It has been shown previously that phenol is formed by the action of ionizing radiations on benzene in aqueous systems.114 The phenol can be determined by the well-known colorimetric method using the Folin reagent. This has been employed also for the chemical dosimetry of ionizing radiations.115 Tn the course of a reinvestigation of this problem in collaboration with Mr. Milling we have found that particularly in the presence of oxygen of the air the formation of phenol is accompanied by the formation of hydrogen peroxide in somewhat more than equivalent amounts. Hydrogen peroxide can be deter- mined by the titanium sulphate reagent (this is not interfered with by the presence of phenol) although it is not clear at present to which extent this is due to other hydroperoxides which may be present in the solution.We have found that under the conditions used previously i.e. saturated solutions of benzene in water and irradiation with X-rays (200 kV) in the presence of air the value for the yield of hydrogen peroxide (G = molecules/100 eV) determined by titanium reagent is G(H202) N 5 - 5 . Dr. G. Stein (Jerzcsalem) said The difference between the action of radiations and Fenton’s reagent and the effect of ferric salts on hydroxylation processes with Fenton’s reaagent and photochemically with ferric salt solutions is known. Tt is referred toeven in thepaperunder discussion and has beeninvestigatedin detail.116 114 Stein and Weiss J.Chem. SOC. 1949 681. 115 Day and Stein Nature 1949 164 671. 116 Stein and Weiss J. Chem. SOC. 1951 3265. GENERAL DISCUSSION (FeOH)2+ + R -+ ROH 4 Fez'- H2Ot 4- c -+ H20". 288 Thus it has been shown that the difference in substitution ratios in the hydroxyl- ation of benzoic acid by X-rays and by photochemical radical formation 117 is due to this reason. It is also shown there that complexing the ferric salt for example by fluoride influences fundamentally the process of hydroxylation by Fenton's reagent of phenol thus indicating a much more important role for the ferric ion than previously assumed. Dr. J. Weiss Durham (University Newcastle) (communicated) 1 should like to draw attention to a paper by Dr.Stein and myself 118 where we have already discussed the hydroxylating effect of ferric ions in general and of the (FeOH)2+ complex in particular viz. the reactions with free phenyl or aryl radicals according to and where we have also suggested that certain differences in the hydroxylation by irradiation on the one and Fenton's reagent on the other hand may be due to reactions of this type. Dr. A. G. Maddock (Cambridge University) said Dr. Stein has raised the question of the chemical effects resulting from the F-centres and other lattice defects produced in solids by ionizing radiations. Caillot and Sue119 have obtained some direct evidence of such effects. They have shown that the ratio of P5+/P3+ for P32 produced by the (n cx) reaction in sodium chloride is influenced by the creation or relaxation of the F-centres in the crystal.I should like to make brief mention of some effects investigated by Dr. J. Green in my laboratory. It is possible that the reactions I shall describe may be adapt- able to a high dose actinometer of the kind desired by Dr. W. Wild since very large irradiations are necessary to produce accurately measurable resulis. We find that subsequent y-irradiation of neutron irradiated salts of oxyanions such as chromates or permanganates increases the fraction of the active species found on solution as the original oxyanion (the retention) ; while the net effect on the inactive oxyanions leads to macroscopic reduction. This effect resembles the thermally activated increase in retention that we have described previously.It is hoped that a quantitative study of both processes will determine whether the increase is entirely due to recombination of the fragments of the oxyanions or whether some electronic mechanism is involved. I understand that Prof. G. E. Boyd (private communication) has obtained similar and more detailed results with potassium bromate. Dr. John L. Magee (Notre Dame Univerdj Indiana) said Although the H and OH radical formation mechanism reviewed by Dr. Stein in eqn. (2-4a) has had wide acceptance there is real reason to doubt its validity. In fact the dom- inant electron recapture process may be where the symbol * designates a highly excited molecule and excited water mole- cule dissociation may be the principal source of radicals.Arguments which suggest this possibility will be discussed at length in a future publication 120 and will only be outlined here. (i) Slow secondary electrons can probably be thermalized in water without capture. It has been shown 121 that in a medium which has a threshold energy for electron capture the probability for capture of electrons as they are being ther- malized is usually very small. There are reasons for believing that H20 has a 117 Loebl Stein and Weiss J . Gem. Soc. 1951 405. Bates Evans and Uri Nature 1950,166 869. 118 Stein and Weiss J . Chem. Sor. 1951 3265. 119 Caillot and Sue Compt. rend. 1950 230 1864. 120 Samuel and Magee forthcoming publication. 121 Magee and Burton J. Amer.C k m Soc. 1951 73 523. GENERAL DISCUSSION (iii) The above discussion assumes that the dissociation reaction H20f 4 H+ + OH 289 threshold.122 The work of Dainton et af. mentioned in this Discussion,l23 on the photon-induced electron transfer from various ions to H20 clearly indicates such a threshold. Cii) Slow secondary electrons will fall below zero energy relative to the H20+ ions at moderately small distances. As an electron collides with the H2O molecules and loses energy it pursues essentially a random-walk path and does not increase its distance from H20+ very quickly. The average loss of energy per collision is not known accurately but the attained distance is not extremely sensitive KO this quantity. With reasonable estimates the electron will go 30A or less.Once the electron drops below zero energy it has been “captured” and an excited H20* is formed. Further electron collisions with the HzO medium will quickly degrade energy and draw the electron closer to the positive ion. does not habe time to occur before the capture is effected. This reaction is exo- thermic only by virtue of the solvation of the H+ ion and a time of at least 10-11 sec is required for its completion. The total time required to stop the electron which moves with velocities of 107-108 cmisec is about 10-13 seconds or so. The H20+ ion can of course exchange its charge with neighbouring H20 molecules but the qualitative situation is unaffected by this process. Dr. G. Stein (Jerusalem) (partly communicated) Far too little is known about electron-capture processes in liquid water and in aqueous solutions to come to a definite decision between the mechanism suggested by Dr.Magee and the one discussed by Lea and Gray Doubtless both processes do occur and the question is which one accounts for the bulk of the chemicaI changes observed. Dr. Magee’s mechanism would in fact assume that the chemical effects in the radiation chem- istry of aqueous solutions are due to radicals formed by the dissociation of excited water molecules exclusively. In addition to the points made by Dr. Magat in this Discussion there are some further considerations which indicate that aqueous systems deserve special treatment in this respect. On ejection of the electron the residual positive ion will undergo in water a process of hydration which will take a time of the order of 10-11 sec as men- tioned by Dr.Magee. This vaIue can be arrived at in several independent ways. In this exothermic process the free energy of the positive trap is considerably decreased and dissociation is caused with the formation of a free OH radical and a hydrated H unless recombination takes place before completion of the process. Whilst this reaction proceeds two other processes occur with the ejected electron. I t is thermalized coming to a stop in a time of the order of 10-14-10-13 sec and at the same time it also undergoes a process of hydration in which it polarizes the medium digging a potential well in it.124 So even if the electron drops below zero energy relative to the original H20’ it will be able to move back to it only at a 1 elocity determined by its surrounding polarized dielectric medium approach- ing as the reaction proceeds only the migration velocity of the hydrated OH- ion.Accordingly above a certain electron energy recombination will no longer take place but an electrical equilibrium of hydrated H+ and OH- ions is created (accompanied by the formation of OH radicals and H atoms respectively) at a distance from each other. Although the electron energy of the threshold is not known exactly it is lower than either in the gas phase or in other organic liquids. Thus in a medium like water the process of hydration will influence the fate of any ions created in a manner which does not occur in different media.122 However the work of Bradbury mentioned in ref. (2) seems to indicate a low threshold. 123 Dainton this Discussion. 124 Cf. Pekar J. Physics Moscow 1946 10 347. GENERAL DISCUSSION 2 OH -+ H 2 0 2 290 on H20 vapour and H 2 0 + 0 2 mixtures certainly indicates that water vapour In addition to this formal consideration the work of Bradbury and Tatel 125 especially when condensing does efficiently capture electrons. In the radiation chemistry of aqueous solutions it appears that the energy re- quired to produce one radical pair available for chemical reactions is less than 32.5 eV. This has been attributed to the possibility that in liquid water dissociation of excited water molecules will occur more readily than in the gas phase where radiative loss of energy may take place.However there is some evidence126 which indicates that in liquid water reactions due to radical formation from excited molecules is efficiently prevented through energy loss to the medium. It is perhaps possible to assume that the decrease in the energy required in aqueous media is due to the fact that some of the electrons which have been ejected are in fact prevented froin recombining through the process of hydration in the water system thtis providing additional radicals spatially separated. It is hoped to discuss some of these points in greater detail in the future and to induce some further points in favour of the Lea-Gray mechanism. Dr. M. Magat (Paris) (communicated) I would like to know how Dr. Magee accounts for the difference in the production of H 2 0 2 from de-aerated water by a- and X-rays in his theory.In the " classical " picture of Lea and Gray the difference was ascribed to the separation in space between H and OH radicals the high density of OH radicals in an o! track favouring the reaction as compared with the reaction (1) (2) H 4- OH -+ H20 this latter being important for X-rays where no high local OH concentrations are built up. This difference can also be accounted for on the basis of the assumption made by Dr. Haissinsky and myself that OH radicals are produced in the following reactions H 2 0 + + OH 4- H' H20- + H 2 + 0- H20 -+ OH 4- OH- For a-rays the OH radicals combine to produce H 2 0 2 and the equivalent amount of H 2 is evolved.With X- or y-rays the OH radicals react with H 2 molecules H2 + OH -+ H 2 O -1- H as a result of the uniform distribution of positive and negative ions. In Magee's theory where H and OH radicals are produced in equal amounts from the same ion it is difficult to imagine how the difference in ionization density could influence HzOz production. Dr. John L. Magee (Notre Dame University) (communicated) In the theory I have described the initial efficiency for the formation of H and OH radicals is indeed the same for X-rays and cx-particles. The geometrical structure of the track however causes a difference in the yields of H202 and H2 for the two cases. The great density of radicals along the a-particle track favours radical combina- tion reaction and therefore most of the radicals unite to form H202 H 2 or H20.The smaller density of radicals in X-ray tracks allows most of the radicals to escape initial combination by diffusion and therefore favours the back reaction 127 which destroys most of the mole~ules of H 2 0 2 and H 2 which were initially formed. This description is not in disagreement with views expressed by Allen in this Discussion. 125 Bradbury and Tatel J . Chern. Physics 1934 2 835 126 Farkas 2. physik. Chem. B 1933 23 89. 127 Allen J. Physic. Chem. 1948 52 478 ; this Discussion. GENERAL DISCUSSION 128Butler and Conway J. Clienr. Soc. 1950 3418 ; 1952 834. 129 this Discussion. 29 1 Dr. J. A. V. Butler (Chester Beatty Res. Inst. S. W.3) said Dr. Conway and 1 have found somewhat similar effects to those described by Miss Alper after the irradiation of nucleic acid solutions in the presence of oxygen by X-rays.l2* There is however a marked difference in that we have been unable to ascribe the after- effect to hydrogen peroxide.The situation is a rather complex one. The sensi- tivity of nucleic acid solutions to hydrogen peroxide appears to depend on small amounts of impurities e.g. metal ions cysteine etc. Pure specimens appear to be insensitive to hydrogen peroxide but they become sensitive to it after irradiation. The question is then whether the amount of hydrogen peroxide actually formed during the irradiation is sufficient to produce the observed effects on the irradiated nucleic acid. We have found a considerable discrepancy between Lhe amount actually formed and the amount which has to be added to produce the observed effect and have suggested an alternative mechanism for the after-effect.With reference to Miss Alper’s experiments in which she finds hydrogen peroxide suffi- ciently effective there is the possibility that the hydrogen peroxide is activated by some of the organic substances (e.g. amino-acids) present. Miss Alper also finds that hydroxyl radicals produced by photodecomposition of hydrogen peroxide are ineffective in inactivating bacteriophage. This is a rather surprising finding and 1 wonder if under the conditions of her experiment the hydrogen peroxide is actually decomposed by the U.V. light. It might happen that owing to a slight turbidity of the solution or its absorption very little of the U.V.light can penetrate into the solution and that in fact few hydroxyl radicals were formed. It is also possible that the organic substances present exert a protective action. In reply to Dr. Baxendale our attempts to make added hydrogen peroxide more effective in its action on nucleic acid were unsuccessful. It is conceivable that hydrogen peroxide formed by irradiation in the absence of the nucleic acid and then added to the latter would be more effeciive. We hope to try this experi- ment. Finally. Dr. Weiss states that the after-effect with nucleic acid is a hydrolysis. We have found no evidence that this is the case. Dr. B. E. Conway (Chester Bentty Res. Inst. S. W.3.) said In Miss Alper’s experiments on the inactivation of bacteriophage by ionizing radiation it is of interest that H202 itself decreases the extent of survival yet photochemical decom- position of the H202 in the presence of the phage to produce OH radicals (and some HO2 radicals) does not further decrease the extent of survival.With sodium deoxyribonucleate both X-irradiation and treatment with H202 and U.V. brings about an apparent depolymerization as well as several different types of chemical change. The action of H202 in vivo and possibly of the X-irradiation is therefore probably different from that on isolated nucleic acid and as Miss Alper suggests may be principally effected by H atoms. The spontaneous action of H202 itself is then however difficult to account for unless a specific peroxidation occurs.Miss T. Alper (Huniniersmith Hospital London) (conzmunicuted) The remarks of Dr. Butler and Conway refer in part to results mentioned in the Discussion but not published in my paper,l29 viz. to the finding that sensitization of phage to the action of hydrogen peroxide did not occur as a result of exposure to OH radicals formed by the photochemical decomposition of hydrogen peroxide. Under the conditions of one experiment measurement of the hydrogen peroxide concentration before and after U.V. irradiation showed that it had decreased by 10 % and it seems reasonable to assume a concentration of OH radicals sufficiently high to have brought about injury of the phage similar to that found with ionizing radiations if the latter acted through the medium of OH radicals.If the failure of OH radicals to produce this injury were due as Dr. Butler suggests to protection by organic substances such protection would of course also be exerted if the OH radicals were produced by ionizing radiation. It should be noted that the solid GENERAL DISCUSSION Chemistry mid Physiology of the Cell N~icleus (Brookhaven August 1951). 292 content of the phage suspensions used was less than that of the DNA solutions used by Butler and Conway,130 in which OH radicals were found to be effective. Dr. J. Weiss (Durham University Newcastle) said 1 do not think there is any real difficulty in understanding the after-effect at least with nucleic acids and related compounds which have been studied by Dr.Scholes and myself in consider- able detail. As we have already stated elsewhere 131 we have come to the conclusion that with nucleic acids the after-effect is due to a slow hydrolysis of some primarily formed unstable phosphate esters although we do not know very much about the exact nature of these unstable esters. Thus we have been able to conclude that the depolymerization of the nucleic acid which continues after cessation of irradia- tion can be largely attributed to the slow hydrolysis of intermediate labile phosphate esters formed under the influence of the radiations. I think that this explanation of the after-effect is also in agreement with the observations of Butler and Conway referred to by Dr. Butler. 130 Butler and Conway J.Chem. Suc. 1952 834. 131 Scholes and Weiss Nature 1950 166 640. Report of the Conference on the TIKVAH ALPER 243 GENERAL DISCUSSION Dr. N. Uri (University of Chicago) (communicated) One of the fundamental quantities in the energetics of the radiation chemistry of aqueous solutions is the bond dissociation energy H . . . OH. An accurate determination has been achieved by the fine experimental work of Dwyer and Oldenberg.1 It is however over-looked that in the evaluation of this bond dissociation energy and that of 0 . . . H in the OH radical they have not used the now generally accepted values for the bond dissociation energy of H . . . H and of the heat of formation of water. The value of 118.2 kcal can therefore not be accepted as absolutely correct. A good approximation is however (decimals are hardly justified) 120 kcal for H .. . OH and 100 kcal for 0 . . . H. Dr. G. Stein (Jerusalem) said There are a number of reactions in radiation chemistry which indicate that effects due to H atoms i.e. reduction processes, do proceed readily under suitable conditions. As the reduction of methylene blue which is reversible on the admission of molecular oxygen shows these re-duction processes are often due to €3 atoms formed primarily rather than in a secondary interaction between H2 molecules and OH radicals. As found the addition of benzoic acid to methylene blue solutions in the absence of 0 2 results in a considerable increase in the decoloration. Benzoic acid is known to react readily with OH radicals. Were the OH radicals the source of the H atoms which reduce the methylene blue the addition of benzoic acid would result in a decrease, and not an increase of the yield as observed.Hardwick 2 has shown recently that ceric sulphate solutions are reduced readily, the reduction yield being the same in aerated and in evacuated solutions and higher in solutions containing H2. Milling Stein and Weiss (to be published) have recently also studied this reaction as well as the reduction of ceric per-chlorate. In this work using 200 kvp X-rays results in good agreement with 1 Dwyer and Oldenberg J . Chern. Phys. 1944 12 351. ZHardwick Can. J. Chem. 1952,30 23 244 GENERAL DISCUSSION those of Hardwick were obtained. The results could be interpreted in terms of a reaction mechanism which did not necessitate the assumption of a reduction of ceric salts by OH radicals.It is perhaps possible that the difference between the results of these workers and those of Haissinsky is to some extent due to the very qoft radiation employed by the latter. Reduction processes may become masked under conditions where a1 terna tive acceptors are available for the H atoms. Thus in the presence of molecular oxygen, the reaction H + 0 2 + H02 will effectively remove the H atoms and prevent reduction processes from occurring, unless the acceptor is capable of reacting with the HO2 radical or its ion 02- and of being reduced by these. This seems to be the case with ceric ions and such reactions have been previously assumed by Weiss and by Baxendale for the re-duction of the ferric ion.On the whole it appears that when due allowance has been made for the particular experimental conditions the bulk of the experimental evidence now available is capable of explanation by assuming the formation of OH radicals and H atoms followed by the appropriate reactions of these. Dr. M. Magat (Paris) said In the short note in which Dr. Haissinsky and myself suggested that the reaction would explain a large amount of experimental evidence we stated that this reaction is thermodynamically possible but did not indicate the argument for i t ; this I would like to present here. We can calculate the exothermicity of this and of the Weiss reaction : (H20 + e)aq + OH-, f Q2 by the following cycle Q + A + D - E - S, where A the heat of evaporation is 10 kcal in both cases D is the dissociation energy E the electron affinity and S the solvation energy.According to Gaydon 3 the best values for D1 and D2 are 11 5.6 and 117.8 kcal. For El and E2 we took the " best values " quoted by Massey,4 50.7 and 48.5 kcal. The solvation energy is more difficult to evaluate but one can show that S(O-) > S(OH-1, independently of the absolute values. Indeed in both cases all the 4 solvating water molecules have a hydrogen pointing towards the negative ion. This result is independent of the particular charge distribution that is assumed for the H20 molecule such as the one recently proposed by Lennard-Jones and Pople 5 or the one used earlier by myself6 and others. The a pviori possible configuration H/O H\ .. . H - 6 is less favourable by at least 30 kcal which is a value much higher than the errors usually made in electrostatic solvation calculations. This means that there exists for the OH- solvation a repulsion between the hydrogen atoms of the OH ion and the hydrogen of one of the four solvating water molecules. This repulsion is difficult to calculate but if we assume that it is 4 kcal the error will probably not exceed 2 kcal. 3 Gaydon Dissociation Energies. 4 Massey Negative ions (Cambridge) ; see also this Discussion. 5 Lennard-Jones and Popple Proc. Roy. Soc. A. 1950. 6 Magat Ann. Physique 1936 5 108 GENERAL DISCUSSION 245 7f one introduces the two sets of values in the eqn. (3) one finds that Ql - Q2 = 8 4 2 kcal reaction (1) being more exothermic than reaction (2).(This last conclusion would remain even if S(0-) = S(OH-).) From the fact that the reaction which we have suggested is more exothermic than the reaction (2) usually assumed it cannot be deduced that reaction (1) will predominate. But it shows that there is no a priori impossibility for the reaction to occur. A knowledge of the activation energies for the two reactions would help in making a decision. But this is difficult to obtain and although we are attempting an evaluation any statement at the moment would be premature. Prof. F. S. Dainton (Leeds University) said Dr. Haissinsky has referred to the interesting suggestion which he and Dr. Magat 7 have made that the molecular hydrogen formed from water and aqueous solutions originates chiefly from the electron capture reaction (1 5 ) 3320 + e -+ H2 -1- OG, 0; + H20 -+ OH.& + OH.(1 5 ) (1 6) In this connection the following two considerations are of importance. Firstly, we may compare on energetic grounds reaction (15) with reaction (14) which is the more commonly accepted process, (14) Using the values Eo = 2.2 eV EOH = 2.1 eV DOH = 99.4 kcal D H ~ = 103-4 kcal, we obtain SOH- - So- = 6 kcal - (AE14 - A&) ; and putting D,o+ 213 + 0 = 219 kcal, AE1 = 16 kcal + AE14 - AE15 = 22 kcal - (SOH- - So-). Thus for reaction (14) to be preferred to reaction (15) on energetic grounds, AE14 > AEls i.e. SOH- - So- < 6 kcal and hence the energy of activation of reaction (16) must exceed 16 kcal. Hence the factors which made H2 + 0-more probable than H + OH- as products of electron capture by water are those which made reaction (16) less likely.This is evident from inspection of the equations concerned ; in fact if reaction (15) occurs readily Oiq is to be regarded as a more stable entity than OH;. We therefore conclude that if (15) occurs lo the exclusion of (14) it is most improbable that it will be succeeded by (16). The second consideration arises from the fact that electron transfer to water from a donor which may be a reducing cation MZf or anion Ag- to a water mole-cule of its hydration shell can readily be achieved by thermal or photochemical means.8 In the absence of any radical acceptor these electron transfer reactions always result in the evolution of hydrogen gas and the oxidation of the reducing ion e.g.the divalent ions V2+ Fez+ Cr2+ E u ~ + are converted to the corresponding trivalent ions. According to reaction (14) the molecular hydrogen results from the dimerization of the H atoms formed in this process and we may write e.g., (a) According to the Magat-Haissinsky reactions for the formation of the hydrogen gas we should write M2+ 2H20 3 M3f OH- + H2 I- OH (15 + 16) 7 Haissinsky and Magat Compt. rend. 1951 233 954. 8 Dainton and James J. Chim. Phys. 1951,48 1 . and that this reaction is followed by H20 + e -+ H + OH,,. M2+ H2O -+ M3+. H20- -+ M3f OH- + H(+ ?- ; H2) 246 GENERAL DISCUSSION which would presumably be followed by leading to a net process and in passing we note that when the initial electron transfer is stimulated by light, +a = 0.5$6.Quantum yields greater than unity would therefore afford some support to the Magat-Haissinsky scheme. These have not been observed but this fact alone is not of great consequence. A much stronger argument against reaction (15) is provided by the observation that radical acceptors such as water-soluble vinyl compounds suppress the gas evolution without affecting the oxidation of the reducing ion and are at the same time themselves polymerized. This is to be expected if the electron transfer reaction is represented by (a) whereas according to the Magat-Haissinsky scheme for the breakdown of HzO- the hydrogen gas evolution should be unaffected by the presence of the monomer. For these two reasons I think that although reactions (1 5) and (1 6 ) may occur they play a minor role compared to that of reaction (14) and cannot take place to an extent adequate to account for the observed radiation yields of molecular hydrogen.Dr. Haissinsky has also suggested that methylene blue and other dyestuffs may be preferentially reduced because the dye molecules compete unsuccessfully with molecular hydrogen for hydroxyl radicals. OH + H2 + H20 + H probably has a much lower velocity constant than any other reaction involving OH radicals. It is known 9 that in the gas phase the energy of activation is about 14 kcal ; and in so far as the energy of activation of the H atom abstraction reaction follows the general rule of proportionality the dissociation energy of the bond broken i.e. DH . . . x the energy of activation for the case when X = H will be greater than for any Y-H bond in the dye molecule.Other possible reactions between dye and OH are probably additive involving even smaller energies of activation-see Dr. Waters' comment. An important point often neglected in considering the decoloration of the dyes is the fact that even in de-aerated solution the net effect on prolonged irradiation may be oxidution and not reduction; (Dr. Collinson has shown this to be true for rnethylene blue and Dr. Dale has men-tioned that thiourea protects MB against radiation bleaching) ; and to this extent the observations are in keeping with the known redox potential of the dye and the presumed e.r.p. of water. Dr. G. W. R. Bartindale (College of Technology Munchester) said In spite of the objections of M.Haissinsky I should like to support the following mechanism for the decomposition of the ion H20- : followed by Mz+ + OH -+ M3' OH-2M2+. 2H20 + 2M3+ OH- + HZ (4 However the reaction HOI+LH- X-+HzO,+ X H20- + H- + OH, H- + H20 -+ H2 + OH-. (1) (2) The products of reaction (1) are both known to exist ; H- is the anion of the salts LiH CaH2 etc. Reaction (2) is also well known as exemplified by the action of water on lithium or calcium hydrides. Further no drastic rearrangement of bonds is involved in either reaction (1) or (2) whereas in the reaction proposed by Haissinsky viz., H20 + e -+ H2 + 0-, two bonds are broken and a fresh one formed. 9 e.g. Frankenburger and Klinkhardt Trans. Firradiry SOC. 1931 27 231 ; and von Elbe and Lewis J.Amer. Chem. SOC. 1932 552 821 GENERAL DISCUSSION 247 Mr. J. Wright (A.E.R.E. Harwell) said The observations concerning hydrogen peroxide production in boric acid solutions in the pile are of interest in connection with our own work on ferrous sulphate solutions containing boric acid. It seems quite probable that deviation from additivity of chemical efl'ects will vary according to the system studied. The lower G value for H202 production by the B(n a) reaction in the pile compared with that for H202 production by radon a-particles reported by Dr. M. Lefort earlier in this Discussion may be a measure of the deviation from additivity for this system which should clearly therefore not be neglected in discussing the results. In our work with ferrous sulphate + H3B03 solutions we have found a linear relation between ferrous oxidation and dose, varying the dose both by increasing the time of irradiation and (in the centre of the pile) by increasing boron concentration.Deviation from linearity with boron concentration in the thermal column irradiations was in the opposite sense from that observed by Dr. Haissinsky. At the maximum H3B03 concentration used in our work (0-08 M) the absorption of thermal neutrons was only 3 % per cm and could be neglected but this might become important for higher concentrations and for different geometrical arrangements of the irradiation vessel. The increased H and OH radical production due to the presence of pile y-radiation may lead to the establishment of lower equilibrium concentrations of H202 which could explain the deviation from linearity with neutron flux observed by Dr.Haissinsky. In connection with the scheme proposed in eqn. (25)-(29) for the effect of boric acid we have found no effect of this substance on the rate of oxidation of ferrous sulphate in 0.8 N H2SO4 and find it difficult to believe that the chain reaction represented by eqn. (26) and (27) is effective in removing OH radicals under these conditions. Tests have been carried out with fission-product gamma radiation and by bombardment with 1-2 MeV electrons. Dr. C. B. Amphlett (A.E.R.E. Harwell) (communicated) The statement that complex formation in sulphuric acid solutions of ceric ion will affect only the rate of reduction and not its mechanism is likely to be misleading since (i) there is no evidence that this is so with ceric solutions and (ii) it may be extended by inference to other systems where it is clearly incorrect.The difference in Ei for the cerous-ceric couple as a function of the acid anion is very great e.g. in 1N solutions the values are - 1-70V (HC104) - 1.61 (HNO3) - 1.44 (H2SO4) - 1-28 (HCl),lo all potentials being expressed in the U.S. convention. The overall change of 0.42 V in going from HCI04 to HCl solutions is as great as that between the systems FeI1/Fe1Ir and Fe phy/Fe ph;' (ph = o-phenanthroline) so that with respect to a suitable system with an Ei value midway between - 1-28 and - 1.70 V the course of a cerous-ceric reaction could be reversed it is not inconceivable therefore that the mechanism of the reaction may be altered by complexing.In this connection it seems worthwhile emphasizing that by neglecting the true state of the ions in solution we may be obscuring the correct interpretation of the results. In the photochemical reduction of perchloric acid solutions of ceric ion the reduc-tion step is usually written as involving the CeOH3+ ion thus :I1 CeOH3+ + OH -+ Ce3+ -i- H202 etc., this providing a pH dependence similar to that given by step (2) in Haissinsky's paper. In sulphate solutions however spectrophotometric data 12 show that the sulphate complexes contain neither hydrogen nor hydroxyl ion so that the analogous step must be differently written. Although evidence is gradually ac-cumulating on the effects of complexing upon the rates and mechanism of ion -+ ion reactions in solution (in which sulphate complexing sometimes plays a notable part,l3) there is as yet little systematic data upon similar effects in radical + ion 10 Smith and Goetz Ind.Eng. Chem. (Anal.) 1938 10 191. 11 Evans and Uri Nature 1950 166 602. 12 Hardwick and Robertson Can. J. Chem. 1951 29 828. 13 e.g. Sykes J. Chem. Soc. 1952 124 248 GENERAL DISCUSSION reactions which are of fundamental importance in the interpretation of the radia-tion chemistry of aqueous solutions. Such evidence is likely to come from three sources viz. (i) a study of the pH effects in redox systems under irradiation in presence and absence of complexing agents (ii) study of the ionic species in the solutions concerned and (iii) efTect of pH and complexing upon suitable isotopic exchange systems.Such evidence as exists concerning the ferrous-ferric system is summarized in my paper,14 where it leads to the concept of increased ease of electron transfer when the hydration shell or other co-ordination shell is broken. Dr. N. Uri (University of Chicago) (cornnzunicatecl) The fundamental difference in the mechanism of the reduction of ceric ion by the OH radical as suggested by Haissinsky on the one hand and later by Evans and Uri on the other is to be sought in the participation of the ion pair Ce4+0H-. According to Evans and Uri, it is the ion pair which reacts with the OH radical. This is essential as otherwise the reaction involves a termolecular collision. At the time when the original Haber-Weiss mechanism was published the significance of ion pairs was not yet recognized and this would have made the assumption of a back reaction in the oxidation of ferrous ion by hydrogen peroxide unlikely.From our knowledge today I consider that this back reaction is an essential part of the Haber-Weiss mechanism. It would be interesting to learn something of the intermediate stages in the reduction of dichromate ion by OH radicals as the mere formulation of overall reactions involving a large number of entities is extremely unsatisfactory in present day kinetics. In those cases where the mechanism has not been estab-lished such as in the chromate ion reduction the oxidation-reduction potential is of little value in the judgment or prediction of reactions occurring. Dr.M. Haissinsky (Paris) (parfly comnzutzicated) In answer to Amphlett and Uri's criticism about the real form of the ion of Cerv in sulphuric acid solution, I may note that the aim of eqn. ( 2 ) of my paper where this ion was formulated as Ce4+ was only to show that the reduction is performed by OH radicals and not by H atoms as it was generally considered previously. The precise form of CeKv can hardly be established by radiochemical methods alone and the question must be studied by other physico-chemical techniques. But it is necessary to note that the potentials of Ce47/Ce3~ in various strong acid solutions are always sufficiently high for assuming the same fundamental mechanism as expressed by eqn. (2) for all these media. It is not the same for the Fe3+/FeZt quoted by Amphlett.The potential of this system is much lower probably at the limit of a possible reduction by OH. Even a small variation of the conditions can then not only modify the reaction mechanism but reverse the direction of the reaction (oxidation or reduction). At the same time I should like to indicate that 1 am not opposed to the e.r.p. concept of Collinson and Dainton which is an interesting development of the ideas of Lefort and myself on the role of the redox potential in radiation chemistry of aqueous solutions. I wanted only to point out that a constant value which would be independent of the conditions of the medium and the nature of the radi-ation cannot be attributed to the e.r.p. Thus I was glad to notice that during this Discussion Collinson and Dainton expressed a similar opinion namely, that " the e.r.p.is a function of the type of radiation of the pH of the 0 2 content, etc." The interesting observations of Dr. Wright on the radiochemical reactions taking place in the pile in the presence of boric acid do not seem to be in contra-diction with our suggestion of a competition of this acid for OH radicals and H202 at least with a-particles. Dr. Wright also found that the oxidation rate of ferrous sulphate in the pile diminishes in the presence of H3B03. If the decrease is more marked in the centre of the pile than in the thermal column it is probably due to the superposition of the action of the y-rays which are more intense in the 14 This Discussion GENERAL DISCUSSION 249 centre on the effect of boric acid.We have shown indeed with Dr. Lefort, that the action of a-rays on various redox systems is due in larger part to H202 acting as an intermediate. Now this reagent is decomposed by y-rays in such a way that the total action of the radiations on the solutes becomes smaller. I think that it is in this sense that one should understand the non-additivity of the a- and y-actions postulated by Wright. The boric acid is not the only " indifferent " compound which modifies the radiochemical yields in aqueous solutions. In addition to the phosphoric acid mentioned by Dr. Conway we observed such effects with Li2SO4 LiC104 MgS04, etc.15 In any case the experimental data on the reactions in the pile are as yet too rudimentary to obtain a satisfactory full understanding.The suggestion of Dr. Bartindale of the formation of H- by decomposition of H20- has already been taken into consideration by Magat and myself in our note quoted in my paper. For energetical considerations we prefer eqn. (2) leading to the formation of 0-. I must however acknowledge that some aspects of the present Discussion seem to pose the question is the activation energy correctly treated in radiation chemistry ? One has the impression that something fundamental concerning perhaps the excited state of the radicals ions and mole-cules in the radiation field still escapes us and this " something " could account, at least partly for the numerous disagreements which have become obvious at this Discussion. For example in spite of the activation energy necessary for the combination OH + OH = H202 which Dr.Weiss insists on the majority of radiochemists consider that the formation of hydrogen peroxide in solution takes place in this way. On the other hand even after the instructive experiments of Stein and Day, and of Collinson on dyestuffs the energetic conditions and the mechanism of the radiochemical transformations of these substances are as yet far from clear. It is therefore also difficult to understand the role of benzoic acid ip Stein's experi-ments. I have already noted that the interpretation of the actions of " protectors " or " accelerators " is very delicate since the final and intermediate products can also participate in the competition for radicals. Finally in spite of the relatively high activation energy of the process H2 + OH, pointed out by Dainton many authors agree that this reaction plays an important role in radiation chemistry of aqueous solutions especially in all the cases where the addition of hydrogen modifies the yields.The high activation energy calculated by Dr. Dainton for the reaction (16)' which is admitted by Magat and myself as following the reaction (15) would be another difficulty of this type. On the con-trary our hypothesis of the direct formation of Hz does not imply that a thermal photochemical ekctron transfer between one ion and a water molecule follows the same mechanism as the decomposition of a free H20- ion. One can conceive that the decomposition of a complex configuration 16 such as Me2+ . H20- gives MeOH2+ 4 H even if the free ion H20- is decomposed into 0- and H2.Only new and probably extensive experiments will allow a choice between the various possible primary reactions. Our suggestion was made principally on the basis of two essential arguments : (i) high radiochemical yields of hydrogen ; (ii) predominance of oxidation reactions and the apparent inertia of the hydrogen. While the high hydrogen yield measured by Lefort and myself for ferrous sulphate was confirmed recently by Rigg Stein and Weiss much lower Hz yields are indicated by various authors for the Ce'" reduction. We shall re-investigate these experiments in order to discover the origin of the disagreement. We shall also examine again the effect of the initial presence of H2 and 0 2 on the rate of reduction of CeIV since our results are in this case also contrary to those of Hardwick.On the other hand the apparent inertia 15 Haissinsky and Pucheault J. Chim. Physique 1952 49 294. 16 See Farkas and Farkas Trans. Faraday Soc. 1938,37 11 13 250 GENERAL DISCUSSION of the hydrogen has so far not found any other satisfactory explanation although it is one of the fundamental characteristics of the radiation chemistry of aqueous solutions. Dr. B. E. Conway (Chester Beatty Res. Inst. S. W.3) said In connection with Dr. Haissinsky’s observation that apparently indifferent substances such as H3B03 diminish the H202 yield in the irradiation of aqueous solutions of a-par-ticles it may be of interest to record a possibly related phenomenon. In some studies with Dr. J. A. V.Butler on radiation chemistry of sodium deoxyribonucleic acid we have investigated the effect of photochemically produced OH (and HO2) radicals from dilute aqueous H202 on mono- di- and tri-ethylphosphate esters as model substances. The U.V. irradiation itself causes no degradation of the esters but the photochemically produced radicals cause fission of the ester bond giving free inorganic Po&. At the same time the efficiency of the photochemical decomposition of H202 is increased for the mono- and di-ethyl esters which are present as anions in the aqueous solution whilst for the neutral tri-ethylphosphate the H202 is less readily decomposed. The effect is magnified if higher concen-trations of the mono- and di-esters are used and may be due to an interference with the secondary chain processes which can occur after the primary photochemical fission viz.: H202 + OH -+ H20 + H02 H02 + 02- + H+ H202 + 02- -+ 0 2 + OH- + OH; these reactions can also occur in the radiochemical production of H202. In view of the facility with which PO$ can be accurately determined the use of a simple phosphate ester in radiation dosimetry might be suggested. Dr. J. Weiss (Durham University) (communicated) I agree with Dr. Matheson that more experiments are required to settle the dependence on the hydrogen peroxide concentration in the photochemical decomposition. However I cannot agree with him on the interpretation of some points in my paper. In the latter I have made an attempt to treat the general case of the non-stationary reaction by introducing certain simplifications and approximations as stated in my paper.It is obvious that while a linear term could not appear under stationary state con-ditions (cf. the treatment of the photochemical decomposition) it will always appear in a general treatment of the non-stationary state. Therefore Dr. Matheson’s criticism on this point is not justified. On the other hand as I have shown in my paper the dependence on [H202]* does not follow as an “ of course ” matter but holds only under certain specific assumptions the nature of which has been clearly stated. My introductory remarks are therefore not in any way a modification of the views expressed in the paper but rather a restatement of the physical conditions corresponding to the more mathematical treatment given in the paper.I think one would certainly agree with Dr. Matheson that the ‘‘ track ” con-cept in a narrow sense cannot be applied to cobalt 60 y-rays. In fact all that matters is that the formation of radicals occurs in discreet “ clusters ”. On the other hand it is doubtful whether any conclusions can be drawn from any very detailed picture about the distribution and size of clusters etc. as presented by Dr. Matheson. All I think that can be decided is whether the interaction of the radicals (i.e. chain-breaking) occurs (i) in or among the clusters created by the same particle (ii) between the radicals in clusters created by diferent ionizing particles or (iii) whether both is the case. In this way the experimental facts give a perfectly clear decision because in as much as the overall decomposition depends on the square root of the dose rate it is clear there must be an appreciable inter-action of radicals from clusters generated by different ionizing particles.How-ever in order to explain the [H202]* dependence one has to assume that one ha GENERAL DISCUSSION 251 not only interaction between radicals created by different electrons but also inter-action between radicals created by the same electron. This assumption may not be justified but all that I intended to demonstrate was that this assumption is necessary to explain the experimental facts on the basis of this theory. However, the very detailed picture regarding the clusters such as presented by Dr. Matheson, does not seem to me to have any real physical basis apart from the fact that efficiency factors of 10-3 or even very much lower are quite common in chemical kinetics.Dr. E. Collinson and Prof. F. S. Dainton (Leeds University) said Dr. Amphlett's paper illustrates very clearly the way in which the reduction potential of a couple is important in determining the final composition of the mixture obtained when an aqueous solution of the simple reducing or oxidizing solute comprising the couple is subjected to prolonged irradiation. A short time ago 17 we suggested that it might be possible to assign to irradiated water a value of a reduction poten-tial designated the " equivalent reduction potential = e.r.p.," which was a function of the type of radiation used the pH and the oxygen content of the water etc.The reducing partner of a couple of reduction potential more positive (U.S. con-vention) than the e.r.p. by at least 0-2V would within the limits of analytical measurement always be completely oxidized whereas the oxidizing partner of couples of E" value more negative by at least 0-2 V would be reduced and system lying within this range e.r.p. f 0.2 V might attain a radiation " equilibrium " capable of approach from either side. The utility of such a concept if it proves to be valid is twofold firstly as a convenient shorthand representation of the net oxidizing power of irradiated water and secondly in providing a criterion by which any theory of the action of ionizing radiation on water may be tested. As an example of the former we would cite the prediction that Tl3+ solutions would be completely reduced to T1+ inert to radiation since Eo at pHo in perchloric acid for the thallous-hallic couple is - 1-25 V.Dr. T. J. Hardwick has in fact told us that his preliminary experiments show this to be the case. The purpose of the following paragraphs is to clarify certain points which could not be discussed within the limited scope of a review article. Imagine a de-aerated aqueous system in which H atoms and OH radicals are continuously generated at equal rates containing a solute in two states of oxidation represented for convenience by the cationic forms M+ and M2+. If the only reactions taking place are OH + M+ -f OH- + M++ (1) and H + M++ + H+ + Mf (2) a stationary state will ultimately be reached given by kl [OHls[M+ls = k2[HIs[M2+1 (3) where the subscript s denotes the concentration of the species at the stationary state.This will be the true thermodynamic equilibrium if in which E& is the standard reversible reduction potential of the M+/M2+ couple and no is the standard reversible reduction potential of the system H(aq) + OHG~) + H Z ~ + OH(,@ + 2. (5) From (3) and (4) the condition for attainment of true equilibrium is 17 Dainton and Collinson Ann. Rev. Physic. Chem. 1952 2 99 252 GENERAL DISCUSSION and this condition will be satisfied when the M+/Mz+ couple is thermodynamically reversible to H and OH. This situation is most likely to occur when the processes concerned are simple electron transfers of the type instanced but in other oxida-tion systems which involve addition and/or subtraction of atoms or groups of atoms, e.g.NO2-/NO3- and Mn2+/Mn04- eqn. (6) may not hold. Therefore even on the basis of the simplest model we would be surprised if all solutes gave the same value of the e.r.p. The species H and OH may enter into other reactions besides (1) and (2) above. The recombination reaction (7) H + OH -+ H20 (7) will not influence the position of the stationary state because it involves the removal of H and OH in a symmetrical way. The effect of reaction (7) competing with (1) and (2) to different extents as the nature of the solute is changed will merely alter the G values for the approach to the stationary state from either side. A correlation between the reduction potential of the solute and the yield is therefore very unlikely.The combination reactions of the radicals 2H0 3 H202 (8) 2H + H2 (9) may have an important effect on the position of the stationary state. will the stationary state be identical with the state obtained from (1) and (2) alone. In solutions where the total solute concentration ([M+Is + [M2+]J is very low the final values of [MfIs/[M2+ls as well as of the yields may therefore differ appreci-ably from those in which the concentration is high. For the same reason some systems might manifest a dependence of the stationary state on the dose rate. A further complication in dilute solutions is that the hydrogen peroxide produced in reaction (8) may cause some slight oxidation (e.g. of Fez+) or reduction (e.g. of CeXv or MnO4-). Assuming that the conditions discussed above are sufficiently closely approached by a range of solutes for the e.r.p.to be independent of the precise chemical nature of the solute we may now consider the effect of other experimental variables such as pH radiation quality and aeration. Owing to the appearance of different chemical species at different pH's a change of pH will generally modify the reduc-tion potential of the solute couple (E"). Assuming the effect of pH on no to be inappreciable the effect of pH on 7~ will be given by the term containing [OH-] and [H+] in eqn. (4). If E" and 7~ are altered unequally by pH the stationary state will also be sensitive to pH. The values of the reduction potential for the reaction Only if kl COHI,[M+I B ~ 8 ~ ~ ~ 1 s z k 2 ~ ~ i s ~ ~ 2 + i .+ k9w]s2 calculated from thermodynamic data are - 0.37 3- 0.05 and $. 0.46 V at pH 0, 7 and 14 respectively and it would be expected that the values of n- whilst not identical with these would vary with pH in a similar way. Generally E" also becomes more positive with increase in pH but the magnitude of the effect varies considerably from solute to solute. The effects of change of pH on the radiation equilibrium of different solutes may therefore differ not only in magnitude but also in direction. Two examples will suffice to illustrate this. The cerous-ceric potential changes from - 1.44 V at pH 0 to + 0.77 V at pH 14 which is much larger than the expected change of n-. Accordingly reduction is observed in acid media and oxidation in alkaline media.18 On the other hand Amphlett's data indicate that in the pH range 0 to 3 the variation of T is greater than the variation in E" and increase of pH therefore favours reduction.18 Haissinsky Lefort and Le Bail J. Chim. Phys. 1951,48,208 GENERAL DISCUSSION 253 In so far as the type of radiation controls the spatial distribution of the radicals and therefore the [OH]/[H] ratio it will be a factor in determining the radiation equilibrium. On these grounds kr.p. a might be appreciably more negative than e.r.p. x or ,, and the limited data available show this to be true. However it should also be pointed out that this shift in e.r.p. could equally well be attributed to pro-duction by a-rays of a higher percentage of a more oxidizing entity than would be produced by X- or y-rays.In aerated solutions reaction (1 1) may occur H + 0 2 e H O 2 (1 1) and if this competes favourably with reactions (2) and (7) the initial yield of oxida-tion will be increased and the position of equilibrium shifted to the oxidation side. Thus the stationary state finally reached will also depend on the competition between reactions (2) and (1 l) and since k2 is likely to differ from solute to solute, it is probable that the displacement of the e.r.p. brought about by aeration will vary from solute to solute. It is also possible that k2 is so large that the displace-ment is quite small and reduction of the same solute may occur in both aerated and de-aerated solutions. Dr. J. H. BaxendaIe (Manchester University) said Dr. Amphett’s paper draws attention to an important feature of the oxidation of aerated ferrous ion solutions by ionizing radiation which still requires clarification.This is the decrease in yield as the acidity is decreased below pH 2. If as is commonly supposed H02 and OH are the only reactive species then it is probable that the reactions shown to occur in the Fez+ + Fe3+ + H202 system 19 should be sufficient to explain the observations. The back reaction Fe3+ + 0 2 - + Fe2+ + 0 2 has been invoked by Rigg et al. but as Amphlett points out the decrease is apparent in the initial yields i.e. when ferric ion is absent a point which would not show up when working at constant dose as was done by Rigg et al. Amphlett suggests that in the higher alkalinities HO2 exists as 02- which is not able to oxidize Fe2f but if this is the case alternative reactions for 0 2 - must be sought.Possible ones are : 0 2 - + 02- (or H02) -+ 0 2 + 022- (or H023 02- + OH -+ 0 2 + OH-. However these would lead to a dependence of the yield on [Fe2+] and dose rate, which is not observed. There is one aspect in which the radiation system may differ from the H202 system viz. the possible existence of a non-homogeneous distribution of radicals in the former. If radicals are produced in isolated regions of high local concen-tration then in order to estimate the importance of the Fe3+ + 02- reaction the relative concentrations of Fe2f and Fe3+ in each of these regions must be con-sidered. Thus if the H02 concentration is sufficiently high it may arise that in at least part of each of such regions the local Fe3+/Fe2+ ratio is high enough to make this back reaction important although when the ratio is averaged over the whole solution it would appear negligible.On this picture one would expect a decreased yield even initially and a more detailed treatment of this aspect of the ferrous ion oxidation might lead to useful information on the distribution of radicals in aqueous systems which is an outstanding problem at present. The anomalous results given by perchlorate acid solutions reported by Amphlett and others also emphasi7e the difference between the radiation and Fe2+ + Fe3+ + H 2 0 2 systems. In our extensive work on the latter 19 no analogous observations were made although the rate of OH radical production was just as high as in the 19 Barb Baxendale George and Hargrave Trans.Faraday Soc. 1951 47 608 254 GENERAL DISCUSSION radiation system. This difference may arise either from the presence of H atoms in the latter or again from high local concentrations of radicals assuming of course that the reactive entities are the same in both cases. Observations using complex ions with organic ligands should be interpreted with some caution. We have found 20 in an investigation of the Fez+ + Fe3+ + H202 system in the presence of ax'-dipyridyl that extensive oxidation of dipyridyl occurs even when it is combined as Fe(dipy)y. Dr. C. B. Amphlett (A.E.R.E. Harwell) (communicated) Dr. Collinson and Prof. Dainton have given an admirable summary of the ideas leading up to the concept of the '' equivalent reduction potential " together with some of the qualitative trends which might be expected under varying conditions.The general scheme outlined by them requires some modification if we have to allow for the introduction of species other than H and OH e.g. H02 H2+ OH+ etc. as they have already indicated; it will also require modification if the primary radicals are generated at unequal rates as in the primary steps proposed by Haissinsky and Magat,21 and by Lefort.22 The participation of species such as H02 and H2+ would lead to additional sources of pH-dependence with respect to suitable systems. In cases where H202 is able to react with one of the oxidation states of the couple its perturbing influence may be quite large because of its high potential oxidizing or reducing power; in particular it may partake to a very important extent in ferrous solutions at low concentrations (< 10-4 M) in the absence of oxygen, where the OH radicals are not efficiently utilized.All these factors will help to make the precise location of e.r.p. H20 difficult to achieve even for simple couples of the type Mn+/M(n + I)+ Apart from these factors the effect of pH upon the final steady-state may vary for a given couple as a function of the anion present if 7~ varies in the same manner irrespective of the anion then the steady-state will be determined by the value of EM (or E'o as we have expressed it in our paper). Measurements of the system Fe2+/Fe3+ in HClO4 and in H2SO4 solutions as a function of pH have shown23 that apart from the difference in the standard potential in these two solutions there is also a qualitative difference in the pH-dependence in the two cases.Sulphate solutions (1.5 x 10-4 M in total Fe) show an increase in potential from - 0.691 V in 0-5 M acid to - - 0.68 V at pH 1 followed by a decrease to a minimum of - 0.708 V at pH 2.5 after which the potential becomes more positive again. Perchlorate solutions on the other hand are independent of pH between 1 M acid and pH 2 (E'o = - 0.732 to - 0.735 V) after which the potential rises steeply and at pH 3 is - - 0.635 V. The behaviour in HC104 solutions is considered to be representative of hydrolysis of Fe3+ to FeOH2+ while the more complex behaviour in H2SO4 solutions presumably reflects the competition for FeI" between hydrolysis and sulphate complexing.It is hoped to proceed further with this work when a reliable value for the sulphate complexing constant is available. Since the steady-state ratio is dependent upon the difference (E - n) we might expect different values for sulphate and perchlorate solutions if the analytical methods available could be made sufficiently sensitive ; it should be noted however that T may vary if H2SO4 is replaced by HC104 since the measure-ments of initial yield indicate an increased oxidizing power in HC104 solutions below pH 2. Although it is true that in general we would not expect any correlation between reduction potential of the solute and the initial yield a correlation may arise in related systems where the kinetics are very similar e.g.in the ferrous-ferric system, whereas if the pH is varied we obtain a parallel effect upon initial ferrous oxidation 20 Barb Baxendale George and Hargrave (unpublished). 21 Haissinsky and Magat Compt. rend. 1951 233 954. 22 Lefort Compt. rend. 1951 233 1194. 23 Amphlett and (Mrs.) Davidge unpublished work. a GENERAL DISCUSSION 255 rate initial ferric reduction rate (in the reverse sense) and a final steady-state ferrous/ferric ratio. In conclusion we see the need for a systematic study of redox systems to cover all these aspects. As we have stated elsewhere,24 the action of X-rays on perchloric acid in aqueous solution leads, in general to the formation of chlorate and oxygen while in the presence of ferrous salt instead of the former an equivalent amount of chloride is formed.It is how-ever not surprising that no interference by perchlorate is observed under ordinary chemical conditions because we have found that the decomposition of the per-chlorate is proportional to its concentration,25 which strongly suggests there is a direct action of the radiation on the perchlorate. In the experiments 24 regarding the oxidation of ferrous sulphate by X-rays in the presence of air pH N 2.5 was the highest pH used. We have ascertained that up to this pH and under the conditions of our experiments there is no initial pH effect. Dr. H. A. Dewhurst (Edinburgh University) (communicated) I would like to describe a few experimental results on the effect of sulphuric acid concentration on the oxidation of ferrous sulphate by C060 y-rays in aerated and air-free solution which are relevant to Dr.Amphlett’s paper. The main results are summarized in the accompanying table. Dr. J. Weiss (Durham University Newcastle) (communicated) [Fez+] = 5 x 10-4 M dose rate = 1-1 x 1017 eV/ml h aerated air-free -__ H2S04 mole/l. Gi Fez+ oxid/100 eV 1.0 20-2 0.4 20.2 5 x 10-1 20.0 5 x 10-2 18-4 8 x 10-4 15.0 5 x 10-4 14.0 HzS04 mole/l. ‘i Fez+ oxid/100 eV 1.0 8-2 0-4 8-2 5 x 10-2 7.6 5 x 10-3 6.1 8 x 10-4 3-2 5 x 10-4 2.0 It is evident from a comparison of these results that the aerated and air-free systems exhibit a somewhat similar dependence on acid concentration. In both systems in the region of acid dependence the oxidation was found to be first order with respect to the ferrous ion concentration.Contrary to Dr. Amphlett’s results 1 have found neither an equilibrium concentration of Fe3f ions nor reduc-tion of Fe3f ions in either aerated or air-free solutions over the range of acid con-centration studied. In the region of acid dependence however the addition of Fe3+ ions to the ferrous solution decreased the initial yield. It was also found that the oxidation curve from any time t could be reproduced only with solutions of the same ferrous and ferric concentrations as at time t. We have repeatedly found that low concentrations of chloride ion have no effect on the oxidation yield in aerated 0-8 N N2SO4 solution and that even at 10-2 M chloride ion the initial yield decreases by only 5 % ; while we have also found that 10-3 M chloride ion has no effect on the oxidation yield in air-free 0.8N H$O4 solution.The results in aerated solution have been confirmed by Dr. E. J. H a r t 3 We have recently had an opportunity to test the effect of chloride ions on aerated ferrous sulphate solutions prepared by Dr. T. J. Hardwick and by Dr. A. 0. Allen and again found no difference in yield. Dr. C. B. Amphlett (A.E.R.E. Harwell) said The initial yields in aerated solu-tions quoted by Dr. Dewhurst are in quite good agreement with those given in my paper. It is interesting to note that the difference in yield between aerated 24 Rigg Stein and Weiss Proc. Roy. SOC. A 1952 211 375. 25 Milling Stein and Weiss to be published. 26 Hart private communication 256 GENERAL DISCUSSION and air-free solutions increases as the pH is increased a fact which is also evident from the earlier work of Fricke and Hart; 27 this may be significant in an inter-pretation of the results since it implies a more effectively pH-dependent mechan-ism in air-free solutions.It would be preferable to obtain air-free yields at rather higher concentrations where the yield is a maximum at high acidities,28 but I am well aware of the difficulties involved at high pH under such conditions. It is difficult to understand why no steady state has been observed at high pH, nor reduction of ferric ion The effect produced by addition of ferric ion which has also been observed by Rigg Stein and Weiss,28 implies competition between Feat and FelIr for the species formed on irradiation and it is difficult to see what can happen to Fell1 other than reduction.The necessity for ferric ions as well as ferrous ions being present in order to reproduce an oxidation curie from any given time is also a consequence of this competition and was noted earlier for the Fe2f + H202 system.29 Since my paper was submitted we have obtained further evidence concerning reduction of ferric ion which will be presented elsewhere. The discrepancy between our results with chloride ion and those of other workers may be due to the presence of bromide in our chloride samples which were un-treated A.R. material; the effect of this upon the radicals produced in irradiated water has already been noted.30 I cannot agree with Dr. Uri that the experimental data are insufficient to show a fundamental inadequacy in the earlier kinetic mechanism.It is insufficient to enable us to decide what the inadequacy is but the fact that we are now considering HO2 dissociation and the participation of FeOH2- (and as Dr. Uri rightly points out even higher species) indicates the insufficiency of the simpler treatment. As Dr. Lefort has also pointed out in a private communication the pH dependence in air-free solution must also be explained and this cannot be done either on the basis of the old mechanism or with the aid of HO2 dissociation. Dr. J. H. Baxendale (Manchester University) (communicated) I would like to enlarge upon my suggestion that if the radicals are produced in localized regions of high concentration then the reactions (9 Fe3+ + H02 -2 Fez+ + H+ + 0 2 , k k Fe2+ + HO2 -2 Fe3+ + H02- (ii) can account for the decreased yield of Fez+ oxidation even initially.To illustrate this I will take a somewhat idealized case. Let us assume that the HO- and HOz reactions do not interfere so that we can treat each separately. Suppose further that the rate constants of reactions (i) and (ii) above (k4 and k3) are so large that : (a) all the HO2 present initially in one of these regions reacts by (i) and (ii) before the H202 produced in (ii) can react appreciably with Fe2+; and (b) no diffusion occurs within the time taken for all the H02 to react. Let the pH be about 3 where k4/k3 is approximately unity.31 Then if the initial [Fez+] is a (Fe3+ = 0) and the initial concentration of HO2 produced in the localized volumes is b it can be shown that when all the HO2 has reacted by (i) and (ii) the total amount of Fe2+ oxidized by H02 (and hence H202 formed) is x where x is given by 2bJa = log (a/(a - 2x)).Examination of this expression shows that if the initial H02 concentration b is, say 1 % of the initial ferrous ion concentration a then x will be within 1 % of b, i.e. all the H02 will react with ferrous ion. If however b approaches a in magni-tude or exceeds it only 40-50 % of the H02 reacts with ferrous ion 50 % being 27 Fricke and Hart J. Chent. Physics 1935 3 60. 28 Rigg Stein and Weiss Proc. Roy. SOC. A 1952 211 375. 29 Barb Baxendale George and Hargrave Trans. Faraday Soc. 195 I 47 469. 30 Allen Ghormley Hochanadel and Davis J. Physic. Chem. 1952 56 575.31 Barb Baxendale George and Hargrave Trans. Faraday Soc. 1951,47,608 GENERAL DISCUSSION 257 the maximum attainable amount in these conditions. This would lead to a fall of about 40 % in the overall yield (taking into account HO* and H202) compared with that given in acid solution. Moreover the yield will be independent of ferrous ion concentration up to concentrations of the order of the initial HO2 concentration. It would also be independent of dose rates until the localizeci regions were so numerous as to overlap. This picture is undoubtedly oversimplified but it has the merit of using only established reactions and since if it is applicable it may give an indication of radical concentrations in irradiated aqueous solutions it would appear to justify further examination.Dr. C. B. Amphlett (A.E.R.E. HarweEZ) (communicated) The suggestion made by Dr. Baxendale to explain the dependence of the initial yield of ferrous oxidation upon pH merits further examination. As he has pointed out the mechanism proposed in my paper requires an additional step in order to dispose of the 0 2 -ions and the experimental results appear to rule out the two possibilities which he has considered; however this does not rule out the possibility of another alternative so that in the absence of further evidence this remains an open question. The mechanism proposed by Baxendale involves a non-uniform production of free radicals which could lead to a high ZocaE ferric ion concentration (and hence a back-reaction involving step (5)) under conditions where the overalZ ferric ion concentration was so low as to make it seem impossible for this reaction to contribute appreciably.On the basis of this mechanism as he has shown we can explain a decrease in the initial yield with increasing pH. If then the initial linear portion of the oxidation curve is due to local competition between ferrous and ferric ions for HO2 radicals then we must invoke another back-reaction to explain the subsequent deviation from linearity; this may be due to the Fe3+ + H02- reaction which will only become appreciable at much higher ratios of Fe3+/Fe2+ and at high pH.32 However we would still not expect the system to attain a steady state at reasonably low values of [Fe3+]/[Fe2+] on the basis of these kinetics unless the rate of depletion of ferrous ion within the volume ele-ments was considerably greater than its rate of replenishment (which may be con-sidered as proceeding both by diffusion of ferrous ion into the elements and also, effectively by diffusion of some radicals out of the latter).Otherwise ferrous ions will continue to diffuse into the elements and be oxidized leading to a steady-state characteristic of the balance between ferrous oxidation and the Fe3+ + H02-reaction which would lead to a greater degree of oxidation than that observed. The general problem of the effects of non-uniform radical distribution is sufficiently important to require further expansion in connection with this system. The effects likely to be observed with X- and with y-radiation have only recently received attention chiefly as a result of the complex variation in dose-rate exponent in polymerization reactions.33 It has also been suggested by Magee,34 on theor-etical grounds that the radical distribution is non-uniform in water and aqueous systems at moderate dose-rates although his theory contains several simplifying assumptions.There are no quantitative data on either the fraction of the total volume which is occupied by the elements in which the radicals are formed or as to the magnitudes of the diffusion rates and distances involved. As a result, we must be cautious in attempting to explain too much on the basis of this model. For example if we consider the oxidation of aerated solutions of ferrous ion in 0.8 N HzSO4 up to the oxygen break-point we can on this basis envisage the local depletion of 0 2 in the volume elements before it is depleted in the bulk of the solution provided that the rate of 0 2 consumption IocaIIy (which is dependent on rate of radical production) is greater than the rate of diffusion of 0 2 into the 32 Barb Baxendale George and Hargrave Trans.Faraday Soc. 195 1 47 59 1. 33 Collinson and Dainton this Discussion. 34 Magee J. Amer. Chem. SOC. 1951 73 3270 258 GENERAL DISCUSSION elements from the solution. This should result in a decrease in the stoichiometry A[Fe2+] = 4[02] at the oxygen break-point for dose-rates where elements do not overlap the oxygen concentration being that in the whole solution. So far as is known the above stoichiometry is observed over a wide range of dose-rates, from - 30 r/min to > 3000 r/min,35 the rate of oxidation being linear up to the break-point within experimental accuracy.This suggests either that the volume elements occupy most of the bulk volume in all these cases or that the diffusion rates (of 0 2 into the elements and of H atoms out of them) are comparable with the local rate of 0 2 depletion. A solution to this problem might be obtained from further work over a much wider range of dose-rates than has hitherto been possible. Concerning the reduction of Feph33-t in aqueous solution we have observed 36 that both ph and its conjugate acid phH+ are destroyed upon irradiation but with much lower G values than those for Feph? in a concentration of - 10-4 M. The reduction curve for Fephr upon irradiation is consistent with competition between Feph:' and some other species for the radicals formed (since Fephy is unaffected by prolonged irradiation this cannot be the competitor).However, a tenfold excess of phenanthroline has no detectable effect upon the initial yield of reduction. Thus while agreeing that the results of experiments on such systems should be interpreted with caution we feel that our yields are representative of the reduction of Feph? ions by species formed on irradiation of the solvent. It is hoped to report this work in more detail at a later date. Mr. N . W. Luft (Waltham Abbey) said Although one might agree that under the conditions of the experiments reported to this Discussion the HO+ ion plays no important role views concerning its existence are rather conflicting.The reaction HO+ + HO- -+ H202 need not necessarily impair the oxidizing power of alkaline solutions since on the other hand HO- ions would promote decom-position of H202. In aqueous solutions of various hydroxy compounds the enthalpy of dissocia-tion HOX -+ HO+ + X- is calculated as AH (kcal) = b + 24 b + 8 b - 9 b - 10 b - 12 b - 40 for X = NO2 C103 OH C10 CI F, where b denotes the enthalpy of formation referred to AHf(H+ aq.) = 0 of HO+ in aqueous solution. This is related to the enthalpy change c in the reaction (1) viz. by b = c + 27 (kcal). Here use is made of the known value 37 of AH = 4.2 kcal for the reaction (2) From the bond energies and ionization potentials 38 of individual charge clouds one would expect c to be small.A crude electrostatic estimate gives c = 18 kcal i.e. b = 45 kcal but the true value might be smaller. This shows that HO+ is probably not important for H202 solutions but may be characteristic of aqueous HOF or H20 + F2 which seems to have exceptional kinetic and oxidation properties and might be used as a means of studying the hydrated HO+ ion. Dr. J. Weiss (Durham University Newcastle) (cornrnmicated) A recent calculation of Prof. C. A. Coulson 39 has shown that the OH+ ion is unlikely to be of any importance in aqueous systems because it should be very unstable with regard to the dissociation according to H202 . . . H+ -+ HO+ . . . OH2 AH = C, H202 . . . H+ + H2O -+ H202 + H20 . . . H+. OH&&. + 0 + Htydr., 35 Fricke and Morse PJiil.Mag. 1929 7 129. 36 Amphlett unpublished work. 37 Evans and Uri Trans. Faraday SOC. 1945 45 224. 38 Mulliken J. Chem. Physics 1935 3 506. Miller J. Chem. Physics 1950,18,79. Wright this Discussion. Rigg Stein and Weiss Pror. Roy. Soc. A 1952 211 375. 39 private communication GENERAL DISCUSSION 259 one of the reasons being that the heat of hydration of the OH+ is presumably very much smaller than that of the proton. This is due to the relatively very small residual positive charge in OH+ caused by the shielding of the proton by the electronic cloud of the oxygen. The situation is quite different with H2+ where one has a relatively large residual charge because there is practically no electronic charge on the remote side of the protons as pointed out by Prof.Coulson elsewhere.40 Furthermore the binding energy of Hz+ which is about 61 kcal/mole also acts in a way as to oppose the dissociation into H f Hi. Dr. N. Uri (University of Chicago) (communicated) I would like to make a few suggestions concerning the mechanism put forward by Garrison and Rollefson in the radiation induced oxidation of ferrous ion in the presence of carbon dioxide. The formation of hydrogen is postulated to occur via the reaction H + H +H2. I consider that the reaction Fe2+(HOH) + H -+ Fe3+OH- + H2 is more likely to occur than the recombination of H atoms present at small concentrations. The reaction which I quoted is exothermic to an extent of 25 kcal. Furthermore, formation of formic acid is more likely to occur via the electron transfer reaction HC02 + Fez+ + Fe3f + HCOO- than by a combination of H atoms and HC02 radicals.Is it not feasible that at larger ferric ion concentrations the formation of formic acid is suppressed by the reaction Fe3f + HC02 +- Fe2+ + H+ + C02, or possibly Fe3+ + C02- -+ Fe2t + C02 if an ionic dissociation of HC02 can be assumed.;! I wonder whether Garrison and Rollefson have any information on the effect of changes in the pH and whether they have tried to evaluate the electron affinity of carbon dioxide or the heat of formation of the HC02 radical. I feel that Amphlett is to be congratulated on having introduced energetic con-siderations into radiation chemistry which previously were insufficiently applied. In a reviewing article I have also pointed out the fact that the ionic dissociation of the H02 radical in aqueous solution has been hitherto completely neglected in radiation chemistry.The same applies to the significance of ion pairs in the reaction kinetics and on this point I would like to remark at a pH above 2-3 not only Fe3+OH- but also higher ion pairs such as Fe3+(OH)22- have been taken into consideration. As our knowledge on these higher ion pairs is still limited I would be reluctant to base any mechanism on experimental data obtained on the basis of experiments carried out at a pH of say 3.8. The published results do not appear to be sufficient for the claim that a fundamental inadequacy of the kinetics has been exposed. Dr. M. Haissinsky (Labordoire Curie Paris) said It seems difficult to under-stand why the reduction of C02 by H atoms is brought about in aqueous solution more easily than in a gaseous phase and that no activation energy is necessary.One would think that the reaction is produced by OH radicals which react primarily with HCO3- ions to give percarbonic free radicals similar to the perboric radicals, 0. HC03- + OH = C/-\;HO + OH-The percarbonic radical reacts then with another OH, This hypothesis implies that 0 2 in a quantity equivalent to HCOOH is formed. In presence of H2 formation of H atoms occurs (H2 + OH = H + H20) and leads to the production of oxalic acid : HCOOH + H = COOH + H2 C020H + OH = HCOOH + 0 2 . or C020H + H = COOH + OH, followed by COOH t COOH COOH - COOH. 40 cf. Rigg Stein and Weiss Proc. Roy. SOC. A 1952 211 375 260 GENERAL DISCUSSION A similar mechanism for the decomposition of HCOOH by OH radicals (work of Hart) can be formulated.Dr. W. M. Garrison and Prof. G. K. Rollefson (University of Culijbniu) (cum-municated) The fact that carbon dioxide and hydrogen atoms do not react readily in the gas phase does not mean that hydrogen atoms are not involved in the formation of formic acid from carbon dioxide in aqueous solution. As we suggested in our paper the reactive species of dissolved carbon dioxide could be carbonic acid. If the probable assumption is made that reactions (2) (3) occur without appreciable or about the same activation energy then in order to reach a stationary concentration of formic acid as low as that observed in our experiments the reactive form of carbon dioxide must be at a concentration comparable to that of the formic acid.Inspection of our data will show that this is consistent with the assumption that carbonic acid is the reactive species since it is known thak about 1 % of dissolved carbon dioxide is present as the hydrate. The hypothesis that the reduction of carbon dioxide observed in our experiments occurs via a reaction involving hydroxyl radicals implies that hydroxyl radicals produced in the 1 M ferrous sulphate solutions are not reduced by Fez+ but instead react preferentially with bicarbonate which is at a concentration of less than 10-5 M. It seems more logical to us to assume that all the hydroxyl radicals are reduced by Fe2+. Dr. J. Weiss (Durham University Newcastle) said Although it has been possible only to give a very rough calculation in my paper nevertheless under certain assumptions a clear and definite physical picture emerges.The fact that the rate of the decomposition is proportional to the square root of the dose rate shows clearly that the only important part of the decomposition reaction is the chain reaction which takes place after the overlapping of the tracks. The question is now-what determines the concentration of the active radicals in this chain re-action which proceeds after the overlapping of the tracks? Clearly this must depend on the processes within the tracks before overlapping of the tracks has taken place. In the tracks (or clusters) the OH radicals are generated according to the eqn. (9) H20 -+ H + OH (and (lo) H202 + H-t H20 - OH).The OH radicals thus formed can now attack the hydrogen peroxide according to the re-action (2) H202 + OH -+ H20 + H02 which is known to be a fast reaction. However at this stage the H02 radicals thus formed apparently do not enter into the second (chain) reaction (reaction (3)) which in fact is known to be a relatively slow process. This is also borne out by the experimental facts which show that under these conditions i.e. within the tracks reaction (3) evidently does not occur to any appreciable extent. In the tracks the H02 radical concentration is appar-ently relatively high and the second chain reaction may not be able to compete with the fast reaction (7) 2 H02 -+ H 2 0 2 + 0 2 . Thus reactions (2) and (7) may lead in the first instance to a nun-chain decomposition of the hydrogen per-oxide within the tracks which may contribute only very little towards the total decomposition.The concentration of the radicals (n) for this process can be calculated to a first approximation by eqn. (IV) given in mq paper where the linear term corresponds to reaction (3) and the quadratic term corresponds to reaction (7). In this simplified equation n represents both the OH radicals and the H02 radicals because as was pointed out in the paper the concentration of the OH radicals is assumed to be proportional to the concentration of the H02 radicals. Now after the overlapping of the tracks the actual chain decomposition sets in and for this reaction the (stationary) concentration of the radicals (n,) is given by the usual type of equation i.e.eqn. (XVIIu) in the paper which can be written simply as (XVIlIa') I N - k'ns2 = 0 i.e. for this chain decomposition the linear term now disappears and one is left only with the bimolecular chain-breaking. In this equation IN is the number of radicals produced per unit of time and volume GENERAL DISCUSSION 261 which is relevant for the intertrack chain reaction. As has been shown in greater detail in the paper as a result of the non-chain process (before the overlap) IN may be given by a relation of the form IN K (dose rate) [H202]-*. Thus from eqn. (XVIIla’) it follows that n K (dose rate)* [H202]-* and the rate of the chain decomposition is then given by rate cc [H202] . ns K (dose rate)* [H202]*. Dr. W. G. Barb (Cuurtaulds Ltrt.Maidenhead) said In table 1 of his paper, Dr. Weiss has listed the kinetic equations to be expected for different termination steps. However there appears to be an error in the rate expression for termina-tion by OH + H02. Dr. Weiss gives this as - d[HzOz]/dt = 21 + 2(k2k3KH02/k5)’ (L/[HTI)’W2021* (i, - d[HzOz]/dt = la + (I2 + 4(k2k&02/k5) (lu/[Hfl)[H202l2)* (ii> If we are dealing with fairly long chains then by making the approximation (x2 + l)$ [N (x + 1 / 2 x ) ] ~ x for large values of x we obtain from (ii), - d[H202]/dt = la + 2(k2k3K~0~/k5)~ (la/[H+I)* [HzOZI. (iii) Ths differs in the first term from eqn. (i). Eqn. (i) could in fact only be obtained from (ii) by the unjustifiable approximation (x2 4- l)* si (x + 1). With reference to Hart and Matheson’s paper it is somewhat difficult to accept termination involving a hydrogen-bonded H02 + H202 complex because (i) it is not clear why such a complex should show enhanced reactivity over (ii) in dilute H202 only a small fraction of the total H02 would be hydrogen-bonded to H202 unless hydrogen-bonding between HO2 + H202 is taken to be considerably more probable than that between H02 + H20.On the other hand it must be admitted that a dependence of rate of reaction on [H202]* seems very difficult to obtain with other mechanisms. I would there-fore ask Dr. Hart whether he thinks the [Hz02]* dependence could possibly be in fact some other more complex dependence intermediate between ~ e r o and first order which might simulate a square-root dependence over a limited range of [H202].Such a relation could perhaps be obtained with the more usual second-order termination reactions. In connection with Ebert and Boag’s paper it seems worth while pointing out that conductivity is not a reliable measure of water purity where traces of organic matter which might react with radicals are concerned ; even reproducibility of results after further purification is not always a reliable guide in this matter.41 I would propose that where doubt exists as to the interference of organic impurities (and this is not to imply that such is necessarily the case in Dr. Ebert’s work) the only reliable measure of these impurities is that employed by Fricke and Hart,42. 43 i.e. irradiating a purified evacuated sample of water and determining the carbon dioxide formed.It may also be that pre-irradiation followed by heating or by treatment with ultra-violet light of a suitable wavelength to destroy any H202, would prove to be a satisfactory method of water purification. Dr. J. Weiss (Durham University Newcastle) said Although the expression now given by Dr. Barb only differs by a factor of 2 in the first term from the equa-tion given in my paper in table l (line l) nevertheless I must insist that my expres-sion as given there is correct under the approximations which I have made which are concerned entirely with the physical nature of the process and do not involve In fact the stationary state treatment applied to Dr. Weiss’ scheme gives HO2 hydrogen-bonded to water ; 41 Barb Baxendale George and Hargrave Trans.Faraday Soc. 1951 47,462. 42 Fricke and Hart J. Chem. Physics. 1936 4 418. 43 Fricke Hart and Smith J. Chem. Physics 1938 6 229 262 GENERAL DISCUSSION any such mathematical approximations as Dr. Barb suggests. This can be shown very simply as follows for the stationary state one obtains (i) (ii) which leads to l a = kdH02I[OHl (iii) and by introducing this into eqn. (ii) and assuming that k2[H202] > k5 [H02] (i.e. approximation for long chains) one obtains d(OH)/dt = 21a - k2[H202][OH] + k3[H202][02-] - k~[H021[OH] = 0 d(H02)W = k21H2021 [OH1 - k3 [H2021 [%-I - kdH021 [OH1 and thus finally with eqn. (iii) and (iv) : which is identical with the equation given in my paper. The reason why I have given this particular approximation is that it leads to the correct approximation for long chains and also to the correct minimum value for the quantum yield for the non-chain process (ymin = 2).Dr. W. G. Barb (Courtaulds Ltd. Maidenhead) (communicated) Dr. Weiss apparently accepts my exact solution (eqn. (ii) in my note) and now seeks to justify his original expression (eqn. (i) in my note) as an approximation. I must therefore point out : (i) No mention of any approximation was made in Dr. Weiss' paper. (ii) I have already shown that eqn. (i) is not a suitable approximation. (iii) Dr. Weiss states that he gave this particular approximation because it gives correct values for the two limiting conditions. In fact the number of otherwise unsatisfactory approximations which can be written solely to satisfy this limiting criterion is easily shown to be infinite.(iv) The fallacy in Dr. Weiss' new derivation is manifest. It involves the successive stages (a) k~[OH][H021 is negligible with respect to k2[OH][H202], (6) k2[OHl[H2021= k3[02-"2021, (c) k5[OH][HO2] is not neghgible with respect to k3[02-][H202]. Prof. W. Mund (Louvain University Belgium) said Dr. J. Weiss has developed theoretically the possible consequences of the heterogeneous distribution of the chain-initiating elementary processes along the path of a particle or a photon. Considering both cases of reactions confined to what may be called an isolated track and of a reaction site expanding from the initial track so as to bring about a general overlapping he seems to assume that in the second case the reactions between radicals or activated molecules originating from different tracks will modify the kinetics of the overall reaction.These theoretical expectations should clearly apply to the irradiation by x-rays where the initial heterogeneity is very pronounced indeed. Now I wish to point out that at least in one typical instance there do exist experimental features of the chain termination which rule out any effect of the overlapping of broadening tracks. The experiments to which I refer could not have been known to Dr. Weiss as they are only about to be published. They are described in a paper by Dr. van Meerssche M. Monigny and myself which is now ready but was not sent in time to be submitted to this Discussion. In brief the polymerization rat GENERAL DISCUSSION 263 of vinyl chloride under the action of the x-rays from a small amount of radon incorporated in the gas phase has been investigated at constant pressure.At four different pressures ranging from 20 to 70 cm Hg the ionic yield was found to be strictly independent of the intensity of irradiation which after one week was reduced to a quarter of its initial value. This points to a purely monomolecular mechanism of chain termination. On the other hand there is strong evidence that at the lower pressures and in smaller vessels an important fraction of the chains is only broken at the walls. The complementary fraction predominant at higher pressures or in larger vessels terminates in the gas phase but only by some monomolecular destruction of the growing radicals.Otherwise an effect proportional to the square root of the amount of radon left in the bulb should manifest itself. Dr. J. Weiss (Durham University Newcastle) said I was very interested in Prof. Mund's results although they refer to a gaseous system and I have only dealt with systems in solutions. I think it follows also from my paper that with first order termination reactions the yield will in every case be independent of the dose rate. This in fact corresponds exactly to one of the cases which is given also in table 1 of my paper where in the corresponding case for the photochemical reaction (table 1 reaction (6)) the quantum yield is independent of the intensity. Dr. M. Lefort (Institut du Radium Paris) said It has not yet been definitely settled which of the two reactions OH + OH = H202 (1) or OH + OH = H20 + 0 O + O = 0 2 is the more probable after irradiation of liquid water by a- X- or y-rays.How-ever there is both theoretical 44 and experimental 45 evidence that in most cases OH radicals have enough activation energy to react through (l) since this reaction is very exothermic. The results of Gunther and Holzapfel in 1939 46 mentioned by Dr. Allen seem to indicate again that (1) is the important reaction. Irradiating carefully de-aerated pure water these authors found a continuous evolution of hydrogen. It is, incidentally six times higher than the constant molecular hydrogen yield assumed by Dr. Allen. They did not find any oxygen. The formation of H2 may be due to the escape of this gas into the large volume above the irradiation cell a process which excludes any back reaction in the solution.But at the same trme OH radicals must be available. If they produce H202 it is possible that under these conditions either the H202 is not decomposed or more probably the decomposition occurs without evolution of oxygen according to a chain reaction involving both OH and H radicals From their results a G-value for H2 of 2.3 can be calculated. They will then react according to (1) or (2). H202 + H = H20 + OH. OH + H202 = H02 + H20 HO2 + H = H202. Whatever the mechanism may be the combination OH + OH = H20 + 0, followed by 0 + 0 = 0 2 cannot be taken into consideration because the oxygen formed in such a way should escape as well as hydrogen into the large volume above the irradiated water.Dr. J. Weiss (Durham University Newcastle) said I cannot see any clear experimental evidence for the recombination of the OH radicals to form hydrogen peroxide. As I have already pointed out in my paper in the gaseous state it has 44 Dainton Ann. Reports 1949 45 27. 45 Lefort J . Chim. Phys. 1950 47 785. 46 Gunther and Holzapfel 2. physik. Chem. B 1939,44,374 264 GENERAL DISCUSSION been shown by Bonhoeffer and Pearson that this recombination does not occur and the photochemical work of Lea on hydrogen peroxide also does not support this view. However I think that we have recently been able to obtain some further evidence which again shows that there can be no appreciable recombination of the OH radicals to form hydrogen peroxide.In these experiments which were carried out by Mr. Milling we have measured the yield of the hydrogen peroxide formed in the irradiation of water in the presence of (i) air (ii) oxygen (1 atm), and (iii) in the presence of mixtures of oxygen and hydrogen. In the experiments in the presence of air or oxygen we found that hydrogen peroxide was formed with an initial yield of G(H202) N 2-5. For this we have suggested the following reaction scheme : H20-+H + OH (1) H + 0 2 -+ HO2 (2) 2H02 -+ H202 + 0 2 (3) 20H + H2O + 0 2 0 3 0 2 (4) If in this case hydrogen peroxide was also formed by the recombination of OH radicals and the same experiment is now carried out in the presence of a mixture of oxygen and hydrogen one should certainly not expect an increase in the yield of hydrogen peroxide because when the OH radicals are transformed to a greater or lesser extent into hydrogen atoms according to ( 5 ) there should be a decrease in the initial yield of hydrogen peroxide.However, the experiments with mixtures of hydrogen and oxygen give the seemingly para-doxical result that there is a considerable increase in the initial yield of hydrogen peroxide in the presence of hydrogen. In fact we have found that the maximum initial yield of hydrogen peroxide in mixtures of hydrogen and oxygen can under suitable coiiditions be nearly twice as much as that obtained in the presence of oxygen alone. This shows quite clearly that OH radicals themselves do not recombine to form hydrogen peroxide but when transformed into hydrogen atoms (according to eqn.(5)) they can again form H02 with molecular oxygen which then leads to an increase in the yield of hydrogen peroxide. In this connection I should like to mention that we have come to the con-clusion that the formation of hydrogen peroxide from H02 radicals very likely proceeds by the reaction between HO2 and 02- according to HO2 + 0 2 - + H02- + 0 2 rather than by the interaction of two H02 radicals (reaction (3)). Dr. M. Haissinsky (Paris) said The increase of H202 formation in the experi-ments of Weiss and Milling by adding hydrogen to the oxygen is not a convincing argument for rejecting the generally admitted mechanism OH+ OH= H202. There is good evidence that the measured overall rates of H202 formation are differences between the amounts really formed and those which are destroyed by the action of free radicals namely, Consequently the addition of H2 protects H202 against decomposition by OH ac-cording to the reaction (5) and contributes to its formation according to (2).The last process is of course a second mode of formation of hydrogen peroxide when oxygen is present. Prof. F. S. Dainton and Dr. J. Rowbottom (Leeds University) said Anyone who has worked with solutions of " pure " hydrogen peroxide cannot fail to admire the skill with which Dr. Hart and Dr. Matheson have conducted their investigation. Nevertheless there is one important feature of their results which seems to us to be anomalous when considered in relation to the results of other workers which H2 + OH -+ H2O + H (6) H202 + OH = H20 + HO2; HO2 + OH = H2O + 0 2 GENERAL DISCUSSION 265 is in conflict with our own observations 47 and which obliges the authors to depart from the commonly accepted chain termination step and to propose one in which H202 molecules acting as third bodies must be at least lo4 times as efficient as water molecules.This feature is the proportionality of the decomposition yield to the reciprocal of the square root of the hydrogen peroxide concentration. They claim that this is also a characteristic of the photochemical reaction. In our submission the balance of the photochemical evidence is overwhelmingly in favour of this chain reaction being first order with respect to hydrogen peroxide. Lea's work 48 (quoted by the authors) supports this as also does that of Tian,@ of Henri and Wurmser,50 of Jeu and Alyea,sl of Kornfeld,s' of Taylor and Anderson.53 The only evidence for dependence of the rate on [H202]0.5 is that provided by Dain and Shvartz,54 and to a less degree by Allmand and Style.55 We have ourselves redetermined the kinetics of the decomposition of unbuffered carefully purified, aqueous solutions caused by the mercury 3650A triplet using a sector technique, and find that over the concentration range 1-22 M the rate is proportional to decreases until the reaction ceases to be a chain reaction when as both Heidt 56 and Lea 48 have demonstrated the rate is proportional to 1:; and independent of the peroxide concentration.Thus with the exception of Dain and Shvartz' results and those of Allmand and Style (which have other anomalous aspects) the published data on the photolysis can be accounted for by the reaction scheme (1) propagation HO f H202 + H20 + HO2 ( 2 ) (3) termination 2 H02 -+ H202 -j- 0 2 .(4) 10.5 [H202]1-0. As the Concentration is reduced the quantum yield h V initiation H202 -+ 20H HO2 (or 0 2 3 + H202 -f 0 2 + OH + H20 (or OH-) Using the same solution we find the y-radiolysis to have the same temperature coefficient and to follow the same kinetic laws as the photolysis. We therefore regard the radiolysis as initiated by reactions (5) and (6) ( 5 ) (6) instead of (l) and followed by reactions (2) (3) and (4). We have not investigated the reaction at very low reactant concentrations because (i) we were anxious to ensure that all the radicals formed in reaction (5) reacted with hydrogen peroxide molecuIes in reactions (6) and (2) and (ii) we anticipated that as the peroxide concentration was reduced the order with respect to H202 would fall and the intensity exponent rise until the ultimate kinetic law was rate cc (R)1.0.It is of considerable interest that Risse 57 observed this changein the kinetics of the X-radio-lysis as long ago as 1929. It is possible that the results of Johnson58 and of Fricke59 (rate K (R)O*5 . [H202]0*5) are due to the fact that their concentration ranges and H2O -L.+ H + OH H + H202 + H20 -k OH, 47 Dainton and Rowbottom Nature 1952 169 370. 48 Lea Trans. Faraday SOC. 1949 45 81. 49 Tian Compt. rend. 1910 151 1040. 50 Henri and Wurmser Compt.rend. 1913 157 126. 51 Jeu and Alyea J. Amer. Chem. Soc. 1933 55 575. 52 Kornfeld 2. Wiss. Phot. 1921 21 66. 53 Anderson and Taylor J. Amer. Chem. SOC. 1923 45 650. 54 Dain and Shvartz Acta physicochim 1935,3 291. 55 Allmand and Style J. Chem. Soc. 1930 596. 56 Heidt J. Amer. Chem. Soc. 1932 54 2840. 57 Risse 2-physik. Chern. 1929 140 133. 58 Johnson J. Chem. Physics 1951 19 1204. 59 Fricke J. Chem. Physics 1935 3 364. 266 GENERAL DISCUSSION dose rates were appropriate to these “ transition ” conditions. This effect would not however account for the difference between Hart and Matheson’s results and our own both of which correspond to conditions in which the kinetic chain length is considerable. This major discrepancy is in our view only to be resolved by further experimentation.Dr. J. Weiss (Durhnni University Newcastle) (coninirinicated) I was very interested in Prof. Dainton and Dr. Rowbottom’s remarks. In fact the mechanism represented by their eqn. (I) to (4) is identical with one of the mechanisms which is given in my paper.60 I cannot see however how at the present state of the experimental evidence all other chain-breaking processes can be excluded in favour of 2H02 -r H202 -I- 0 2 . Dr. E. J. Hart (Argonne Natioizal Lab. Chicago) said In reply to Dr. Barb, experimentally we find that the y-ray induced decomposition follows the (H202)* dependence in the concentration range from 0.008 to 1.0 M. Under our experi-mental conditions the one-half order provides the best fit for the data in this 125-fold change in concentration.The transition stages to lower and higher orders occur outside this range of concentration. Below 0.002 M the behaviour is complex being approximately zero order down to a concentration of 0.00002 M. This is in the range of concentration where chain lengths of the order of unity are obtained. We also find complex behaviour above a concentration of 1.0 M hydrogen peroxide although this concentration range has not been extensively investigated. Mr. N. M. Luft (Waltharn Abbey) said The six-membered ring complexes of hydroperoxide seem to be quite an interesting proposition. If they can be formed by the participation of an HO;! radical then one would conclude they might also arise from H202 molecules alone. The binding energy of such rings (N 25 kcal) would be probably sufficient to ensure stability.There is an unexplained low Raman shift in concentrated H202 at 135 cm-1 which might correspond to ring puckering and the recently reported system61 between 500 and 630 cm-1 could involve ring deformation besides torsion of the monomer. Dr. M. Ebert (Hamn.tersmith Hmpitul W. 12) said Since sending in our paper we have carried out some further experiments with a beam of 500 kV electrons. In this work the water depth was 0.95 mm and the initial ionic yield was found to be about 0.4 at a dose rate of 1-4 x 105 ergs/g sec. The equilibrium H202 con-centration reached was 180 pmole H202/1. The initial yield was thus similar to that found in the work with 1 MeV electrons reported in the paper and the equi-librium concentration was intermediate between those for 1.2 MeV X-rays and for 1 MeV electrons.Miss T. Alper Dr. M. Ebert and Dr. L. H. Gray (Hamniersmith Hospital), Dr. M. Lefort (Paris) Dr. H. C. Sutton and Prof. F. S. Dainton (Leeds University) (comnmnicated) In connection with Dr. Ebert’s paper it seems desirable to publish here the results of some measurements of H202 yields in aerated water which are of special interest in that most of them were obtained in a joint investigation by several workers whose various techniques could be correlated and applied under the same conditions to samples taken from the same batch of water. The out-standing conclusion to be drawn from the results is that there is a definite difference between the effects of X-rays of 30 and 220 keV on the one hand and of 1 MeV electrons on the other despite the comparatively small difference in ion density of these radiations.The joint work was carried out with 1 MeV electrons from the Van de Graaf generator at the Radiotherapeutic Research Centre at Hammersmith Hospital by Miss T. Alper M. Ebert M. Lefort and H. C . Sutton. Samples of water, freshly redistilIed from KMn04 and aerated of inverse conductivity 300 to 600 kQ, were irradiated in open vessels in the manner described in Dr. Ebert’s paper; 40 this DiscuFcion. 61 Giguere Can. J . Res. B 1950 28 485 GENERAL DISCUSSION 267 irradiation and all dosimetric calculaiions are due to J. Boag. Low yields of H202 (< 3 x 10-5 M) in the irradiated samples were measured by T.A. using the method described by Savage 62 in which potassium iodide in acidified aqueous solution is oxidized by the H202 and the iodine liberated is measured colormetric-ally with a starch reagent and by H.C.S.using a technique in which the same reaction is carried out with K1 solutions strictly buffered to pH 5 and the re-sultant iodine is determined as 13- from its absorption at 3520 A (cf. Ghormley 63). Higher yields were estimated by M. L. and M. E. independently using the colori-metric titanium method. All these methods were calibrated from the same standard H202 solution. The results with 30 keV X-rays were obtained by M. L. at the Institute du Radium in Paris they are taken in part from previous pubkations and in part from recent measurements at low doses using Ghormley’s methcd of H202 estimation.The 220 keV X-ray results are taken from an invcstigation of 11. C. S. in the University of Leeds. b”’ t + FIG. 1~.-H202 yields in aerated water. A 10 r/sec 0 1,000 r/sec ‘ 30 ” ‘,MI-. and M.E. b 300 ,, 1 MeV electrons. i I ‘> lo’ooo ” M.L. and M.E. T 60,000 ,, -O- loo ” 1 The results are set out in graphical form in fig. 1 and 2. For convenience in presentation the results with small doses are shown on a larger scale in fig. 2. These results demonstrate that the inirial value of GHzO2 is independent of dose rate but not of radiation quality; in fact it is nearly doubled in going from 1 MeV electrons (Go - 1.10) to X-rays (Go = 2-28). An explanation for this may be envisaged along the following lines. In aerated water the principal reactions involved are probably the following, provided we consider only those radicals which escape recombination and assume that the free H atoms are quantitatively oxidized to HO;! : primary act H20 -+ W t OH ( 1 ) H + 0 2 -f 3 4 0 2 (2) H 2 0 2 formation with radical destruction OH + OH + H202 NO2 f H02 -+ H202 f 0 2 62 Savage Aiialyst 1951 76 224.63 Ghormley O.R.N.L. 130 Oct. 1 1 . 1949. (3) (4 268 GENERAL DISCUSSION FIG. 1~.-H202 yields in aerated water. 30 keV X-rays. A 152 r/sec 'II 76 ) }M.L. 0 304 ), (A) 1 MeV electrons. (R) X-rays. } M.L. A 30 MeV 152 r/sec v V 30 MeV 76 ,, 220 MeV 10 r/sec H.C.S. 0 1,000 y, 0 10,ooo , J GENERAL DISCUSSION 269 radical removal HO2 4 OH -f €320 + 0 2 (5) (6) H202 destruction without radical removal OH + H202 -+ H2O + H02 and/or (71 In the initial stages of irradiation where [H202] is less than 10-5 M the linearity of the yield against dose curve indicates that reactions (6) and (7) are unimportant.The initial GH202 will therefore be determined largely by a competition between reactions (4) and (3) on the one hand (with (4) probably predominating though this treatment is independent of their relative importance) and (5) on the other. The relative importance of these reactions would be expected to depend on the distribution of the OH and H02 radicals in such a way that high local concentra-tions of one radical in regions comparatively free of the other would favour (3) and (4) and lead to enhanced H202 formation. This asymmetry is probably at a maximum in regions of maximum ion density so that we might expect the initial GH202 to increase with the average ion density of the radiation used as is observed.One would not expect a large difference between 30 keV and 220 keV X-rays since the mean energy of the secondary electrons from these sources are about the same but the mean ion density of initially 1 MeV electrons is ap-preciably lower (see Prof. Spiers’ paper). At high doses the H202 yield builds up to a limiting stationary concentration which our results show to be insensitive to changes of dose rate with 1 MeV electrons. Whilst the stationary state has not been attained with X-rays the appearance of the figures shows that the equilibrium concentration of H202 is in this case dependent on dose rate and is much higher in proportion to the initial G than with 1 MeV electrons.We have been unable to correlate this difference with any external features of the irradiations such as enhanced ozone formation from 1 MeV electrons since the H202 yield for a given dose of X-rays was un-influenced by the addition of large amounts of ozone to the surrounding air. It is possible that these differences between the effects of the two radiations are due to differences in the kinetics of H202 decomposition in the two cases; and preliminary investigations of the radiolysis conform with this view. The station-ary state yield is attained when the rates of the formation and radiolysis of H202 are equal. As we have discussed above the formation is largely ascribed to radical interactions within tracks in regions of greatest ion density and as such is greater for more densely ionizing radiations but for a given type of radiation is independent of the dose rate i.e.G formation is constant. The radiolysis on the contrary, is probably initiated by radicals formed in or diffused to the more uniform regions, where the chances of radical-radical interaction are lowered relative to those of reaction with H202. To account for a stationary state concentration independent of dose rate (eg. 1 MeV electrons) the rate of the back (radiolysis) reaction must be proportional to the first power of the dose rate. Lea’s photochemical results 64 show that in a homogeneous system this state of affairs is only possible at high rates of formation of initiating radicals and low H202 concentrations.We must therefore presume that at the dose rates used with 1 MeV electrons this con-dition is fulfilled-a view which is in harmony with very approximate quanti-tative comparisons with Lea’s data. When lower energy radiation is used a smaller proportion of the radicals formed is in the homogeneous regions and available for the initiation of the back reaction and the H202 yield is higher. If the radical concentration in the homogeneous region is sufficiently low the back reaction will proceed in part by a chain mechanism; and again using Lea’s photochemical experiments as a model we expect the rate to become dependent on a power of 64 Lea Trans. Faraday SOC. 1949 45 81 270 GENERAL DISCUSSION the dose rate less than unity.An increase of the dose rate should then displace the stationary concentration of H202 to higher values as is observed with X-rays. The arbitrary distinction drawn between track-like and homogeneous reaction regions is clearly to some extent unrealistic. Nevertheless it provides a useful basis for comparison of different types of radiation. It is of interest that the results of Fricke 45 on radiolysis of dilute H202 in air free solutions with X-rays of effective wavelength of 0-35 A and of Johnson 66 on the same system but using 2 MeV X-rays show differences in dose rate dependence which are also com-patible with such a model. Dr. M. S. Matheson (Argorzne National Labs. Chicago) said In connection with the H202 radiolysis and photolysis we agree with Rowbottom and Dainton that more experimental work is necessary to establish whether the rates in the two cases have a different dependence on H202 concentration.Our own extensive experiments plus those of Fricke 67 and Johnson 68 lead us to believe that in X- and y-radiolysis the over-all rate indeed depends on (H202)i. However the literature results are in disagreement as to the dependence on (H202) in the photochemical decomposition. Of the references subsequent to 1920 cited by Rowbottom and Dainton in their comment as showing that the photolytic chain reaction is first order with respect to hydrogen peroxide few are valid Kornfeld’s 68a data when analyzed fit a square root dependence on H202 as well or better than first order ; Jeu and Alyea69 use the same type of termination with and without added in-hibitor ; Anderson and Taylor 70 give only one sample of data to illustrate the first order law and these data were obtained with all wavelengths between 2000 and 4000A so that much of the light must have been strongly absorbed.Therefore, only the work of Lca 71 (not very extensive on H202 dependence) and that of Dainton and Rowbottom72 (at higher concentrations than those used by us) appears to us at the present time to support fjrst order dependence on H202. If both radiolysis and photolysis rates are proportional to (H202)3 then only a termination step such as we have proposed seems likely to account for the dependence. If the radiolysis rate is proportional to (H202)+ and the photo-chemical rate is proportional to (H202) then one can account for the difference only in the mechanisms of initiation since it is difficult to conceive how the two reactions can differ in the later stages of the reaction.One way of accounting for such a differing dependence is as follows. The radiolysis mechanism is as we have given it but in photolysis the quantum yield of initiation is low and is pro-portional to (H202). However Lea 71 finds a high quantum yield of initialion which seems to rule this out. A second way would be the approach used by Weiss,S wherein in the radiolysis the fraction of the free radicals in a given track which survive to escape from the track and induce the chain reaction is propor-tional to (H202)-1. This second case would account for a radiolytic rate cc(H202)a without a third order termination step.In his paper Weiss73 considers decomposition in a single track using (I&) and (IIIb) i.e. both reactions ( 2 ) and (3) occurring and destruction of radicals by interaction of 20H. By this mechanism the concentration of radicals will be changed by 20H interacting and by diffusion but not by the reaction of OH or H02 with H202 since in these reactions one radical is formed for each radical 65 Fricke J. CAem. Physics 1935 3 364. 66 Johnson J . Ciienz. Physics 1951 19 1204. 67 Fricke J. Clzern. Physics 1935 3 364. 68 Johnson J. Clzem. Physics 1951 19 1204. 68a Kornfeld 2. wiss. Pilot. 1921 21 66. 69 Jeu and Alyea J. Amer. Chenz. SOC. 133 55 575. 70 Anderson and Taylor J. Anzer. C h n . Suc. 1923 45 650. 71 Lea Trans.Faraday SOC. 1949 45 81. 72 Dainton and Rowbottom Nature 1952 169 370. 73 Weiss this Discussion GENERAL DISCUSSION 27 1 consumed. Therefore in eqn. (IV) the term ka(S)n should be replaced by a dif-fusion term. Using (IIIa) and (IIIb) the assumption that (H02) cc (OH) leads to the result that (OH) and (H02) are constant with time within a track since d(OH)/d(HOz) is also constant. In the “reaction due to the interaction of the tracks” again in eqn. (VIII) the ka(S)n term should be zero because reaction with the solute does not destroy radicals. The important item to note here is that in eqn. (XVITI) the term A arises because ka(S)n is erroneously included in eqn. (VIII). If A is made iero then it is no longer possible to obtain a rate proportional to (H202)* from eqn.(XVIII). Nevertheless thc modifications Weiss included in his introductory remarks under certain assumptions may lead to the proper inclusion of the ka(S)n term. Certain considerations however suggest that the “ track ” concept should not be applied to the C060 y-ray induced radiolysis of H202 and that the radio-lysis and photolysis should not differ appreciably in kinetics. (Dainton and Rowbottom7 find similar H202 dependence for the two cases but not however, the same H202 dependence we obtain,) Thus Spiers,74 in his table 1 estimates 10 ion pairs per micron for the mean ion density along the path of the recoil electron from a C060 y-ray. Further as Spiers has noted in the average cluster of ionization along the primary electron path there will be only three ion pairs.These clusters will be spaced about 3000A apart whereas from the radical con-centration in our sector experiments the average distance between radicals through-out the solution is only about 16,OOOA. Even if four of the six radicals in each cluster recombine it is difficult to see why the remaining radical pair should re-combine as readily as a pair generated photochemically since the photochemical pair is generated in a solvent cage. Also it should be noted that if the (H202)* rate dependence in radiolysis is explained by bimolecular termination of two HO2 radicals and initiation cc (H202)-1 then since the (H202) range variation covered in our experiments is over 100-fold the lowest initiation efficiency in our experiments must be < 0.001.To account for such a low efficiency in the cluster picture given above one must assume that only one radical pair escapes from 300 clusters by three radical pairs each and an ion pair escapes only when separated by 0.1 mm along the track from its neighbour. Dr. A. G. Maddock (Cambridge University) said I should like to ask Dr. Ebert and Dr. J. W. Boag if tests were made for traces of the oxyacids of nitrogen after the irradiations in nitrogen-saturated solutions ? Dr. M. Ebert (Hammersmith Hospital W.12) said We agree with Dr. Barb that the tests for impurities employed by Fricke and Hart are possibly the best which can be made. They are of course only possible when using evacuated and sealed irradiation vessels. All our work was done with solutions open to the atmosphere and we found that aeration of the solution with unwashed air did introduce impurities which led to irreproducibIe results.When carefully washed air was used for aeration however reproducible results were obtained which were not affected by the several different techniques of further purification employed. We still think that this is fairly strong evidence that any remaining impurities were not important. In reply to Dr. Maddock we did not make routine tests for the presence of the oxyacids of nitrogen as they would not in any case interfere with the titanium test for H202. We do know hohever. from prior experiments that traces of these oxyacids are formed by the irradiation of nitrogenated water. Mr. N. W. Luft (Waltham Abbey) (cammrmicated) It appears that current views on the reaction HO + HO -+ Hz02 have been greatly influenced by Weiss’s 75 investigations.This author derived an activation energy for the gas phase 74 Spiers this Diccuwion. 75 Weiss Ti-mu. Fnrrrchy SOC. 1940 36 856 272 GENERAL DISCUSSION recombination process of E - p2/r3 - 4.6 kcal by assuming dipole repulsion with p = 1.66 D and Y ~ l 2 A. Apart from possible differences in the dipole moments of the OH radical and OH bond this estimate overlooks the predominant influence of the quadrupoIe potentials of bonds and lone electrons. If all these factors are taken into consideration by an electrostatic method using dipole and quad-rupole moments and polarizabilities then for a range of values and distances equal to or larger than the normal 0-0 separation no potential maximum is obtained for the configuration of minimum energy (dihedral angle $I 110').The poten-tial maximum in the cis-position decreases rapidly with r from its value of ca. 12 kcal at r = 1.48 A. Since in the critical range the two bonding electrons interact strongly approximately according to Morse's law with DO =; 52 kcal it is clear that the true activation energy of HO recombination is zero. The same holds for recombination of NH2 and CH3 radicals although quadrupole repulsions are appreciable in the latter case. The theoretical results for CH3 are supported by recent experiments.76 In view of the known instability of aqueous H202 solutions there js no need to postulate a finite activation energy for HO recombination in order to explain poor H202 yields.Moreover the extent to which OH recombination occurs is limited by competing reactions e.g. the other chain-breaking process HO -i- HO2, which seems to be important in thermal decomposition at high H202 concentrations. Dr. J. Weiss (Durhnnz University Netrnntle) (communicated) My objection to the assumption of a recornbination of OH radicals to give hydrogen peroxide was never based primarily on theoretical grounds but originates from the fact that the experimental evidence suggests strongly that this reaction does not occur.77 My earlier suggestion that this reaction might have a considerable heat of activ-ation due to the repulsion of the dipoles of the OH radicals was mainly to furnish a qualitative theoretical explanation for the experimentally observed facts and was not meant as a calculation of the heat of activation of this reaction which to give with any degree of accuracy would be in any case practically impossible.I still think that experimental evidence for the recombination of OH radicals is lacking. On the contrary some new evidence which I have presented here78 definitely favours the other alternatives. Dr. C. B. Amphlett (A.E.R.E. Harwell) said In connection with the resurts of Dr. Hardwick obtained with the ferrous sulphate system I wish to present the following preliminary yields of ferrous ion oxidation in 0-8N H2SO4 using 0.92 mV electrons. The dose-rates expressed in e.u./min are overall values calculated from the total energy dissipation within the absorbing volume i.e.from the product of beam voltage and cell current the latter being measured via a high resistance and sensitive microammeter connected to earth. Owing to the variations in dose-rate along electron tracks the overall dose-rate will in fact be lower than the maximum dose-rate within the cell. Apart from the value at 0.001 pA the oxidation curves obtained (which were initially linear) all showed characteristics of oxidation in de-aerated solutions i.e. beyond the oxygen break points; oxidation was in all cases practically complete and was then followed by a build up of peroxide the latter attaining a steady-state concentration of ca. 3 x 10-4 M in 0.1 M Fe3+ sohtions in 0.8 N H2S04 at 0.92 mV and 1 pA. The results obtained are given below : [Fez+], moles/l.5 x 10-4 1.03 x 10-2 1.03 X 10-1 cell current pA 0.001 0.0 1 0.1 1.0 overall dose-rate e.u./min N 103 - 104 - 10s - 106 Go (aerated) 17 (de-aerated) - 7.3 6.0 4.2 76 Gomer J. Chem. Physics 1951 19 85. 77 cf. Bonhoeffer and Pearson 2. physik. Chem. By 1931 14 1 ; Lea Trans. Furuday 78 this Discussion. - - -Soc. 1949 45 81 GENERAL DISCUSSION 273 Compared with the maximum 79 de-aerated yield of about 10 the yield is seen to decrease with increasing dose-rate at high dose-rates; this is probably due to increasing importance of recombination reactions both of radicals and of ions. It is interesting to note that at the highest dose-rates studied the yield approaches the value of 3.05 for or-particle irradiations quoted by Hardwick.80 Dr. M. Lefort (Institut dzi Radium Paris) said Dr.Hardwick's suggestion, that the reduction of ceric ions would occur in two different ways when irradiated by soft X-rays (reduction by H202) or by y-rays (reduction by H atoms) may be an explanation of the different results obtained with these two kinds of radiation. Dr. Haissinsky and myself had already reached the conclusion that the reduc-tion was mainly occurring through the hydrogen peroxide formed from water with x-rays. Indeed for this radiation we know that even pure water gives fairly large amounts of H202 and H2 in the absence of air. With soft X-rays on the contrary it does not appear from experiments on pure de-aerated water that there is more H202 produced than with y-rays or electrons of high energy. According to the differences in ion density soft X-rays should give effects more similar to y-ray than to x-ray effects.The ion density with or-rays of 5 MeV is roughly 100 times greater than with electrons of 10 kV (soft X-rays) and only about 7 times greater than with 1 MeV electrons. We agree that we could expect slight quantitative variations in the yield of reduction of ceric ions. We observed variations of this kind for the formation of hydrogen peroxide in aerated water as one changes from soft X-rays to 1 MeV electrons. But we do not think that a complete change in the mechanism could occur. Therefore the important differences in the results obtained with X-rays and y-rays for example, for the evolution and the influence of hydrogen are still not explained. Before drawing any conclusion we think we must repeat the experiments of irradiation, at the same time and under conditions as similar as possible with soft X-rays, hard X-rays and y-rays.Dr. W. G. Barb (Courtaulds Ltd. Muidenhertd) (partly co~?unrmic.aterl) Collinson and Dainton have described certain kinetic features in which their system deviates from the usual vinyl polymerization kinetics and have suggested some of the differences may be due to a non-uniform radical distribution when ionizing radia-tion is used for initiation. It must however be pointed out that phenomena qualitatively similar to the present case are found in a number of polymerizations where the polymer is insoluble in the reaction mixture even though other means than ionizing radiation are used to initiate the reactions.This does not imply that Collinson and Dainton's suggestion about spatially inhomogeneous distribu-tion is necessarily incorrect even though the authors themselves point out a serious objection i.e. the long radical life-time. What does seem imperative is to take the phenomena due to polymer insolubility into account first and then to examine whether any features remain which require the stipulation of phenomena peculiar to radiative initiation. I have had the opportunity of discussing this point with Dr. Magat and Dr. Chapiro as well as with Dr. Bamford and Dr. Jenkins of the Courtauld (Maidenhead) laboratories and find they share my view very cIosely. Essentially the peculiarity in the kinetics of insoluble polymer formation corre-sponds to the absence of stationary state conditions ; a stationary state is not established because the radical termination velocity coefficient is low and prob-ably also time-dependent.The cause appears to be the coiling of the insoluble polymer radicals and their coalescence with polymer molecules both of which considerably reduce the ease of mutual termination while chain propagation is less affected (more detailed discussions of the factors involved are in course of publication 81'82). In such systems one observes some or all of the following 79 Rigg Stein and Weiss Proc. Roy. SOC. A 1952 211 375. 80 Allen Hochanadel Ghormley and Davis AECU-1413. 81 Bamford Barb and Jenkins Nature 1952 169 1044. 82 Bamford and Barb Dircr4ssion.s Farads-v Suc. 1952 in course of publication.I 274 GENERAL DISCUSSION phenomena prolonged acceleration periods,81-84 abnormally large after-effects,sl unusual dependences on monomer concentration and rate of initiation,gL 82,85 ap-preciable temperature effect on termination.86 (References quoted above are pur-posely confined to work in which ionizing radiation was not used.) All of these phenomena have also been found by Collinson and Dainton in the present system, yet they have not been related to the insolubility of the reaction product. It is true that mention is made of coagulation phenomena in connection with abnormal effects at monomer concentrations greater than 0.75 M but the authors consider that coagulation promotes the termination step; in fact such appears to be the case only under rather special conditions,87 and where the polymer forms a definite precipitate the effect is generally one of retarded termination.Again if the authors admit that such physical factors affect the reaction rate at monomer con-centrations above 0.75 M there is no reason to suppose they are necessarily negli-gible below that concentration and that an interpretation of the results should be sought which does not allow for them. Collinson and Dainton draw attention to the fact that very different kinetics are observed in the polymerization of aqueous acrylonitrile solutions when Fe3+OH- is used as a photo-initiator. At first this seems to emphasize the peculiarity of the y-ray and X-ray initiation systems. However closer considera-tion reveals various possible reasons why the radiochemical and photochemical kinetics may differ.Thus the light-scattering by precipitated polymcr produces a type of " skin-effect "88 in the latter case which is probably very different for ionizing radiations. (This light-scattering effect also leads to different kinetics for thermally and photochemically initiated insoluble-polymer systems 1 and throws doubt on the claim of Collinson and Dainton that in their system photochemical initiation leads to a uniform generation of radicals throughout the reaction mixture). Further points to be borne in mind are (i) certain entities are present in one system and not the other e.g. iron ions in the photochemical case and H atoms in the radio-chemical case which could affect the kinetics (ii) the effects of precipitation discussed earlier are also operative in the photochemical case but their quantitative role can vary with several factors which may be different in the two sets of experi-ments e.g.the rate of reaction and the molecular weight of the polymer formed, (iii) a stationary state treatment is not strictly applicable to either system so that the significance of a quantitative comparison is doubtful. Again one cannot claim that these points necessarily explain the observed differences completely ; but they do draw attention to the difficulties in comparing the two sets of results and obtaining evidence of any spatial effects specifically associated with initiation by ionizing radiation. Dr. M. Magat (Paris) said We were very interested in the experimental results of Collinson and Dainton.However we are not quite convinced by their theoretical arguments. We agree with Dr. Barb that caution must be exercised when dealing with precipitating polymers. We have also some objections to the interpretation of the band appearing at 2200cm-1 as being due to CD vibration. Mrs. Prevost-Bernas Miss Fiquet and Dr. Chapiro havee repated the experiments of Collinson and Dainton using X-rays of 37 kV Mo target 2400 r/min on the surface and 0.12 M solutions of acryloni-trile. They found that the band in question was about equally strong when H20 and D20 were used as solvents and appeared moreover when the polymerization was initiated by NaNH2 in liquid ammonia in absence of water. A very strong absorption band appears at the same wavelength when propionitrile is treated 83 Bengough and Norrish Proc.Roy. Soc. A 1950 200 301. 8 4 Prat Mem. Sevv. chim. Z ' h t (Paris) 1946 32 319. 85 Abere Goldfinger Naidus and Mark J . Physic. Chem. 1945 49 211. 86 Burnett and Melville Trans. Faraday SOC. 1950 46 976. 87 Chapiro J . Chim. Phys. 1950 47 747 and 764. 58 Bateman and Gee Pror. Rov. Soc. A 1950 195 376 GENERAL DISCUSSION 275 with NaNH2.89 In the latter case the C r N band disappears almost completely. This leads us to believe that the 2200 cm-1 band is due to a conjugated N=C-C-N vibration although the second harmonic of the CH2 rocking frequency is located in the same region.90 Hence the argument for the CD band becomes a quantitative one i.e. one has to show quantitatively that the ratio of absorption intensity at 2200cm-1 to the absorption of the C r N band is larger in the case of polymerization in D2O than in HzO.Thrs is difficult to decide because the apparent ratio of the % absorption by the 2200cm-1 bond and by the C r N band depends quite strongly on the thickness of the film as was found by Mrs. Prevost-Bernas Miss Fiquet and Dr. Chapiro. If we assume that Beer’s law holds and we compare the curves IA and IIA reproduced in fig. 9 of the paper by Collinson and Dainton then with the irradiation conditions given one would expect the chain length of the polymer for ~ I A to be about 3 to 9 times shorter than for IA. In other words there are at least 3 times as many CD bands for each C-N band in case IIA than in case IA. Assuming that the 2200 cm-1 band is due to CD and using Beer’s law one can calculate the CD/CH ratio from the two curves.One finds then that there is about 7 % less CD for IIA than for IA (the figure of 7 % is probably within the experimental error). These considerations in addition to the fact that the 2200cm 1 band does not appear in the spectrum of the polymethacrylonitrile polymerized by X-rays in H20 and in DzO lead us to believe that these experiments do not prove the primary production of H and D atoms by ionizing radiations. Dr. A. D. Jenkins (Courtaulds Ltd. Maidenhead) (communicated) Among the phenomena reported by Collinson and Dainton to occur in the radiation-induced polymerization of acrylonitrile are a number of features which have also been observed during reactions initiated by free radical catalysts or by ultra-violet light.Thus inhibition periods were found by Koningsberger and Salomon,91 although these workers failed to observe a subsequent period of acceleration. Recent work in these laboratories,92 however has shown that the periods of ac-celeration are inherent features of the polymerization of the undiluted monomer, and that in the complete absence of oxygen there is no inhibition period. This has been proved by carrying out the reaction in dilatometers (filled with monomer and catalyst in vacuo) capable of detecting less than 0.01 % of reaction. Periods of acceleration were found in all polymerizations of carefully purified acrylonitrile and these remained unchanged if monomer recovered from a 60 % polymerization was used.In fact an acceleration with time has been shown to be a common feature of all polymerization reactions which produce a polymer insoluble in the reaction medium.93 Photo-initiated reactions were found to commence immediately the light was switched on both by observation of the level of the meniscus and by visual ob-servation. (Scattering of light by the polymer particles enabled the latter to be detected much earlier in a photo-reaction than was otherwise possible.) In the presence of oxygen reaction was completely inhibited for long periods. It can therefore be safely assumed that the periods of inhibition observed by Collinson and Dainton were either artefacts arising from the method of direct visual observation which they employed or if genuine due to the incomplete removal of oxygen.Prat 94,95 has found that vinyl chloride similarly exhibits an inhibition period in the presence of oxygen. 89 Lander unpublished results. 90 Barchevitz private communication. 91 Koningsberger apd Saloinon J . Polymer Sci 1946 1 200. 92 Bamford and Jenkins. in course of publication. 93 Bamford Barb and Jenkins Natrrre 1952 169 1044. 94 Prat ilfeni. Serv. Chim. I’Etat (Paris) 1940 32 319. 95 Prat Con7pt. rend. J . Znt. Plastiques (Paris) 58 (Jan. 1949) 276 CjENERAL DISCUSSION With regard to the molecular weights of the polymers it should be noted that Lanzl 96 has published values for the Houwink constants for polyacrylonitrile these being K = 1.75 x 10-3 100 ml/g and ,8 = 0.66. The extreme values of the function used by Collinson and Dainton, d (log N) - d (log 4) d(log M ) d (log R) d (log R) d (log R)' II__--are not dependent on /3 but it is clearly not satisfactory to assume direct propor-tionality between molecular weight and intrinsic viscosity.Dr. J. H. Baxendale (Munchester University) said The work of Baxendale, Evans Kilham and Bywater 57,98 on the polymerization of methyl methacrylate initiated by Fez- $- H202 in aqueous systems showed that the polymerization kinetics are consistent with the assumption that monomer polymer and aqueous phase form an homogeneous solution in the presence of emulsifying agent. In such systems polymerization rates were found proportional to the square root of the rate of production of the initiating radicals. However in view of the insolu-bility of polymethyl methacrylate it is clear that the detailed picture is more complex than this and that polymerization does not occur in true aqueous solution.It would seem that the ideas put forward95 to explain emulsion polymerizations of the same system must also apply to the solution case viz. that polymerization occurs in a swollen polymer-monomer phase which is held dispersed by the emulsifying agent. In view of this there is some doubt as to the exact significance of the rate constants used but the observed conformity to the kinetic scheme seems to show the absence of any specific effects due to polymer insolubility. However experiments done in collaboration with Dr. G. W. Madaras show that such is not the case for the analogous polymeriiation of acrylonitrile.Here the results cannot be fitted to the same kinetic scheme or any of the usual modi-fications of it. The rate and extent of polymerization are much more dependent on monomer concentration and in addition the polymerization continues slowly for a much longer time than is the case for methyl methacrylate. Data obtained in collaboration with Dr. G. L. McLeavy show that vinylidene chloride behaves similarly to acrylonitrile. We had attributed these differences to the fact that acrylonitrile and vinylidene chloride are both insoluble in their polymers so that although propagation and termination may occur in true solution while the polymer radical is small enough to be soluble new features due to the immobility of the growing radicals are introduced when the radicals are coagulated.In view of the complications observed with these systems it is doubtful whether the usual kinetics can be applied as has been done by Collinson and Dainton. Dr. A. Chapiro (Paris) said Collinson and Dainton have mentioned two of our previous results (i) that the effect of benzene on y-ray initiated polymerization is not appreciable and (ii) that the polymerization rate of pure monomer is pro-portional to the square root of the intensity. I must point out that these results were obtained with styrene in homogeneous polymerization and that without further experiments they cannot be extended to the polymerization of acrylonitrile where the polymer precipj tates during the reaction. This precipitation of the polymer makes the kinetic interpretation of the reaction more difficult as has already been shown by Dr.Barb in this Discussion. I must add that in my own experiments 100 the y-ray initiated polymerization of styrene and methyl methacrylate could be interpreted by normal kinetics assuming 96 Lanzl see Houtz Textile Res. J. 1950 20 786. 97 Baxendale Evans and Kilham Trans. Farariay SOC. 1946 42 668. 98 Baxendale Bywater and Evans Trans. Faradaj? SOC. 1946 42 675. 99 Baxendale Evans and Kilham J. Polymer Sci. 1946 1 466. 100 Chapiro J. Chirn. Phys. 1950 47 747 and 764 GENERAL DISCUSSION 277 a homogeneous distribution of free radicals as long as the reaction took place in solution. In very dilute solutions of the monomer in presence of polymer precipitants phenomena similar to those observed by Collinson and Dainton were found.Dr. N. Uri (University of Chicago IZZ.) (cornmunicated) Collinson and Dainton suggest that the assumption of oxygen as the inhibitor of polymerization would imply that very little 0 2 is removed in the process of H T 0 2 . 1 wonder whether the authors had considered the possibility of HOz being itself an inhibitor e.g. by effective termination of growing polymer chain. On the whole it appears from the results reported in the literature that larger atoms or radicals such as Br or SCN are very effective chain terminators. Prof. F. S. Dainton (Leeds University) (cornnzunicated) Dr. P. Smith’s results,lOl concerned with the dependence of the rate of polymerization in aqueous media on catalyst concentration where H202 is the catalyst indicate quite clearly that in accordance with Uri’s suggestion HO2 radicals are effective chain terminating agents and we did consider this possibility in relation to the present work.How-ever for the reasons given in our paper we regard this reaction as not playing a major role in radiation induced polymerization. Dr. E. Collinson and Prof. F. S. Dainton (Leeds University) said Criticisms of our suggested interpretation of the results for the radiation polymerization of acrylonitrile have been levelled by Dr. Magat Barb Baxendale and Jenkins. These fall under three main headings : (i) that similar kinetic effects can be obtained in non-radiation systems in which the polymer is insoluble in either the monomer or the solvent ; (ii) that it is not permissible to compare radiation and photochemical results ; (iii) that polymerizations of acrylonitrile in non-radiation systems cannot be explained on the basis of stationary state homogeneous kinetics as can those of, say methyl methacrylate for which the polymer is soluble in the monomer.Though we acknowledge the possibility that polymer insolubility may play some part in the radiation polymerizations we think that the differences between the results of the photo and radiation polymerizations are so marked and yet each set of results so clear cut that we cannot attribute the principal features to this cause alone. We also find no reason to believe that such considerations play a major-part except in the case of high monomer concentrations. The effects which can be obtained for systems of insoluble polymers in which polymerization is not initiated by radiation and which appear in our results have been instanced by Dr.Barb. The crux of the matter would seem to be not whether these effects have each or all been found in non-radiation work generally but whether they can be reproduced for this particular monomer in aqueous solution, together with the dose rate and monomer dependence found by us using non-radiation methods. In our paper we referred to the work of Mr. James on the polymerization of acrylonitrile initiated by ultra-violet light irradiation of the (FeOH)2r ion in aqueous solution. In view of the criticism of this comparison we propose to give some of the facts in more detail. Mr. James’ work shows every indication of being explicable on the basis of stationary state kinetics.The results are so self-consistent and reproducible and differ so clearly from those of the radiation polymerizations that an explanation of the difference on the basis of a skin effect would seem to be strained in the extreme. As was pointed out in our paper the dependence of polymerization rate on intensity and monomer concentration in this case is given by rate cc [rn~]Z.$,~ contrasting markedly with the reIation rate cc (R)x [m# for the radiation work. The I:bs relation was established by the use of (i) a high speed sector of variable angle, (ii) by varying the initial concentration of FelI1 with a constant 10. 101 P. Smith Dim. (Cambridge 1952) 278 GENERAL DISCUSSION The overall rate of radical generation giving the I:bs dependence was about 10- 3 times the overall rate of radical generation giving a polymerization rate propor-tional to (R)* in the radiation case.Thus the chance of the photochemical rate of radical generation being just that required to give the relation was very small. The dependence of the photochemical polymerization rate on [m# was very clear cut in all the work over a concentration range covered by at least 0.3 to 1-2 M. In this connection it is important to note that the only published results (other than our own) concerned with the photo-initiated polymerization of acrylo-nitrile in aqueous solution are those of Evans et a1.102 and are in accord with Mr. James' findings. Not only do the principal kinetic results for the two sets of experiments differ so markedly but also there are other differences of detail.In the photochemical work inhibition periods assessed visually were never greater than 30 sec. A short acceleration period was followed by a period of maximum rate covering about 7 % of the total polymerization before the rate began to fall. Moreover, the acceleration period was not due to the production of a continuously increasing number of long-lived growing polymer chains but to an increasing light absorption in the reaction vessel caused by the light scattering of the precipitated polymer. (A similar effect was observed in the polymerization of methyl methacrylate.) The evidence for this was twofold. In the first place the acceleration period invariably continued up to the time when a certain weight of polymer (which was independent of either [ml] or Jabs) was formed.Secondly the rate of formation of FeIr was linear and somewhat larger in the case of a monomer concentration sufficient to give turbidity than in the case of a monomer concentration high enough to prevent back reaction of Fexl and OH radicals but too low to give rise to turbidity. More-over for the hydrogen peroxide photosensitized polymerization of acrylonitrile, Mr. P. Smith has observed a similar acceleration period. By measurement of the intensity of light transmitted he has shown the acceleration period to end at the same time as the amount of light transmitted falls to zero. The percentage of light transmitted can in fact be used instead of dilatometry for following the initial stages of the reaction.A further indication of the origin of the acceleration period was the fact that on stopping a radiation polymerization allowing the post-irradiation effect to go almost to completion and then restarting the new rate was equal to that obtaining immediately prior to cessation of irradiation even though the polymer was coagulated. In the photochemical case this was only true if the polymer was not coagulated or filtered from the solution. In both of the last cases an acceleration period was found before the steady rate was re-attained. The post-irradiation polymerization mentioned for the radiation results also occurred but to a much less degree in the photopolymerization work. However, with regard to Dr. Baxendale's comment our experience is that in both radiation and photopolymerizations methyl methacrylate gives much inore marked after-effects than does acrylonitrile.In one radiation case the rate of the post poly-merization of methyl methacrylate was as high as 34 % of the irradiation rate and the total polymer was trebled during 16 days following the cessation of radiation. If post-polymerization is any criterion of the degree to which polymer insolubility controls the kinetic behaviour of a polymerization it would appear from this that the kinetics of the acrylonitrile polymerization should be more amenable to treat-ment on the stationary state method than those of methyl methacrylate. We may also note that both pure acrylonitrile 103,104 and methyl methacrylate or solutions in methanol up to 50 % monomer104 show linear percentage contraction against time curves over the initial stages of the polymerization (i.e.up to 10 %) and that methyl methacrylate shows an increasing rate with time at some degree of poly-merization above this in spite of the solubility of the polymer in the monomer. 102 Evans Santappa and Uri J. Polymer Sci. 1951 7 243. 103 Prkvot Compt. rend. 1950 230 288. 104 Chapiro Cousin Lander and Magat Rec. truv. chirn. 1950 68 1056 GENERAL DISCUSSION 279 It is of course true that there are certain entities present in the photopolymeriza-tion case which are absent in the radiation case but it is difficult to believe that these entities can so radically alter the kinetics. Change of pH had a negligible effect on the rate of the radiation polymerizations in the range pH 7 to 0.8 N sulphuric acid.Change of ionic strength had a negligible effect on the photo-polymerizations suggesting that the rate of coagulation of colloidal polymer particles was not a determining factor. An important effect which was observed, however was that both reducing ions hydrogen peroxide can act as chain terminating agents. The former by the reaction where mj" represents a growing polymer radical and Pi a dead polymer chain and the latter by an unknown reaction which is probably mj" + H202 -+ Pj f- H02. followed by the production of the chain-terminating agents 0 2 and HO2 from H202. The effect of adding reducing ions e.g. FezL to the photoinitiated polymer-ization of acrylonitrile is to raise the intensity exponent and decrease the rate.Throughout all such variations of intensity exponent due to Fe3+ however the rate remained proportional to [ml]'. It may well be that this effect of ferrous ion was a contributory factor to the anomalous results with acrylonitrile found by Dr. Baxendalc but which were not observed by Evans et a1.102 In reply to Dr. Barb's criticism of our statement that coagulation above 0.75 M may promote the termination step this is undoubtedly what occurs with styrene polymeri7ed in akohol as the monomer concentration is increased beyond a certain poi n t .lo7 The acrylonitrile case differs from the latter inasmuch as the polymer is in-soluble in the monomer but as pointed out in the paper the fall in intrinsic viscosity of the polymers prepared in solutions of greater concentration than 0-75 M is in agreement with an increased termination rate.We did not invoke a consideration of coagulation effects at concentrations below 0.75 M because the considerable irreproducibility of the results above 1 M which one might expect to accompany such effects contrasted markedly with the consistent [m1]2 relation found for all systems in which the monomer concentration was less than 0.75 M. This indicated that coagulation effects played Iittle part at the lower concentrations. It is also noteworthy that the difficulty of termination envisaged for two mutually interacting insoluble polymer radicals disappears if termination is by H or OH. We now know 105 that the lifetime of the chains is likely to be about 0.1 sec so that one of our objections to a radical termination mechanism loses much of its force.Dr. Chapiro suggests that similar results to ours were obtained for very dilute solutions of styrene in precipitants for the polymer. However apart from super-ficial resemblances it is not possible to make a comparison of the two sets of results since the results for the styrene system do not extend to a determination of the dependence of rate on dose rate and monomer concentration. We apologize for misquoting Dr. Chapiro's work ; the statement that the rate of polymerization of pure acrylonitrile is dependent on the square root of the dose rate actually occurs in a paper by another member of the same laboratory.103 With regard to our use of benzene to give an indication of the importance of direct action in the polymerizations we consider this to be permissible.The important question is whether benzene acts as an inhibitor of vinyl polymerizations. As judged by the results of Dr. Chapiro for styrene it does not and there is therefore no obvious reason why it should be an inhibitor for the acrylonitrile polymerization. If this is so then in the light of the fact that benzene is known to be one of the 105 D. G. L. James unpublished results 280 GENERAL DISCUSSION most radiation resistant substances (as is shown by the work of Burton 106 and by the results of Chapiro 107 for styrene) a polymerization carried out in benzene should give a fair indication of the polymerization initiated by direct action on the mono-mer.The polymer is equally insoluble in benzene and water so that no hypothe-tical interfering effects can arise from differing solubilities of the polymer in these two media. We agree with Dr. Jenkins’ conclusion regarding the inhibition period; we have in fact reached the same conclusion in the paper. We hope to obtain further clarification of this difficult problem of polymeriza-tion in aqueous solution from a comparison of the photo thermal and radiation induced polymerization of methacrylic acid. The data quoted by Dr. Jenkins on the relation between intrinsic viscosity and molecular weight were unknown to us at the time of writing the paper. If the same value of p = 0.66 applies to unfractionated samples as to fractionated samples then the assumption that [TI = KF is certainly unienable but there is reason to believe that the value of /3 is nearer unity for unfractionated samples such as ours.The relation 7lSp/c = KniM for this polymer in dimethyl formamide is quoted on p. 55 The Clzeniistry QfAcrylonitriZe (American Cyanamide Co. 1951). Dr. Magat’s results indicate that the band at 2200 cm-1 which is well known to be typical of the C-D frequency may also arise from the attack of sodamide in liquid ammonia on the cyanide group. Though it is not yet certain just what grouping gives rise to the absorption in these polymers we are grateful to him for pointing out the ambiguity. Dr. G. Stein (Jerusalenz) said Interest in radiation chemistry tended to con-centrate on aqueous solutions in recent years. For the interpretation of phe-nomena in biological systems in particular it might be of advantage to consider first of all the similarities between solutions and solids on the one hand and the information obtained in the radiation chemistry of solids on the other.Biological systems might in particular behave in some respects rather like solids. Concepts such as electron trapping centres conductance bands etc. well known from the study of electronic phenomena in ionic crystals can be introduced into the study of the biological action of radiations. These would emphasize the possibility of direct electron-capture processes in these systems as well as their likelihood even in aqueous solutions containing suitable acceptors. One may perhaps use these concepts based on the thermal or photochemical release of trapped electrons from shallow traps to interpret some recent work of Swanson and Yost 108 on the effect of temperature on the induction of activated stable states in the chromosomes of Tradescanfia.Another aspect of interest is the evident similarity between the reactions of radicals produced in aqueous solution by ionizing radiations and some reactions due to enzyme systems. The examples quoted in the paper from our work in-dicate that by this means further evidence can be obtained regarding the mechanism of free radical reactions. Such free radical reactions may then be of importance in biological systems not necessarily through the formation of free OH radicals there as well but rather through the fact that semiquinone or other organic radicals (which can be obtained under specific conditions through the action of OH radicals on complex organic molecules in aqueous solution) may play a role in biological systems where they are formed under the influence of enzymes.Dr. M. J. Day (Royal Victaria Infirmary Newcastle) said Some further observations made in collaboration with Dr. Stein on the radiation chemistry of aqueous solutions of methylene blue may be of interest. X-rays generated at 200 kV with an average dose rate of about 3,000 r/min gave the following results. 106Burton J. Physic. Chem. 1948 52 564. 107 Chapiro J . Chim. Phys. 1950 47 747. 10s Swanson and Yost Proc. Nat. Acnd. Sci. 1951 37 796 GENERAL DISCUSSION 28 I (i) Presence of oxygen in agreement with other workers we find that irre-versible destruction of the dye occurs and eventually a dark coloured precipitate forms.Presumably the effects are due to OH and H02 radicals which bring about irreversible changes in the conjugated ring system of the dye. (ii) Absence of oxygen further experiments show that the irreversible effect mentioned above still occurs (though with reduced yield) but reduction of the dye also takes place. re-oxidation with partial recovery of the colour occurring when oxygen is admitted to the system. We assume that the OH radical is responsible for the irreversible change while the H atom (or possibly electron capture) brings about the reduction of the dye to its leuco-base. (iii) Presence of 0.5 % sodium benzoate and oxygen (air) up to about 60,000 r there is no detectable effect on the dye.If irradi-ation is continued beyond this point however the colour is discharged but there is a fairly rapid recovery process which is thought to be due to re-oxidation of leuco-methylene blue by hydrogen peroxide formed in the initial period. The admission of air accelerates this process and the solution recovers at least 98 "/o of its original photometric extinction coefficient. If the system be re-irradiated the same cycle of reversible reduction and re-oxidation can be repeated. (iv) Presence of 0.5 % sodium benzoate ; absence of oxygen reduction of methylene blue to its leuco-base occurs in high yield and subsequent admission of oxygen results in virtually complete restoration of the colour. The irreversible decolora-tion observed without sodium benzoate is not observed.The simplest interpretation of (iv) would seem to be that the benzoate reacts with OH radicals leaving H atoms free to reduce methylene blue. In the presence of air (iii) H atoms react preferentially with oxygen forming H02 which enters into reaction with benzoate leaving the dye unaffected. The saturation concentra-tion of oxygen is depleted by this process after about 60,000 r and the reaction subsequently proceeds almost as in the absence of oxygen. The fact that the addition of an OH acceptor protects the dye from the irre-versible effect but enhances the reduction effect is contrary to Haissinsky's theory of indirect reduction by OH radicals and is strongly in favour of a direct effect of primary H atoms.The radiation effect is seen to be very dependent on experi-mental conditions which may explain some of the confusion referred to by Dr. Dale in his paper. Dr. H. C. Sutton (Leeds University) said Dr. Stein has mentioned the work of Forssberglog on catalase deactivation as a specific instance of a biological system in which irradiation effects have been attributed to H atoms. Since this work has been referred to a number of times in this Discussion it may be of interest to mention here some preliminary experiments on deactivation of catalase the results of which are opposed to this view. These experiments were carried out by Miss L. Mee and myself in the Department of Radiotherapeutics Cambridge University. Firstly an attempt was made to deactivate air free solutions of crystalline catalase by H atoms produced chemically.The methods used were ultra-violet photolysis of NaI solutions in the presence of catalase thereby producing H and I atoms and the diffusion of cathodically produced H atoms through a palladium foil into an air-free catalase solution after the method of Parravano.110 Both methods failed to effect any significant deactivation other than that observed in control experiments. Nevertheless such methods are genuine sources of H atoms ; in particular the second method caused complete reduction of methylene blue to its leuco base at an appreciable rate. Secondly protection effects in the X-irradiation of aqueous catalase solutions were found to lead to the same conclusion. It was confirmed that 0-2 % potassium iodide solution completely inhibits the X-ray effect a result which can scarcely be 109 Forssberg Nature 1947 159 309.110 Parravano J . Amer. Chem. Soc. 1951 73 628 282 GENERAL DISCUSSION interpreted in terms of a reaction between H atoms and iodide ions but which is understandable if the OH radical is the effective agent. Finally it was shown that saturating the catalase solution with 0 2 or N2 prior to X-irradiation caused only a slight protective effect whilst pre-saturation with H2 caused a marked protection such that the amount of deactivation for a given dose was reduced about sevenfold, compared to that observed in oxygen saturated solutions. This result also suggests that OH is the effective agent and that H2 protects the catalase by converting OH into H.It should be emphasized that this work is only preliminary and takes account of only certain aspects of the very comprehensive work of Frossberg but it is nevertheless clear that the results are opposed to the view that the H atom is the effective agent in this system. This result is of peculiar interest since this is one of the very few biological systems in which a primary role has been claimed for H atoms. Dr. M. Haissinsky (Laboratoiue Curie Paris) said Dr. Stein in his paper, and Rigg Stain and Weiss in a recent paper advance arguments based on pH dependence in favour of or against primary reactions leading to the formation of free radicals or molecules. I do not think that such arguments can be decisive if they are taken separately in each particular case.I have quoted in my paper several examples showing that oxidation can either increase or decrease with change of pH. It seems that it is rather the chemical nature of the whole system under conditions of radiochemical competition which determines the influence of pH. For example the increase of H2 evolution with acidity mentioned by Stein as an indicabion of H2+ ions can also be explained in the following manner. The preceding discussion has shown that many acids even if they are not apparently attacked can react with OH radicals leading to the formation and decomposition of unstable transitory compounds (boric phosphoric and even sulphuric acids), An equivalent amount of H2 would be evolved. The choice between the two interpretations could be made if the specific influence of the anion on gas evolution were more precisely known.The best example is given by the radiochemical behaviour of the two similar acids arsenious and phosphorus. Both are oxidized by X-rays but the oxidation increases with the pH for the former while it decreases for the latter. Now, according to the general chemical proprieties and the free energy variations both would be more easily oxidized in basic solutions. The only notable difference between these acids from redox point of view is that arsenites are readily oxidized by H202 but that phosphites are inert towards this compound (in spite of the strongly negative (calculated) potential). One can then assume that Po$- is also more resistant than AsOa- to the action of OH radicals and when competing for these with another compound the former will have less chances to capture them.This would be the case in basic solution where the tendency of H202 to be de-composed by OH radicals is marked. It follows that arsenites can be oxidized in this medium either by Hz02 or by OH at a rate greater than in acid solution as can be expected from their general chemical behaviour. On the contrary, phosphites which do not react with hydrogen peroxide are less oxidized even by the OH radicals as the latter react more strongly with H 2 0 2 . Dr. G. Stein (Jerusalem) said In connection with the dependence of H2 evolu-tion on the pH this phenomenon may serve to elucidate the mechanism of the process only if it is first ensured that the compounds added to the water in order to vary :he pH are themselves unaffected by the reactive intermediates.Unless this is so the reactions of the added substance with the active radicaIs formed may completely obscure the situation. Both arsenious and phosphorous acid un-doubtedly strongly react with some of the radicals formed and are thus unsuitable to serve as examples. Regarding boric acid a similar situation may exist as ap-parent also from the remarks of Dr. Wright in this Discussion. Tt seems however, that sulphuric and phosphoric acids may serve a$ suitable additions under suitabl GENERAL DISCUSSION 283 chosen conditions. Evidence exists which indicates that the addition of these anions does not influence the yield in the radiation chemistry of metal ions through reactions with the radicals formed by the radiation.Using these anions in pre-liminary experiments (Milling Stein and Weiss) no elidence was found to in-dicate the formation of products from these anions. It appears therefore that these anions may serve to elucidate the dependence of some of the primary processes on pH. Dr. N. Uri (University of Chicago Ill.) (conzmunicated) In the well-informed paper presented by Stein there is one statement which I think is inaccurate and 1 would like to draw attention to it as similar statements occur not infrequently in the literature. It is said that the molecule with the greatest electron affinity will retain in the end all those electrons or H atoms which have not undergone other irreversible processes. A similar impression on the parallelism of the electron affinity and the H atom abstraction arises also from other passages in this paper.1 shallquote only a few examples in order to show that the assumption of such parallelism is not permissible the electron affinity in solution of the C1 atom is somewhat larger (by about 3 kcal) than that of the OH radical. On the other hand the abstraction of an H atom by an OH radical is considerably more exothermic (by at least 12 kcal) than that by a C1 atom. Thermodynamically this difference will express itself in the different heats of ionic dissociation of water and hydrochloric acid. The energy change in the abstraction of H atoms by C1 atoms or phenyl radicals will be about equal but a phenyl radical has a very much lower electron affinity in soluLion than a C1 atom.Some recent observations orr " promoter " and " sup-pressor " effects in oxidations by Fenton's reagent are based on this very pheno-menon-that oxidation by eleciron transfer of ferrous ion and abstraction of hydrogen from an organic substrate do not go hand in hand.1'1 C1 atoms would show much more preference in oxidation by electron transfer than OH radicals, where this competes with oxidacion by hydrogen abstraction. The same applies to the acetate radical. The energy changes involved i n such reactions as are very different. It might appear preferable to avoid mentioning the two different modes of oxidation as if they were entirely parallel occurrences. Dr. G. Stein (Jerusalem) (conmzunicated) In the systems considered by Dr. Uri there certainly exists a considerable difference betweFn electron transfer and dehydrogenation processes.In those systems the electron or the H atom have to be abstracted from an existing molecule or ion by the attacking species and considerable differences will exist in such case between a process like the abstrac-tion of an electron from a hydrated ion with the formation of a higher oxidation state or on the orher hand the dehydrogenation of for example an aromatic molecule with the formation of a free radical. The situation is however quite different in the radiation chemistry of aqueous solutions. In this system the ejected electron or the hydrogen atom formed through the reaction of the electron with the aqueous medium are not bound to any existing molecule and arz free to be captured by a suitable acceptor.In fact the detachment process has been performed in the primary radiation chemical process so that onr has to consider not an oxidation or a coupled oxidation-reduction process but on the contrary, an isolated reduction brocess alone. Moreover owing to the spatial separation between the electron or H atom and the original molecule from which it has been detached the back reaction is considerably less influenced by the nature of the original molecule than in photochemistry. As a result in the presence of some suitable substrates such as methylene blue or metal ions (ferric ceric) we have been unable to differentiate experimentally whether the reduction process occurring 111 Kolthoff and Medalia J . Amer. Clwn. Soc.1949 71 3777. 3784 284 GENERAL DISCUSSION was due to direct electron capture or to reaction with an H atom already formed from the electron. Regarding systems in which the primary reduction will be followed by further reversible processes the possibility of differences in the sense pointed out by Dr. Uri is not excluded in aqueous systems. However in the multicomponent systems known to us such effects have not been observed and although we have considered this point we could not dekise a decisive experiment. In non-aqueous and especially solid systems the process of reversible electron trapping will certainly yield the results as discussed in the paper. Regarding electron or H atom capture by 02 it has been impossible to differ-entiate between the two processes 0 2 + e + 0 2 - a97 where the electron is captured by the 0 2 molecule the ion thus formed becoming hydrated and 0 2 + H -+ H02, where the H atom has already been formed by the preceding hydration of the original electron.Accordingly one deals here again with two isolated reduction processes which in this particular case are formally parallel. Dr. W. A. Waters (Oxford University) said In connection with the use of dyestuffs as radical deteclors it should be noted that these are in general conjugated quinonoid systems A=C-C-C-C-B where A and B are hetero-atoms (0 0 in indigo; N N in methylene blue perhaps S). Though in general it is true that free radicals very easily add on to quinones the nature and facility of the addition depends on the chemical character of the radical concerned.Thus H* adds easily to 0 and N but less easily to C whilst *OH will not add to 0 probably not to N but very easily to C. Thus from radiochemical fission of water one would expect reduction in pre-ference to oxidation (as found). The *OH attack would be on Ihe inner C=C structure and would necessarily lead to the complete destruction of the whole conjugated system. This has been established for indigo sulphonic acid which breaks down to isatin sulphonic acid. Again the reaction must occur in stages e.g. D 4- *H -f DH* or DH2 +-:OH + DH. S(H20, giving semi-quinonoid (or similar) radicals. In kinetic work it is often forgotten that in general these semi-quinonoid radicals do not persist-they could be spotted immediately from their very intense colours-but react much more readily than either the dyes D or their stabler leuco compounds DH2.It seems that these radicals DH* resemble free metallic ions such as Fez+ or Fe3+ in being immediately reversible oxidation-reduction systems each individual radical having its own redox potential. Some radicals for instance can easily reduce Fe3+ whilst others easily oxidise Fez+. Reactions due to these intermediates can be expected to set in whenever organic radicals are produced by irradiations of solutions of organic compounds and consequently in the interpretation of radiochemical processes care should be Laken to see that one is not being confused by secondary reacLions of radicals produced subsequently to the primary chemical decomposition. With regard to the action of *OH radicals on benzene etc.Dr. Stein has used the hypothesis that direct attack on H occurs viz. : HO* -/- C6H6 -+ H20 + *C6H5, but many atoms and radicals e.g. C1* CH2 (diradical) are known to act by primary addition to the 7~ electron system. An alternative interpretation of radical substitution via addition should therefore be borne in mind and until the primar GENERAL DISCUSSION 285 mechanism is made evident the theoretical interpretation of substitution laws for actions of radicals upon aromatic systems should be regarded as an open question. Dr. G. Stein (Jerusalem) (partly communicuted) The remarks of Dr. Waters are most illuminating regarding the different possibilities of attack by the various radicals on dyes. They emphasize the observations of Dr.Collinson reported in this Discussion and the experiments of Mr. Day and myself. There can be little doubt that both modes of reaction can be demonstrated under suitably chosen conditions. Regarding the mechanism of aromatic substitution by radicals it is indeed reasonable to assume that the primary attack is by way of an addition of the radical to the r r electron system as pointed out by Dr. Waters. Our own interest has been in establishing experimentally whether this formation of a transition complex is followed by (i) a displacement reaction (ii) a dehydrogenation reaction, or (iii) further addition of a second incoming radical to the addition complex forming e.g. a dihydro-dihydroxy compound. In these cases the final substitution would be caused (i) by the first radical in the first stage (ii) by a second radical in a distinct second stage or (iii) by two radicals acting jointly in the transition compound.From the experimental evidence obtained 112 it seems strongly indi-cated that the substitution reaction proceeds in two distinct stages as in (ii). In the first dehydrogenation occurs presumably via an addition complex with the formation of an organic radical derived from the aromatic molecule . followed by a second step in which the organic radical thus formed interacts with a second molecule or free radical thus bringing about the final substitution step, e.g.7 . /OH Dr. h. Collinson (Leeds University) (partly communicated) In connection with Stein’s paper I should like to mention some results obtained on the aqueous methylene blue system irradiated by 220 kV X-rays.Our interest in this system was stimulated by the fact that it was reported that this solute is reduced by irradiation in de-aerated solutions in spite of its reduction potential being well over to the reducing side a result which appeared anomalous from the point of view of the concept of the equivalent reduction potential.113 The justification for classing the decolorization as a reduction seemed to depend on the occurrence of a restora-tion of colour on admitting oxygen and although this seemed sound evidence it was thought that it would be useful to attempt a more direct check on the products of irradiation. Under the conditions employed the methylene blue dye exhibited absorption bands at 6575 6100 2910 and 2460A.By quantitative reduction of the dye solution and spectrophotometric measurement under vacuum of the resulting solution a spectrum of the leuco dye was obtained. This possessed only one absorption band in the wavelength region 2200 to 7000 A viz. a sharp band at 2560A. Irradiation by ultra-violet light of a solution of methylene blue con-taining hydrogen peroxide followed by spectrophotometric measurement yielded an absorption spectrum of the oxidation products due to the action of OH radicals upon the dye. This spectrum had no marked absorption from 7000 to 3500A, 112 Stein and Weiss J. Clzem. Soc. 1949,3245 and following work quoted in the paper 113 Collinson and Dainton Ann. Rev. Physic. Cliem. 1951 2 99. under discussion 286 GENERAL DISCUSSION the absorption rising slowly and continuously at lower wavelengths.Irradiation of aerated methylene blue solutions gave rise to the same spectrum as that obtained from the attack of OH radicals no trace of a band at 2560 A being present. Colour could not be restored to a solution decolorized in this way either by oxygec, hydrogen peroxide zinc amalgam or sodium hydrosulphite. The progress of irradiation of de-aerated dye solutions could be followed by measuring the spectrum at relevant points after successive irradiations the solution being under vacuum throughout. Not only could the rate of removal of the dye colour be measured by this method but also the rate of development of the leuco dye. The rate of decolorization in de-aerated solution was much faster than that in aerated solution and it was found that the predominant action was undoubtedly one of reduction.However at no stage could the decolorization be attributed entirely to leuco dye formation. The proportion of decolorization due to the formation of leuco dye decreased as the irradiation continued and at complete decolorization in a neutral solution the leuco dye concentration was about half that of the original dye. In acid solutions the leuco dye concentration at complete decolorization was a higher proportion of the original dye concentration. On continuing irradia-tions beyond the point of complete decolorization the absorption band at 2560 A slowly disappeared the rate of destruction of the leuco dye being about 0.1 times the initial rate of decolorization of the dye and the rate of destruction in acid solution being lower than the rate of destruction in neutral solution.The ultimate spectrum was the same as that obtained from irradiation in aerated solution or by the action of OH radicals. Therefore the ultimate result of irradiation of an aqueous solution of methylene blue even when de-aerated is one of irreversible oxidaiion. However the fact that predominant reduction occurs in the early stages of an irradiation still seems anomalous from the point of view of the e.r.p. Several features of this rather complex system may combine to give this result but the most important factor is probably that ihe hydrogen atom or othcr reducing agent can react in only one specific way with the dye molecule namely to reduce it to the leiico dye whilst the OH radical or other oxidant may attack the molecule at several points some of which may cause an inappreciable change in the absorption spectrum.Moreover, the hydrogen atom may havc no further point of ready attack once the leuco stage has been reached whilst the oxidized products and the leuco dye may still be open to attack by OH radicals even after the dye molecule has been broken down into smaller fragments. The fact that the leuco dye disappears as oxidation products apparently without passing through the dye stage supports the view that the attack of OH is mainly on other parts of the molecule than those respons-ible for the reversible colour change. Hence the increased decolorization yield in the presence of benzoate reported by Stein seems more likely to be due to prevention of radical recombination than to prevention of re-oxidation of the leuco dye by OH radicals.Dr. G. Stein (Jerusalem) (partly conmiunicated) Dr. Collinson’s interesting experiments show conclusively that the decoloration of methylene blue may take place by two mechanisms. One of these an irreversible decoloration is presumably an oxidation reaction. The other a reversible decoloration has now been shown to consist of a reduction process. Our aim in this respect was to show that, under suitably chosen experimental conditions the reversible reductive decolor-ation may predominate and to devise experiments which enable one to elucidate the mechanism of this reaction. The fact that a reductive decoloration can be obtained with a high yield and can be rcversed almost completely as shown by Mr.Day’s results communicated at the meeting is not necessarily in contradiction with the useful concept of the equivalent reduction potential of Dainton and Collinson. The e.r.p. is the result of the establishment of an equilibrium in a system where the various reactive species (OH H02 H H2+) can mutually reverse each others actions. In ou GENERAL DISCUSSION 287 system however this reversibility is upset by the occurrence of irreversible re-actions. Thus the introduction of benzoic acid reacting irreversibly with the OH radicals makes impossible the establishment of the usual reversible equilibrium, necessary for the creation of an e.r.p. in the system.We would like to emphasize that certain differences may occur between the oxidation of a substrate by Fenton’s reagent and oxidation by radiation in aqueous solution. In addition to the possible interference by H atoms in the latter the presence of oxidizable and reducible metal ions in the former could give rise to reactions which are not present in the radiation case. We have found evidence for such reactions in a detailed quantitative study of the oxidation of benzene by Fenton’s reagent. In vacuo the products of this oxidation have been shown previously to be diphenyl and phenol. Reactions which have been previously considered to give rise to hydroxylated products are (4 and Ph* + OH .+ PhOH (b) Our observations are that with 10-4 M Fez+ and H202 70 % diphenyl and 30 % phenol are produced and the oxidation balance precludes the presence of other products.The products are unaffected by change in hydrogen peroxide concen-tration so that reaction (a) can be excluded. However the initial presence of ferric ion alters the relative amounts of diphenyl and phenol appreciably in the sense that with increasing ferric ion concentration the proportion of phenol increases. Simultaneously there is a decrease in the net amount of ferrous ion oxidized. These observations are consistent with the oxidation of the phenyl radical to phenol by Fe3f or FeOH2f. It is possible that even in the absence of ferric ion initially the ferric ion formed during the reaction produces all the phenol. Hence phenol can be formed by entirely different mechanisms in the radiation and Fenton sys terns.The reduction of phenyl radicals by ferrous ion must also be assumed to account for our observations. It is clear that analogous reactions are possible when other substrates are used and these must be borne in mind when comparing the effects of the radiation and Fenton systems. Dr. J. Weiss (Durham University Newcastle) (commzmicated) It has been shown previously that phenol is formed by the action of ionizing radiations on benzene in aqueous systems.114 The phenol can be determined by the well-known colorimetric method using the Folin reagent. This has been employed also for the chemical dosimetry of ionizing radiations.115 Tn the course of a reinvestigation of this problem in collaboration with Mr.Milling we have found that particularly in the presence of oxygen of the air, the formation of phenol is accompanied by the formation of hydrogen peroxide in somewhat more than equivalent amounts. Hydrogen peroxide can be deter-mined by the titanium sulphate reagent (this is not interfered with by the presence of phenol) although it is not clear at present to which extent this is due to other hydroperoxides which may be present in the solution. We have found that under the conditions used previously i.e. saturated solutions of benzene in water and irradiation with X-rays (200 kV) in the presence of air, the value for the yield of hydrogen peroxide (G = molecules/100 eV) determined by titanium reagent is G(H202) N 5 - 5 . Dr. G. Stein (Jerzcsalem) said The difference between the action of radiations and Fenton’s reagent and the effect of ferric salts on hydroxylation processes with Fenton’s reaagent and photochemically with ferric salt solutions is known.Tt is referred toeven in thepaperunder discussion and has beeninvestigatedin detail.116 Dr. J. H. Baxendale and Dr. J. Magee (Manchester University) said Ph* + H202 -+ PhOH + OH 114 Stein and Weiss J. Chem. SOC. 1949 681. 115 Day and Stein Nature 1949 164 671. 116 Stein and Weiss J. Chem. SOC. 1951 3265 288 GENERAL DISCUSSION Thus it has been shown that the difference in substitution ratios in the hydroxyl-ation of benzoic acid by X-rays and by photochemical radical formation 117 is due to this reason. It is also shown there that complexing the ferric salt for example, by fluoride influences fundamentally the process of hydroxylation by Fenton's, reagent of phenol thus indicating a much more important role for the ferric ion than previously assumed.Dr. J. Weiss Durham (University Newcastle) (communicated) 1 should like to draw attention to a paper by Dr. Stein and myself 118 where we have already discussed the hydroxylating effect of ferric ions in general and of the (FeOH)2+ complex in particular viz. the reactions with free phenyl or aryl radicals according to (FeOH)2+ + R -+ ROH 4 Fez'-, and where we have also suggested that certain differences in the hydroxylation by irradiation on the one and Fenton's reagent on the other hand may be due to reactions of this type. Dr. A. G. Maddock (Cambridge University) said Dr.Stein has raised the question of the chemical effects resulting from the F-centres and other lattice defects produced in solids by ionizing radiations. Caillot and Sue119 have obtained some direct evidence of such effects. They have shown that the ratio of P5+/P3+ for P32 produced by the (n cx) reaction in sodium chloride is influenced by the creation or relaxation of the F-centres in the crystal. I should like to make brief mention of some effects investigated by Dr. J. Green in my laboratory. It is possible that the reactions I shall describe may be adapt-able to a high dose actinometer of the kind desired by Dr. W. Wild since very large irradiations are necessary to produce accurately measurable resulis. We find that subsequent y-irradiation of neutron irradiated salts of oxyanions such as chromates or permanganates increases the fraction of the active species found, on solution as the original oxyanion (the retention) ; while the net effect on the inactive oxyanions leads to macroscopic reduction.This effect resembles the thermally activated increase in retention that we have described previously. It is hoped that a quantitative study of both processes will determine whether the increase is entirely due to recombination of the fragments of the oxyanions or whether some electronic mechanism is involved. I understand that Prof. G. E. Boyd (private communication) has obtained similar and more detailed results with potassium bromate. Dr. John L. Magee (Notre Dame Univerdj Indiana) said Although the H and OH radical formation mechanism reviewed by Dr.Stein in eqn. (2-4a) has had wide acceptance there is real reason to doubt its validity. In fact the dom-inant electron recapture process may be H2Ot 4- c -+ H20". where the symbol * designates a highly excited molecule and excited water mole-cule dissociation may be the principal source of radicals. Arguments which suggest this possibility will be discussed at length in a future publication 120 and will only be outlined here. (i) Slow secondary electrons can probably be thermalized in water without capture. It has been shown 121 that in a medium which has a threshold energy for electron capture the probability for capture of electrons as they are being ther-malized is usually very small. There are reasons for believing that H20 has a 117 Loebl Stein and Weiss J .Gem. Soc. 1951 405. 118 Stein and Weiss J . Chem. Sor. 1951 3265. 119 Caillot and Sue Compt. rend. 1950 230 1864. 120 Samuel and Magee forthcoming publication. 121 Magee and Burton J. Amer. C k m Soc. 1951 73 523. Bates Evans and Uri Nature, 1950,166 869 GENERAL DISCUSSION 289 threshold.122 The work of Dainton et af. mentioned in this Discussion,l23 on the photon-induced electron transfer from various ions to H20 clearly indicates such a threshold. Cii) Slow secondary electrons will fall below zero energy relative to the H20+ ions at moderately small distances. As an electron collides with the H2O molecules and loses energy it pursues essentially a random-walk path and does not increase its distance from H20+ very quickly.The average loss of energy per collision is not known accurately but the attained distance is not extremely sensitive KO this quantity. Once the electron drops below zero energy it has been “captured” and an excited H20* is formed. Further electron collisions with the HzO medium will quickly degrade energy and draw the electron closer to the positive ion. With reasonable estimates the electron will go 30A or less. (iii) The above discussion assumes that the dissociation reaction H20f 4 H+ + OH does not habe time to occur before the capture is effected. This reaction is exo-thermic only by virtue of the solvation of the H+ ion and a time of at least 10-11 sec is required for its completion. The total time required to stop the electron which moves with velocities of 107-108 cmisec is about 10-13 seconds or so.The H20+ ion can of course exchange its charge with neighbouring H20 molecules, but the qualitative situation is unaffected by this process. Dr. G. Stein (Jerusalem) (partly communicated) Far too little is known about electron-capture processes in liquid water and in aqueous solutions to come to a definite decision between the mechanism suggested by Dr. Magee and the one discussed by Lea and Gray Doubtless both processes do occur and the question is which one accounts for the bulk of the chemicaI changes observed. Dr. Magee’s mechanism would in fact assume that the chemical effects in the radiation chem-istry of aqueous solutions are due to radicals formed by the dissociation of excited water molecules exclusively.In addition to the points made by Dr. Magat in this Discussion there are some further considerations which indicate that aqueous systems deserve special treatment in this respect. On ejection of the electron the residual positive ion will undergo in water a process of hydration which will take a time of the order of 10-11 sec as men-tioned by Dr. Magee. This vaIue can be arrived at in several independent ways. In this exothermic process the free energy of the positive trap is considerably decreased and dissociation is caused with the formation of a free OH radical and a hydrated H unless recombination takes place before completion of the process. Whilst this reaction proceeds two other processes occur with the ejected electron. I t is thermalized coming to a stop in a time of the order of 10-14-10-13 sec and at the same time it also undergoes a process of hydration in which it polarizes the medium digging a potential well in it.124 So even if the electron drops below zero energy relative to the original H20’ it will be able to move back to it only at a 1 elocity determined by its surrounding polarized dielectric medium approach-ing as the reaction proceeds only the migration velocity of the hydrated OH-ion.Accordingly above a certain electron energy recombination will no longer take place but an electrical equilibrium of hydrated H+ and OH- ions is created (accompanied by the formation of OH radicals and H atoms respectively) at a distance from each other. Although the electron energy of the threshold is not known exactly it is lower than either in the gas phase or in other organic liquids.Thus in a medium like water the process of hydration will influence the fate of any ions created in a manner which does not occur in different media. 122 However the work of Bradbury mentioned in ref. (2) seems to indicate a low 123 Dainton this Discussion. 124 Cf. Pekar J. Physics Moscow 1946 10 347. threshold 290 GENERAL DISCUSSION In addition to this formal consideration the work of Bradbury and Tatel 125 on H20 vapour and H 2 0 + 0 2 mixtures certainly indicates that water vapour especially when condensing does efficiently capture electrons. In the radiation chemistry of aqueous solutions it appears that the energy re-quired to produce one radical pair available for chemical reactions is less than 32.5 eV.This has been attributed to the possibility that in liquid water dissociation of excited water molecules will occur more readily than in the gas phase where radiative loss of energy may take place. However there is some evidence126 which indicates that in liquid water reactions due to radical formation from excited molecules is efficiently prevented through energy loss to the medium. It is perhaps possible to assume that the decrease in the energy required in aqueous media is due to the fact that some of the electrons which have been ejected are in fact prevented froin recombining through the process of hydration in the water system thtis providing additional radicals spatially separated. It is hoped to discuss some of these points in greater detail in the future and to induce some further points in favour of the Lea-Gray mechanism.Dr. M. Magat (Paris) (communicated) I would like to know how Dr. Magee accounts for the difference in the production of H 2 0 2 from de-aerated water by a- and X-rays in his theory. In the " classical " picture of Lea and Gray the difference was ascribed to the separation in space between H and OH radicals, the high density of OH radicals in an o! track favouring the reaction 2 OH -+ H 2 0 2 H 4- OH -+ H20 as compared with the reaction this latter being important for X-rays where no high local OH concentrations are built up. This difference can also be accounted for on the basis of the assumption made by Dr. Haissinsky and myself that OH radicals are produced in the following reactions : (1) (2) For a-rays the OH radicals combine to produce H 2 0 2 and the equivalent amount of H 2 is evolved.H 2 0 + + OH 4- H' H20 H20- + H 2 + 0- -+ OH 4- OH-With X- or y-rays the OH radicals react with H 2 molecules as a result of the uniform distribution of positive and negative ions. In Magee's theory where H and OH radicals are produced in equal amounts from the same ion it is difficult to imagine how the difference in ionization density could influence HzOz production. Dr. John L. Magee (Notre Dame University) (communicated) In the theory I have described the initial efficiency for the formation of H and OH radicals is indeed the same for X-rays and cx-particles. The geometrical structure of the track however causes a difference in the yields of H202 and H2 for the two cases.The great density of radicals along the a-particle track favours radical combina-tion reaction and therefore most of the radicals unite to form H202 H 2 or H20. The smaller density of radicals in X-ray tracks allows most of the radicals to escape initial combination by diffusion and therefore favours the back reaction 127 which destroys most of the mole~ules of H 2 0 2 and H 2 which were initially formed. This description is not in disagreement with views expressed by Allen in this Discussion. H2 + OH -+ H 2 O -1- H 125 Bradbury and Tatel J . Chern. Physics 1934 2 835 126 Farkas 2. physik. Chem. B 1933 23 89. 127 Allen J. Physic. Chem. 1948 52 478 ; this Discussion GENERAL DISCUSSION 29 1 Dr. J. A. V. Butler (Chester Beatty Res. Inst. S. W.3) said Dr. Conway and 1 have found somewhat similar effects to those described by Miss Alper after the irradiation of nucleic acid solutions in the presence of oxygen by X-rays.l2* There is however a marked difference in that we have been unable to ascribe the after-effect to hydrogen peroxide. The situation is a rather complex one. The sensi-tivity of nucleic acid solutions to hydrogen peroxide appears to depend on small amounts of impurities e.g. metal ions cysteine etc. Pure specimens appear to be insensitive to hydrogen peroxide but they become sensitive to it after irradiation. The question is then whether the amount of hydrogen peroxide actually formed during the irradiation is sufficient to produce the observed effects on the irradiated nucleic acid. We have found a considerable discrepancy between Lhe amount actually formed and the amount which has to be added to produce the observed effect and have suggested an alternative mechanism for the after-effect. With reference to Miss Alper’s experiments in which she finds hydrogen peroxide suffi-ciently effective there is the possibility that the hydrogen peroxide is activated by some of the organic substances (e.g. amino-acids) present. Miss Alper also finds that hydroxyl radicals produced by photodecomposition of hydrogen peroxide are ineffective in inactivating bacteriophage. This is a rather surprising finding and 1 wonder if under the conditions of her experiment the hydrogen peroxide is actually decomposed by the U.V. light. It might happen that owing to a slight turbidity of the solution or its absorption very little of the U.V. light can penetrate into the solution and that in fact few hydroxyl radicals were formed. It is also possible that the organic substances present exert a protective action. In reply to Dr. Baxendale our attempts to make added hydrogen peroxide more effective in its action on nucleic acid were unsuccessful. It is conceivable that hydrogen peroxide formed by irradiation in the absence of the nucleic acid and then added to the latter would be more effeciive. We hope to try this experi-ment. Finally. Dr. Weiss states that the after-effect with nucleic acid is a hydrolysis. We have found no evidence that this is the case. Dr. B. E. Conway (Chester Bentty Res. Inst. S. W.3.) said In Miss Alper’s experiments on the inactivation of bacteriophage by ionizing radiation it is of interest that H202 itself decreases the extent of survival yet photochemical decom-position of the H202 in the presence of the phage to produce OH radicals (and some HO2 radicals) does not further decrease the extent of survival. With sodium deoxyribonucleate both X-irradiation and treatment with H202 and U.V. brings about an apparent depolymerization as well as several different types of chemical change. The action of H202 in vivo and possibly of the X-irradiation is therefore probably different from that on isolated nucleic acid and as Miss Alper suggests may be principally effected by H atoms. The spontaneous action of H202 itself is then however difficult to account for unless a specific peroxidation occurs. Miss T. Alper (Huniniersmith Hospital London) (conzmunicuted) The remarks of Dr. Butler and Conway refer in part to results mentioned in the Discussion, but not published in my paper,l29 viz. to the finding that sensitization of phage to the action of hydrogen peroxide did not occur as a result of exposure to OH radicals formed by the photochemical decomposition of hydrogen peroxide. Under the conditions of one experiment measurement of the hydrogen peroxide concentration before and after U.V. irradiation showed that it had decreased by 10 % and it seems reasonable to assume a concentration of OH radicals sufficiently high to have brought about injury of the phage similar to that found with ionizing radiations if the latter acted through the medium of OH radicals. If the failure of OH radicals to produce this injury were due as Dr. Butler suggests to protection by organic substances such protection would of course also be exerted if the OH radicals were produced by ionizing radiation. It should be noted that the solid 128Butler and Conway J. Clienr. Soc. 1950 3418 ; 1952 834. 129 this Discussion 292 GENERAL DISCUSSION content of the phage suspensions used was less than that of the DNA solutions used by Butler and Conway,130 in which OH radicals were found to be effective. Dr. J. Weiss (Durham University Newcastle) said 1 do not think there is any real difficulty in understanding the after-effect at least with nucleic acids and related compounds which have been studied by Dr. Scholes and myself in consider-able detail. As we have already stated elsewhere 131 we have come to the conclusion that with nucleic acids the after-effect is due to a slow hydrolysis of some primarily formed unstable phosphate esters although we do not know very much about the exact nature of these unstable esters. Thus we have been able to conclude that the depolymerization of the nucleic acid which continues after cessation of irradia-tion can be largely attributed to the slow hydrolysis of intermediate labile phosphate esters formed under the influence of the radiations. I think that this explanation of the after-effect is also in agreement with the observations of Butler and Conway referred to by Dr. Butler. 130 Butler and Conway J. Chem. Suc. 1952 834. 131 Scholes and Weiss Nature 1950 166 640. Report of the Conference on the Chemistry mid Physiology of the Cell N~icleus (Brookhaven August 1951)

ISSN:0366-9033    

DOI:10.1039/DF9521200243

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

IV. PROTECTION AND SENSITIZATION PROTECTION EFFECT AND ITS SPECIFICITY IN IRRADIATED AQUEOUS SOLUTIONS BY WALTER M. DALE Christie Hospital and Hol t Radium Institute Manchester 20 Received 14th January 1952 Systems which are initially one-solute systems become multi-solute as a result of irradiation. Dissolved oxygen can be a second solute leading to reaction products like HOz and H202 which can cause chemical effects. Radiation effects in two-solute systems have been investigated on two different but complementary lines. First the estimation of gaseous end-products and secondly the estimation of a change of the initial solute. A second solute in a two-solute system acts as a competitive acceptor of free radicals thus reducing the radiation effect on the first solute i.e.protecting it. This protective effect can be used as a measure of the protective power of various solutes relative to the first solute (the indicator). The specifi- city of this effect in particular that of sulphur is demonstrated in experiments with the enzyme carboxypeptidase and with alloxazin-adenine dinucleotide as the indicators. The protective power per unit mass of protector is sometimes dependent on the con- centration of the protector. One has to assume in such cases that protector molecules can hand on energy to the indicator. In order to discuss radiation effects in multi-solute systems I start with some general considerations which form the background against which the experi- mental results have to be seen. Practically all systems which are initially one-solute systems become multi- solute solutions as a result of irradiation.A one-solute system would require the absence of dissolved oxygen and secondly that only one reaction takes place whose reaction products are not further capable of reacting with free radicals resulting from the decomposition of water i.e. the reaction product has to be inert towards radiation. Some solutions of inorganic solutes capable of being reduced or oxidized come nearest to this ideal one-solute system though there is evidence that reduction and oxidation can take place simultaneously and therefore the stipulation of an inert reaction product is not strictly fulfilled. Dissolved oxygen as mentioned before acts as a second solute which by secondary reactions causes the formation of hydrogen peroxide or of the radical H02.These then lead in some cases to an enhancement of the radiation effect. There are however examples-for instance the inactivation of the enzyme carboxypeptidase and also of ribonuclease 1-which are independent of the presence of oxygen. I should like here to comment briefly on the statement often made that it is extremely difficult to remove oxygen effectively. This is quite true when the aim of the experiment is to establish an absolute quantitative relationship for reactions in presence or in absence of oxygen. It is not difficult however to reduce the oxygen tension in small volumes sufficiently to decide whether or not there is an effect of oxygen. Another point I should like to mention here is the radiation effect on solutions of methylene blue about which there seems to be some confusion in the literature.All investigators agree that bleaching occurs when the dye is exposed to ionizing radiations. This is often taken as being due 29 3 PROTECTlOF4 EFFECT AND ITS SPE<'IFICITY 294 to a reduction of the dye to its leuco-base. One has however to distinguish between the bleaching occurring in an oxygenated and in an oxygen-free solution. In oxygen-free solution the blue colour can be partially restored after irradiation when the solution is shaken with air or oxygen. In this case one deals with a predominant reduction by radiation of the methylene blue to leuco-methylene blue. In an oxygenated solution however an irreversible change of the mole- cule takes place which also appears as bleaching of the dye but the solution once bleached can never regain its colour on being shaken with air or oxygen.In other words an irreversible change has taken place which is more than a simple reductive change. Mention is now made of some difficulties with which an analysis of radiation reactions is faced when it aims at a complete insight into the primary reaction as well as into secondary reactions difficulties which become ever greater the more complex the substances and systems which are involved. This explains why many investigators have chosen the simplest solutes possible hoping to free themselves of the limitations imposed by this complexity. It is however note- worthy that even in systems in which free radicals are uniformly generated by chemical means several alternative reactions often present themselves and that the analysis cannot go further than to attribute a greater probability to some reactions than to others.The formation of free radicals by ionizing radiations introduces the additional difficulty of the discontinuity of the primary act and it is then justified to ask how far these reactions can be treated as occurring in a homogeneous system. One might be led to conclude from these remarks that no useful purpose could be served by investigating more complex systems at all but I think this would be unnecessarily pessimistic. The investigation of radiation effects has proceeded on two distinct lines. First the estimation of end products in the form of the gases H2 and C02 a line which was followed mainly by Fricke and his colleagues and secondly the deter- mination of the disappearance of an initial solute sometimes supplemented by the identification of some intermediate product into which the primary solute was changed by irradiation.Disappearance includes for a biologically active sub- stance the change from an active to an inactive molecule. Neither of the two lines of approach is without limitations. At first sight it may seem as if the first method the determination of gaseous end-products is unequivocal and will at least give an overall balance of the reaction. However uncertainty of the action of H atoms and OH radicals must sometimes exist since H atoms can dehydrogenate i.e.oxidize as well as hydrogenate and as pointed out recently by Evans and Uri,2 by Haissinsky,3 and by Weiss,4 OH radicals can oxidize as well as reduce. Furthermore carbon-carbon bonds can be broken leading to reaction products of lesser carbon content but of more stable nature. On balance solutes may by interaction with radicals catalyze the recombination of H and OH. The second line of approach followed in my laboratory and applicable to substances of more complex structure does not concern itself with the overall balance of end products and intermediate reactions but with the loss of a specific property of the solute molecule in fact for biologically active molecules for instance enzymes viruses proteins we are more interested to know the efficiency with which their specific activity is affected by radiation than the contribution they make to the total output of gaseous end products an output which according to Fricke,s is very small for proteins.In short each of the two methods exists in its own right and both are complementary. The important feature of experiments with multi-solute systems is that they offer a means of measuring to what extent a second solute added to the first is able to influence the radiation effect on the first by acting as a competitive acceptor of free radicals and so decreasing the radiation effect on the first solute i.e. WALTER M . DALE 295 protecting it. Addition of substances differing in composition then gives a picture of how their capacity to act as acceptors changes in accordance with their specific structure.Experiments of this kind were carried out by Fricke and his co- workers on solutions of simple organic solutes mainly of the aliphatic series determining the total output of C02 and H2. Without considering these experiments here in detail one can say that the reaction concentration curves of single solutes are often of a complicated nature and although one would think that simple structures of molecules should also show simple concentration reaction relationships this is by no means the case. Thus Fricke in an earlier interpretation of his concentration-reaction curves assumed that in certain cases a different chemical reaction takes place in a dilute solution from that which occurs in a more concentrated solution although in each case the solute was changed by radiation only to a minute extent.He has now also suggested that the CO2 yield dependence on concentration for the same experimental results could be interpreted in terms of chain reactions sustained by radicals formed from the solute.7 This explanation seems to be more satis- factory since it is not easy to believe that a different chemical reaction should take place simply on increasing the concentration of the solute. Such difficulties existing in one-solute systems are increased when solutes occur in multi-solute systems. Before I proceed to mention our experiments on two-solute systems I shall quote a few examples in which an increase of the radiation effect caused by the presence of a second solute is reported.There are experiments of Frickes in which an increase in reduction takes place when solutions of potassium dichromate mixed with various aliphatic acids are irradiated There are also other cases where X-rays cause reduction coupled with an increase of the effect by adding substances capable of removing oxidizing OH radicals. Stein and Day have used the reduction of oxygen-free solutions of methylene blue for X-ray dosimetry and could enhance the bleaching effect of X-rays by adding sodium benzoate.9 The same mechanism is suggested by Forssberg 10 for explaining the increase in the inactivation of catalase in presence of reduced glutathione and cysteine. One would assume then that in cases where H atoms are responsible for an observed radiation effect the addition of reducing substanccs will enhance and where OH radicals are responsible the addition of reducing substances will depress a radiation effect.This assumption however would not fit in with the dual role of reducing and oxidizing property of the OH radical mentioned earlier. In our experiments with two-solute systems 11 12 the first solute was the enzyme carboxypeptidase and the determination of its inactivation by radiation in presence and in absence of a second solute formed the basis on which the ability of various second-solutes to react with free radicals could be compared. Since there was always a decrease of the radiation effect in presence of the substances added we speak of the second solute as the protector of the enzyme the latter being called the indicator.We also have used alloxazin-adenine dinucleotide as the indicator. This dinucleotide is only the prosthetic group of the enzyme D-amino acid oxidase and therefore itself inactive but becomes active in presence of a specific protein. One can determine the chemical change of the dinucleotide after irradiation by building up the complete system dinucleotide (D) specific protein (P) and the substrate alanine (A) and measuring the oxygen consumed for oxidation of the alanine.12 The two indicators carboxypeptidase and dinucleo tide differ widely in their molecular weight 35,000 and 920 respectively but both have in common that the efficiency of radiation (ionic yield) remains constant over a wide range of initial concentrations.The following experiment (table 1) illustrates the protective effect of various concentrations of leucylglycine added to dinucleotide before irradiation. The PROTECTION EFFECT AND ITS SPECIFICITY 296 oxygen uptake increases with decreasing concentration of leucylglycine until it reaches the value of the control. Similar experiments with carboxypeptidase as indicator had results shown in table 2.13 It will be seen that the first six substances of very different molecular weights but otherwise of similar average composition have practically the same pro- tective power per microgram of protector and therefore the protective power per molecule which is proportional to the molecular weight is an additive manometer 2 3 1 4 5 6 substance TABLE I * X-ray dose = 4000 r.L == leucylglycine [ ]* = irradiated contents D f P f A [D + 10-4 mole L]* + P -1- A [D + 10-6 mole L]* 4- P 4 A [D + 10-7 mole L]* t P i- A [ D -1 10-8 mole L]* t P + A [Dj* -t P t A TABLE 2 -f mol. wt. 48 x 106 7.6 s 106 40,000 - 89 180 45 88 60 142 116 tobacco mosaic virus bushy stunt virus cryst. egg albumin denat. egg albumin alanine glucose formate H-COONa COONa oxalate COONa iVH2 thiourea C=S I I NH2 urea I I I NH2 c=o NH2 alloxan CO CO NH-CO I I NH-CO mesoxalate I I COONa I c=o I COONa * reproduced by permission of the Biochemical Journal.7 reproduced by permission of the British J . Radiology. 7 pI. 0 2 in 1st 10 min 28.6 27-1 19.3 12-8 10.0 10.7 rel. protective power per pg 30 20 17 20 39 34 320 1.5 -1 120 0.5 13 WALTER M. DALE 297 function of the molecular groups constituting this molecule. The next six sub- stances however contain special groups of atoms which form a large part of the total molecule and these have a very marked and specific effect on the protective power. Such specific effects are masked by the average uniformity of the com- position of large molecules. Of special interest is the change of protective power caused by the substitution of a sulphur atom for an oxygen atom in urea and this has led to an investigation of other sulphur-containing compounds and of sulphur itself.14 Table 3 summarizes the results.Column 1 gives the weight of protector per ml used in the experiment and column 2 the respective sulphur content. Column 3 gives the protective power ( Q p ) per microgram of protector and column 4 the protective power Qs of such amounts of protector as contain 1 microgram of sulphur in each case. One can therefore estimate from column 4 how the non-sulphur residue in any one com- pound affects the protective power of 1 microgram of sulphur contained in it taking colloidal sulphur as reference. Elemental sulphur is about as protective 3 TABLE 3 $ 1 p g protector per ml 4 QS 130 58 5.0 5.0 1.6 1-6 thiourea dimethylthiourea colloidal sulphur 2 pg sulphur per ml 2.1 1.54 1-06 (for one S) QP 55 18 110 24 110 118 sodium thiosulphate 5.3 as thiourea and sodium thiosulphate but the introduction of two methyl groups into thiourea causes a considerable decrease in protective power of the sulphur The protective power of a protector can be expressed by the quotient Q = (DP+I - Dz)/’DI - I/P where Dr is the X-ray dose required to change the indicator by 63 % in absence of protector and Dp+ I the X-ray dose required to change the indicator by 63 % in presence of protector and I and P the concentration (pglrnl) of indicator and protector respectively.Q therefore represents the proportion of the X-ray dose taken up by the protector to that taken up by the indicator for equal weight of protector and indicator.In uncomplicated cases Q is independent of the concentration of protector used. In other cases however one finds that Q decreases with increasing concentration of protector as shown in fig. 1. Such a decrease of Q can be explained by assuming that energy taken up by the protector is handed on to the indicator. It is however not to be assumed that the decrease of Q is caused by a direct action of radiation on the protector molecule. Such a direct effect would require much higher concentrations of protector than those at which the decline of the protection effect becomes evident and would lead to improbably high figures of the ionic yield when calculated on the basis of a direct mode of action.We have developed formulae based on the collision frequencies of a radical with protector and indicator molecules of given molecular weight which are valid for all cases regardless of the existence or non-existence of a handing-on effect and the values of the ratios of the probabilities of destruction of a radical by collision with a protector and an indicator molecule respectively are collected in table 4. The ratios pp/pR are a measure of the protective power per molecule of the protector for the given indicator. It will be seen that the protective power of thiourea is 10,000-fold greater than that of urea and further that a change from the indicator carboxypeptidase (C.P.) to the dinucleotide (D.N.) does not appreci- ably change the values of the protective power although these indicators are rather 1 reproduced by permission of the BritisJi J.Cnnccr. K PROTECTION EFFECT AND ITS SPECIFICITY 298 different. It seems as if it were advantageous to use indicators which do not exhibit complicated concen trat ion-radia t ion dosage relationships. Finally the protection effect also operates in systems which are not true solu- tions. This has been shown in experiments with suspensions of biological units protector thiourea sodium formate dime thy1 thiourea glucose dimethylurea egg albumin alloxan sodium mesoxalate sodium oxalate urea 0 \ 0 8 A ‘a Q e.g. bacteria and sperm of arbaciae to which various substances were added as protectors.Hollaender and his colleagues have used the survival rate of coli bacteria when irradiated in presence and in absence of alcohol SH compounds etc.,ls and Evans and his colleagues 16 used the change of fertilizing power of arbacia sperm as indicator of radiation effects. Such experiments have clearly shown the protective action of substances dissolved in the surrounding medium. The interpretation however is complicated by the comparatively enormous 2.0 x 10-3 7.5 :. 10-4 “ Changing “ Changing quotient quotient ” for for C.P. C.P. Curves points Curves are are theoretical theoretical ; points expt. expt. A dimethylurea A Jf Jf dimethylurea with with C.P. C.P. 30 30 pg/ml B B 0 glucose with glucose with C.P. C.P. 30 30 pg/ml pg/ml ucose ucose with with C.P.C.P. 90 90 pg/ml pg/ml \ 0.24 size of the particles and the existence of cell membranes acting as permeability barriers and introducing surface phenomena. If the substance does not penetrate into the interior of the particle its effect is confined to competitive action in the surrounding fluid and the surface layers of the particle and can be understood by assuming that these surfaces play an active part in the maintenance of the bio- logical activities of the cell. A full explanation of the mechanism will therefore require an analysis of permeability as well as an investigation of the question whether such surfaces can be the link between radiation and the effects observed. f reproduced by permission of the British J . Cancer. conc. i n /ig,ml.TABLE 4 f mol. wt. 76 68 104 180 88 -40,000 142 152 134 60 Q p p l p ~ with D.N. p p l p ~ with C.P. 2.1 0.74 1.9 - 0-21 - 0.21 - 4-7 3-1 0.79 0.4 2.1 x 10-2 2.4 x 10-2 - - - 299 In contrast to systems of suspended particles is the condition of gels in which a colloid forms a continuous network of micelles between which the solvent circulates. Zt is important from the biological point of view that no protective action by the gelatinous network was observed by Skoog17 and Gordon and Quastler 18 when they irradiated Auxin in agar blocks or by Day and Stein 9 in their experiments with methylene blue in gelatine gels. These systems are of biological importance as models of the inhomogeneous physical structure of the interior of cells referred to in our experiments.13 WALTER M .D A L E 1 Dainton and Holmes Nature 1950 165 266. 2 Evans and Urj Nature 1950 166 602. 3 Haissinsky and Lefort Compt. rend. 1950 230 1156; J . Chim. Phys. 1950 47 588. 4 Weiss Reunion Ann. SOC. Chim. Phys. (1951). 5 Fricke Cold Spring Harbor Symp. 1935 6 164. 6 Fricke Hart and Smith J . Chem. Physics 1938 6 229. 7 Fricke Symp. 4 (1950) Sept. 18-20 Army Medical Center Md.). 8 Fricke and Brownscornbe J . Amer. Chem. Soc. 1933 55 2358. 9 Stein and Day Nature 1950 166 146. 10 Forssberg Nature 1947 159 308. 11 Dale Brit. J. Rad. 1943 16 171. 12 Dale Biochem. J. 1942 36 80. 13 Dale Brit. J. Rad. 1947 Suppl. 1 46. 14 Dale Davies and Meredith Brit.J . Cancer 1949 3 31. 15 Burnett Stapleton Morse and Hollaender Proc. SOC. Expt. Biol. Med. 1951,77 636. 16 Evans Slaughter Little and Failla Radiol. 1942 39 663. 17 Skoog J. Cell. Cohp. PJiysiol. 1935 7 227. 18 Gordon and Quastler (private communication). IV. PROTECTION AND SENSITIZATION PROTECTION EFFECT AND ITS SPECIFICITY IN IRRADIATED AQUEOUS SOLUTIONS BY WALTER M. DALE Christie Hospital and Hol t Radium Institute Manchester 20 Received 14th January 1952 Systems which are initially one-solute systems become multi-solute as a result of irradiation. Dissolved oxygen can be a second solute leading to reaction products like HOz and H202 which can cause chemical effects. Radiation effects in two-solute systems have been investigated on two different but complementary lines.First the estimation of gaseous end-products and secondly the estimation of a change of the initial solute. A second solute in a two-solute system acts as a competitive acceptor of free radicals thus reducing the radiation effect on the first solute i.e. protecting it. This protective effect can be used as a measure of the protective power of various solutes relative to the first solute (the indicator). The specifi-city of this effect in particular that of sulphur is demonstrated in experiments with the enzyme carboxypeptidase and with alloxazin-adenine dinucleotide as the indicators. The protective power per unit mass of protector is sometimes dependent on the con-centration of the protector. One has to assume in such cases that protector molecules can hand on energy to the indicator.In order to discuss radiation effects in multi-solute systems I start with some general considerations which form the background against which the experi-mental results have to be seen. Practically all systems which are initially one-solute systems become multi-solute solutions as a result of irradiation. A one-solute system would require the absence of dissolved oxygen and secondly that only one reaction takes place whose reaction products are not further capable of reacting with free radicals resulting from the decomposition of water i.e. the reaction product has to be inert towards radiation. Some solutions of inorganic solutes capable of being reduced or oxidized come nearest to this ideal one-solute system though there is evidence that reduction and oxidation can take place simultaneously and therefore the stipulation of an inert reaction product is not strictly fulfilled.Dissolved oxygen as mentioned before acts as a second solute which by secondary reactions causes the formation of hydrogen peroxide or of the radical H02. These then lead in some cases to an enhancement of the radiation effect. There are however examples-for instance the inactivation of the enzyme carboxypeptidase and also of ribonuclease 1-which are independent of the presence of oxygen. I should like here to comment briefly on the statement often made that it is extremely difficult to remove oxygen effectively. This is quite true when the aim of the experiment is to establish an absolute quantitative relationship for reactions in presence or in absence of oxygen.It is not difficult however to reduce the oxygen tension in small volumes sufficiently to decide whether or not there is an effect of oxygen. Another point I should like to mention here is the radiation effect on solutions of methylene blue about which there seems to be some confusion in the literature. All investigators agree that bleaching occurs when the dye is exposed to ionizing radiations. This is often taken as being due 29 294 PROTECTlOF4 EFFECT AND ITS SPE<'IFICITY to a reduction of the dye to its leuco-base. One has however to distinguish between the bleaching occurring in an oxygenated and in an oxygen-free solution. In oxygen-free solution the blue colour can be partially restored after irradiation when the solution is shaken with air or oxygen.In this case one deals with a predominant reduction by radiation of the methylene blue to leuco-methylene blue. In an oxygenated solution however an irreversible change of the mole-cule takes place which also appears as bleaching of the dye but the solution once bleached can never regain its colour on being shaken with air or oxygen. In other words an irreversible change has taken place which is more than a simple reductive change. Mention is now made of some difficulties with which an analysis of radiation reactions is faced when it aims at a complete insight into the primary reaction as well as into secondary reactions difficulties which become ever greater the more complex the substances and systems which are involved.This explains why many investigators have chosen the simplest solutes possible hoping to free themselves of the limitations imposed by this complexity. It is however note-worthy that even in systems in which free radicals are uniformly generated by chemical means several alternative reactions often present themselves and that the analysis cannot go further than to attribute a greater probability to some reactions than to others. The formation of free radicals by ionizing radiations introduces the additional difficulty of the discontinuity of the primary act and it is then justified to ask how far these reactions can be treated as occurring in a homogeneous system. One might be led to conclude from these remarks that no useful purpose could be served by investigating more complex systems at all but I think this would be unnecessarily pessimistic.The investigation of radiation effects has proceeded on two distinct lines. First the estimation of end products in the form of the gases H2 and C02 a line which was followed mainly by Fricke and his colleagues and secondly the deter-mination of the disappearance of an initial solute sometimes supplemented by the identification of some intermediate product into which the primary solute was changed by irradiation. Disappearance includes for a biologically active sub-stance the change from an active to an inactive molecule. At first sight it may seem as if the first method the determination of gaseous end-products is unequivocal and will at least give an overall balance of the reaction.However, uncertainty of the action of H atoms and OH radicals must sometimes exist since H atoms can dehydrogenate i.e. oxidize as well as hydrogenate and as pointed out recently by Evans and Uri,2 by Haissinsky,3 and by Weiss,4 OH radicals can oxidize as well as reduce. Furthermore carbon-carbon bonds can be broken, leading to reaction products of lesser carbon content but of more stable nature. On balance solutes may by interaction with radicals catalyze the recombination of H and OH. The second line of approach followed in my laboratory and applicable to substances of more complex structure does not concern itself with the overall balance of end products and intermediate reactions but with the loss of a specific property of the solute molecule in fact for biologically active molecules for instance enzymes viruses proteins we are more interested to know the efficiency with which their specific activity is affected by radiation than the contribution they make to the total output of gaseous end products an output which according to Fricke,s is very small for proteins.In short each of the two methods exists in its own right and both are complementary. The important feature of experiments with multi-solute systems is that they offer a means of measuring to what extent a second solute added to the first is able to influence the radiation effect on the first by acting as a competitive acceptor of free radicals and so decreasing the radiation effect on the first solute i.e. Neither of the two lines of approach is without limitations WALTER M .DALE 295 protecting it. Addition of substances differing in composition then gives a picture of how their capacity to act as acceptors changes in accordance with their specific structure. Experiments of this kind were carried out by Fricke and his co-workers on solutions of simple organic solutes mainly of the aliphatic series, determining the total output of C02 and H2. Without considering these experiments here in detail one can say that the reaction concentration curves of single solutes are often of a complicated nature, and although one would think that simple structures of molecules should also show simple concentration reaction relationships this is by no means the case. Thus Fricke in an earlier interpretation of his concentration-reaction curves, assumed that in certain cases a different chemical reaction takes place in a dilute solution from that which occurs in a more concentrated solution although in each case the solute was changed by radiation only to a minute extent.He has now also suggested that the CO2 yield dependence on concentration for the same experimental results could be interpreted in terms of chain reactions sustained by radicals formed from the solute.7 This explanation seems to be more satis-factory since it is not easy to believe that a different chemical reaction should take place simply on increasing the concentration of the solute. Such difficulties existing in one-solute systems are increased when solutes occur in multi-solute systems.Before I proceed to mention our experiments on two-solute systems I shall quote a few examples in which an increase of the radiation effect caused by the presence of a second solute is reported. There are experiments of Frickes in which an increase in reduction takes place when solutions of potassium dichromate mixed with various aliphatic acids are irradiated There are also other cases where X-rays cause reduction coupled with an increase of the effect by adding substances capable of removing oxidizing OH radicals. Stein and Day have used the reduction of oxygen-free solutions of methylene blue for X-ray dosimetry and could enhance the bleaching effect of X-rays by adding sodium benzoate.9 The same mechanism is suggested by Forssberg 10 for explaining the increase in the inactivation of catalase in presence of reduced glutathione and cysteine.One would assume then that in cases where H atoms are responsible for an observed radiation effect the addition of reducing substanccs will enhance and where OH radicals are responsible the addition of reducing substances will depress a radiation effect. This assumption however, would not fit in with the dual role of reducing and oxidizing property of the OH radical mentioned earlier. In our experiments with two-solute systems 11 12 the first solute was the enzyme carboxypeptidase and the determination of its inactivation by radiation in presence and in absence of a second solute formed the basis on which the ability of various second-solutes to react with free radicals could be compared.Since there was always a decrease of the radiation effect in presence of the substances added we speak of the second solute as the protector of the enzyme the latter being called the indicator. We also have used alloxazin-adenine dinucleotide as the indicator. This dinucleotide is only the prosthetic group of the enzyme D-amino acid oxidase and therefore itself inactive but becomes active in presence of a specific protein. One can determine the chemical change of the dinucleotide after irradiation by building up the complete system dinucleotide (D) specific protein (P) and the substrate alanine (A) and measuring the oxygen consumed for oxidation of the alanine.12 The two indicators carboxypeptidase and dinucleo tide differ widely in their molecular weight 35,000 and 920 respectively but both have in common that the efficiency of radiation (ionic yield) remains constant over a wide range of initial concentrations.The following experiment (table 1) illustrates the protective effect of various concentrations of leucylglycine added to dinucleotide before irradiation. Th 296 PROTECTION EFFECT AND ITS SPECIFICITY oxygen uptake increases with decreasing concentration of leucylglycine until it reaches the value of the control. Similar experiments with carboxypeptidase as indicator had results shown in table 2.13 It will be seen that the first six substances of very different molecular weights but otherwise of similar average composition have practically the same pro-tective power per microgram of protector and therefore the protective power per molecule which is proportional to the molecular weight is an additive TABLE I * X-ray dose = 4000 r.L == leucylglycine [ ]* = irradiated manometer contents pI. 0 2 in 1st 10 min 1 D f P f A 28.6 2 27-1 3 [D + 10-6 mole L]* 4- P 4 A 19.3 4 [D + 10-7 mole L]* t P i- A 12-8 6 [Dj* -t P t A 10.7 [D + 10-4 mole L]* + P -1- A 5 [ D -1 10-8 mole L]* t P + A 10.0 TABLE 2 -f rel. protective power per pg substance mol. wt. tobacco mosaic virus 48 x 106 30 bushy stunt virus 7.6 s 106 20 cryst. egg albumin 40,000 17 20 denat. egg albumin -alanine 89 39 glucose 180 34 formate H-COONa 45 320 oxalate I 88 1.5 COONa COONa iVH2 thiourea C=S NH2 I I NH2 I I NH2 urea c=o NH-CO I I alloxan CO CO 60 142 I I NH-CO COONa 116 I I COONa mesoxalate c=o -1 120 0.5 13 7 * reproduced by permission of the Biochemical Journal.7 reproduced by permission of the British J . Radiology WALTER M. DALE 297 function of the molecular groups constituting this molecule. The next six sub-stances however contain special groups of atoms which form a large part of the total molecule and these have a very marked and specific effect on the protective power. Such specific effects are masked by the average uniformity of the com-position of large molecules. Of special interest is the change of protective power caused by the substitution of a sulphur atom for an oxygen atom in urea and this has led to an investigation of other sulphur-containing compounds and of sulphur itself.14 Table 3 summarizes the results.Column 1 gives the weight of protector per ml used in the experiment and column 2 the respective sulphur content. Column 3 gives the protective power ( Q p ) per microgram of protector and column 4 the protective power Qs of such amounts of protector as contain 1 microgram of sulphur in each case. One can, therefore estimate from column 4 how the non-sulphur residue in any one com-pound affects the protective power of 1 microgram of sulphur contained in it, taking colloidal sulphur as reference. Elemental sulphur is about as protective TABLE 3 $ 1 2 3 4 QP QS thiourea 5.0 2.1 55 130 dimethylthiourea 5.0 1.54 18 58 sodium thiosulphate 5.3 1-06 (for one S) 24 118 p g protector pg sulphur per ml per ml colloidal sulphur 1.6 1-6 110 110 as thiourea and sodium thiosulphate but the introduction of two methyl groups into thiourea causes a considerable decrease in protective power of the sulphur The protective power of a protector can be expressed by the quotient, Q = (DP+I - Dz)/’DI - I/P, where Dr is the X-ray dose required to change the indicator by 63 % in absence of protector and Dp+ I the X-ray dose required to change the indicator by 63 % in presence of protector and I and P the concentration (pglrnl) of indicator and protector respectively.Q therefore represents the proportion of the X-ray dose taken up by the protector to that taken up by the indicator for equal weight of protector and indicator. In uncomplicated cases Q is independent of the concentration of protector used.In other cases however one finds that Q decreases with increasing concentration of protector as shown in fig. 1. Such a decrease of Q can be explained by assuming that energy taken up by the protector is handed on to the indicator. It is however not to be assumed that the decrease of Q is caused by a direct action of radiation on the protector molecule. Such a direct effect would require much higher concentrations of protector than those at which the decline of the protection effect becomes evident, and would lead to improbably high figures of the ionic yield when calculated on the basis of a direct mode of action. We have developed formulae based on the collision frequencies of a radical with protector and indicator molecules of given molecular weight which are valid for all cases regardless of the existence or non-existence of a handing-on effect and the values of the ratios of the probabilities of destruction of a radical by collision with a protector and an indicator molecule respectively are collected in table 4.The ratios pp/pR are a measure of the protective power per molecule of the protector for the given indicator. It will be seen that the protective power of thiourea is 10,000-fold greater than that of urea and further that a change from the indicator carboxypeptidase (C.P.) to the dinucleotide (D.N.) does not appreci-ably change the values of the protective power although these indicators are rather 1 reproduced by permission of the BritisJi J. Cnnccr. 298 PROTECTION EFFECT AND ITS SPECIFICITY different.It seems as if it were advantageous to use indicators which do not exhibit complicated concen trat ion-radia t ion dosage relationships. Finally the protection effect also operates in systems which are not true solu-tions. This has been shown in experiments with suspensions of biological units, TABLE 4 f protector mol. wt. p p l p ~ with C.P. p p l p ~ with D.N. thiourea sodium formate dime thy1 thiourea glucose dimethylurea egg albumin alloxan sodium mesoxalate sodium oxalate urea 76 68 104 180 88 -40,000 142 152 134 60 4-7 2.1 3-1 0.74 1.9 -0.79 0-21 0.4 -0.24 0.21 2.1 x 10-2 -2.4 x 10-2 -2.0 x 10-3 -7.5 :. 10-4 -e.g. bacteria and sperm of arbaciae to which various substances were added as protectors.Hollaender and his colleagues have used the survival rate of coli bacteria when irradiated in presence and in absence of alcohol SH compounds, etc.,ls and Evans and his colleagues 16 used the change of fertilizing power of arbacia sperm as indicator of radiation effects. Such experiments have clearly shown the protective action of substances dissolved in the surrounding medium. The interpretation however is complicated by the comparatively enormous 8 A Q 0 Curves are theoretical ; points expt. pg/ml ucose with C.P. 90 pg/ml \ 0 ‘a “ Changing quotient ” for C.P. A Jf dimethylurea with C.P. 30 B 0 glucose with C.P. 30 pg/ml Q conc. i n /ig,ml. \ size of the particles and the existence of cell membranes acting as permeability barriers and introducing surface phenomena.If the substance does not penetrate into the interior of the particle its effect is confined to competitive action in the surrounding fluid and the surface layers of the particle and can be understood by assuming that these surfaces play an active part in the maintenance of the bio-logical activities of the cell. A full explanation of the mechanism will therefore, require an analysis of permeability as well as an investigation of the question whether such surfaces can be the link between radiation and the effects observed. f reproduced by permission of the British J . Cancer WALTER M . D A L E 299 In contrast to systems of suspended particles is the condition of gels in which a colloid forms a continuous network of micelles between which the solvent circulates. Zt is important from the biological point of view that no protective action by the gelatinous network was observed by Skoog17 and Gordon and Quastler 18 when they irradiated Auxin in agar blocks or by Day and Stein 9 in their experiments with methylene blue in gelatine gels. These systems are of biological importance as models of the inhomogeneous physical structure of the interior of cells referred to in our experiments.13 1 Dainton and Holmes Nature 1950 165 266. 2 Evans and Urj Nature 1950 166 602. 3 Haissinsky and Lefort Compt. rend. 1950 230 1156; J . Chim. Phys. 1950 47 588. 4 Weiss Reunion Ann. SOC. Chim. Phys. (1951). 5 Fricke Cold Spring Harbor Symp. 1935 6 164. 6 Fricke Hart and Smith J . Chem. Physics 1938 6 229. 7 Fricke Symp. 4 (1950) Sept. 18-20 Army Medical Center Md.). 8 Fricke and Brownscornbe J . Amer. Chem. Soc. 1933 55 2358. 9 Stein and Day Nature 1950 166 146. 10 Forssberg Nature 1947 159 308. 11 Dale Brit. J. Rad. 1943 16 171. 12 Dale Biochem. J. 1942 36 80. 13 Dale Brit. J. Rad. 1947 Suppl. 1 46. 14 Dale Davies and Meredith Brit. J . Cancer 1949 3 31. 15 Burnett Stapleton Morse and Hollaender Proc. SOC. Expt. Biol. Med. 1951,77 636. 16 Evans Slaughter Little and Failla Radiol. 1942 39 663. 17 Skoog J. Cell. Cohp. PJiysiol. 1935 7 227. 18 Gordon and Quastler (private communication)

ISSN:0366-9033    

DOI:10.1039/DF9521200293

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

WALTER M . D A L E 299 EFFECTS OF RADIATIONS ON AQUEOUS SOLUTIONS OF INDOLE BY C. B. ALLSOPP AND MISS J. WILSON Guy’s Hospital Medical School London S.E. 1 Received 24th January 1952 As estimated with Ehrlich’s reagent indole appears to be readily decomposed by small doses of X-radiation (i.e. up to 20 r). The quantity decomposed increases with (i) in- creasing concentration of indole and (ii) decreasing dosage-rate. The initial decomposi- tion is rather greater in solutions prepared and irradiated in an atmosphere of nitrogen than in aerated solutions the subsequent reaction appears to be quantitatively the same. The radiation-chemical and photo-chemical decompositions of indole (in presence of air) follow similar courses. These effects are considered to be consistent with an initial oxidation to indole-5 6-quinone followed by oxidative condensation of this substance with unchanged indole.Preliminary accounts have already been given of the sensitivity of aqueous solutions of indole to gamma rays 1 and to soft X-rays.2 The results described here include the effects of X-rays half-value layer 1.2 mm of aluminium over the concentration range from 3-0 to 130 mg/l. of indole and the dosage-rate range from 2 to 63 rontgens/min in the presence and absence of oxygen together with some observations on the effects of monochromatic radiation of wave-length 2537A on indole solutions exposed to air. SOLUTIONS OF INDOLE 300 EXPERIMENTAL A N D RESULTS The solutions were exposed to X-rays in small stoppered glass tubes containing 1 rnl under conditions already described.2 Indole was estimated absorptiometrically with Ehrlich’s reagent (pdirnethylaminobenzaldehyde).3 The X-ray tube is air-cooled and has a maximum running time of 10 min before becoming overheated this factor limited the range of both dose and dosage-rate which could conveniently be employed.Since the irradiations were made under standardized conditions throughout the doses are expressed in rontgens as measured in air with a Victoreen dosemeter. They are propor- tional to the energy dissipated in the irradiated solutions. Each point on the curves of fig. 1 to 5 represents the mean of at least six separate observations and the statistical error is covered by the size of each point. VARIATION OF THE QUANTITY OF INDOLE DECOMPOSED WITH X-RAY DOSE.-Fig.1 Shows two curves representing the largest and smallest effects which we have observed which relate the quantity of indole changed per rontgen to the total dose in rontgens. These curves are characteristic of all the experiments. For very small doses there appears to FIG. 1.-Variation of quantity of indole changed per rontgen with total dose in rijntgens (a) indole concentration 100 mg/l. ; dose-rate 2.8 r/min ; (6) indole concentration 30 mg/l. ; dose-rate 70 r/min. be a rapid decomposition but the quantity decomposed per rontgen falls off at first rapidly and later for doses above about 10 r more slowly. Over the range of doses studied it never becomes constant as it should if at any stage the quantity decomposed were proportional to the total dose.The initial parts of the curves correspond to the decomposition of several hundred molecules of indole for each ion-pair formed in the solution. This result has been confirmed by direct measurements of the change in intensity of the absorption band of indole at 2700 8 during irradiation. The mean of three spectro- photometric determinations of the decomposition in a solution containing 130 mg/l. caused by 2-8 delivered in 1 min was 6.2 % as compared with 7.2 % estimated absorptio- metrically with Ehrlich’s reagent. The difference is less than the experimental error in the spectrophotometric determination. EFFECT OF DOSAGE RATE.-The effect of dosage rate is shown in fig. 2 in which the quantity of indole decomposed by a dose of 1 r is plotted against dosage rate in r per min for a solution containing 30 mg/l.of indole. The decomposition is greatest at the lowest dosage rates. It decreases by about 80 % between rates of 2 and 40 r/min but then appears to be approaching a constant value. EFFECT OF CONCENTRATION.-NO simple quantitative relationship has been deduced between the decomposition observed at the smallest dose measured and the concentration of indole although the graph relating them is a smooth curve. In fig. 3 it is plotted for convenience on logarithmic scales. The maximum decomposition of indole per rontgen = 1 r. C . B . ALLSOPP AND MISS J. WILSON increases very rapidly with the concentration in the curve shown by a factor of 100 over the concentration range studied.EFFECT OF OXYGEN.-In the experiments described so far no attempt was made to exclude oxygen from the solutions. Experiments have however been made in solutions in which the pressure of oxygen was reduced. For this purpose the manipulations were FIG. 2. - Variation with dosage-rate of the quantity of indole changed by one rontgen indole concentra- tion = 30 mg/l. ; total dose FIG. 3.-Variation with concentration of the quantity of indole changed per rontgen dosage-rate = 2-8 r/niin ; total dose = 2.8 r. I .?.5 x 10-5 carried out by remote control under a sealed bell-jar in an atmosphere of nitrogen. The indole was previously weighed into a graduated flask and placed under the bell-jar so that air could be swept out by a current of nitrogen before the oxygen-free water was brought to it.Nitrogen was bubbled through the solution in the atmosphere of nitrogen until all the indole was dissolved which required about four days. 1 rnl portions were trans- ferred to irradiation tubes which were stoppered all without any exposure to air. The nitrogen-filled solutions were irradiated simultaneously with ordinary solutions and the irradiated specimens were analyzed side by side. K* 301 302 SOLUTIONS OF INDOLE Typical results are shown in fig. 4. For very small doses the quantity of indole de- composed was always a few per cent higher at the reduced oxygen pressure at higher doses there was little significant difference. The nature of this difference did not seem to FIG.4.-Effect of excluding oxygen on X-ray decomposition of indole indole concentration = 30 mg/l. Full line = oxygen present ; broken line = irradiation in nitrogen ; dosage-rate (a) 70 r/min ; (6) 25 r/min. fild dose punto absorbed ,200 I iQ0 xi0 IS FIG. 5.-Photochemical decomposition of indole indole concentration = 30 mg/l. quanta absorbed per second under each cm2 of exposed surface (a) 1.0 x 1014; (b) 4.0 x 1014. justify any more elaborate measures to exclude oxygen. It is most probably due to in- creased stability of the unirradiated indole solution in the absence of oxygen indole being known to undergo slow auto-oxidation. No significant difference was observed between the effects produced when the indole solutions were irradiated simultaneously in Pyrex and fused silica tubes respectively.303 PHOTOCHEMISTRY OF INDOLE.-The solutions were exposed to various intensities of ultra-violet radiation X = 2537A in shallow layers in open Petri dishes placed under a neon-sensitized mercury lamp 4 under conditions described elsewhere.5 Incident light intensities in quanta/cm2 sec were measured by the uranyl oxalate actinometer method of Leighton and Forbes.6 The results from two experiments are shown in fig. 5 where for comparison with fig. 1 the decomposition is plotted as milligrams of indole decomposed per 1020 quanta absorbed against the total number of quanta absorbed this latter number being deduced from the incident intensity the molecular extinction coefficient E = 1000 at X = 2537 A,7 and the geometry of the irradiated solution.The general form of the curves is similar to those of fig 1 and they have the same feature that the greater decom- position is caused at the lower intensity. No smooth curve corresponding to fig. 2 has however been obtained in the photochemical experiments. (IV) isatin (I) indole C. R. ALLSOPP AND MISS J . WILSON (11) indoxyl (V) indole 5 6-quinone DISCUSSION In interpreting these results the following considerations are relevant (i) Ehrlich’s reagent is not specific for indole but is a general reagent for pyrrole groups; the almost steady concentration which is recorded at the higher X-ray doses thus indicates that the final product of the chemical reactions contains sub- stantially unchanged pyrrole.(ii) At the concentrations which have been used the action of the X-rays must be indirect and the chemical reactions are most probably initiated through the production of hydroxyl and hydrogen radicals from the water ; in the presence of oxygen other oxidizing radicals may also be present. (iii) In irradiated solutions the effect of oxidizing radicals usually pre- dominates 8 and the reactions now observed may be expected to be oxidative in character. (iv) The very large ionic yields in the early stages of the reactions suggest that some form of chain mechanism is involved. Indole can be oxidized either on the pyrrole ring e.g. at the 2- or 3-positions or on the phenyl ring at the 5- or 6-positions. An investigation of the first alter- native was made possible by a gift of materials from I.C.I.Ltd. Indole (I) and indoxyl (11) gave red colours with Ehrlich’s reagent. Dioxindole (111) prepared from isatin by reduction with alkaline sodium hydrosulphite,9 and isatin (IV) did not. The ability to produce the characteristic red colour with Ehrlichs reagent disappears with the double bond between the 2- and 3-carbon atoms. Indoxyl is extremely unstable and its formation could not possibly account for the apparent stability of indole to larger radiation doses. The oxidation deriva- tives of indole in the 2- and 3-positions readily oxidize to indigo; but no blue colour has been observed in any irradiated solution. It seems improbable therefore that this type of oxidation could account for our observations.(111) dioxindole Indole derivatives in which oxidation has taken place at the 5- or 6-positions as in the quinone (V) occur during the enzymatic oxidation of tyrosine,lo and there is spectroscopic evidence that they are also present among the products of photochemical oxidation of tyrosine.7 Harley-Mason 11 has described reactions between quinones and indoles to form compounds such as 4 3’-indolyl-1 2- benzoquinone (VI) the condensation occurring at the 3-position. He has put (VI) 4 3’4ndolyl-1 2-benzoquinone 304 forward strong evidence that a melanin may be produced by polymerization of indole-5 6-quinone (V) by repeated oxidative condensation involving the 3- position of one molecule and the 4- or 7-positions of another as in (VII).Tt is significant that such a polymerization would not destroy the pyrrole rings and so reaction with Ehrlich's reagent would still be possible although we have not as yet been able to test this. As the size of the molecule increases the nature of the Ehrlich colour would be slightly modified such changes were described in our previous paper.' To explain our present results therefore we suggest that oxidation of indole to 5 6-dihydroxy-indole or to indole-5 6-quinone might perhaps be the first stage in these radiation-induced reactions and that this is followed by polymer- ization. The largest effect is produced at lowest intensities of X-rays and at highest concentrations. A low intensity of X-rays would produce a low initial concentra- tion of hydroxyl radicals and the oxidized molecules would be formed relatively far apart from one another.The most likely polymerization since indole seems to disappear most at higher concentrations would then be an oxidative condensa- tion of the quinone with unchanged indole which could be propagated as in (VII) although condensation of quinone with quinone is not excluded. The suggestion is a tentative one and much further work will be necessary before this mechanism can be confirmed but as a working hypothesis it does not appear to be incon- sistent with the observations so far recorded. From the biological viewpoint the explanation is attractive in that it suggests that the pigmentation resulting from exposure to ionizing radiations may originate in the chemical degeneration of protein leading ultimately to melanin formation.Our study of the effects of X-rays on other indole compounds may throw light on this. Our results also suggest that it might be of value to re-examine with low intensities and low doses of ionizing radiation some of the substances of biological interest which appear to be " radiation resistant " when subjected to large intense doses. These experiments form part of a programme of research financed by the British Empire Cancer Campaign to which body the authors express their thanks. They are also indebted to I.C.I. Ltd. for a gift of materials and to Dr. M. A. T. Rogers and to Dr. F. H. Brain for advice on the preparation of indoxyl and dioxindole. 4 Melville Trans. Faraday Soc.1936 32 1525. SOLUTIONS OF INDOLE (VII) suggested polyrncrization of indole-5 6-quinone (Harley -Mason) 1 Allsopp Medical R4.r. Corrricil Reports (Metlical U.vcs of Rarlirrni) 1937 no. 2.32 12 ; 1938 no. 236 1 I. 2 Allsopp and Wilson f. Cfiiin. Phys. 1951 48 195. 3 Allsopp Biocfiem. f. 1941 35 965. 5 Allsopp and Szigeti Cancer Res. 1946 6 14. 305 C . B . ALLSOPP AND MISS J . WILSON 6 Leighton and Forbes J . Ameu. Chern. SOC. 1930 52 3139. 7 Allsopp unpublished work. 8 see for instance Haissinsky and Lefort Cmnpt. rend. 1950 230 1156. 9 Maschalk Rer. 1912 45 582. 10 see Raper J . Chem. SOC. 1938 1952. 11 see Bu’Lock and Harley-Mason J. C h n . Suc. 1951 703. WALTER M . D A L E 299 EFFECTS OF RADIATIONS ON AQUEOUS SOLUTIONS OF INDOLE BY C.B. ALLSOPP AND MISS J. WILSON Guy’s Hospital Medical School London S.E. 1 Received 24th January 1952 As estimated with Ehrlich’s reagent indole appears to be readily decomposed by small doses of X-radiation (i.e. up to 20 r). The quantity decomposed increases with (i) in-creasing concentration of indole and (ii) decreasing dosage-rate. The initial decomposi-tion is rather greater in solutions prepared and irradiated in an atmosphere of nitrogen than in aerated solutions the subsequent reaction appears to be quantitatively the same. The radiation-chemical and photo-chemical decompositions of indole (in presence of air) follow similar courses. These effects are considered to be consistent with an initial oxidation to indole-5 6-quinone followed by oxidative condensation of this substance with unchanged indole.Preliminary accounts have already been given of the sensitivity of aqueous solutions of indole to gamma rays 1 and to soft X-rays.2 The results described here include the effects of X-rays half-value layer 1.2 mm of aluminium over the concentration range from 3-0 to 130 mg/l. of indole and the dosage-rate range from 2 to 63 rontgens/min in the presence and absence of oxygen together with some observations on the effects of monochromatic radiation of wave-length 2537A on indole solutions exposed to air 300 SOLUTIONS OF INDOLE EXPERIMENTAL A N D RESULTS The solutions were exposed to X-rays in small stoppered glass tubes containing 1 rnl under conditions already described.2 Indole was estimated absorptiometrically with Ehrlich’s reagent (pdirnethylaminobenzaldehyde).3 The X-ray tube is air-cooled and has a maximum running time of 10 min before becoming overheated this factor limited the range of both dose and dosage-rate which could conveniently be employed.Since the irradiations were made under standardized conditions throughout the doses are expressed in rontgens as measured in air with a Victoreen dosemeter. They are propor-tional to the energy dissipated in the irradiated solutions. Each point on the curves of fig. 1 to 5 represents the mean of at least six separate observations and the statistical error is covered by the size of each point. two curves representing the largest and smallest effects which we have observed which relate the quantity of indole changed per rontgen to the total dose in rontgens.These curves are characteristic of all the experiments. For very small doses there appears to VARIATION OF THE QUANTITY OF INDOLE DECOMPOSED WITH X-RAY DOSE.-Fig. 1 Shows FIG. 1.-Variation of quantity of indole changed per rontgen with total dose in rijntgens : (a) indole concentration 100 mg/l. ; dose-rate 2.8 r/min ; (6) indole concentration 30 mg/l. ; dose-rate 70 r/min. be a rapid decomposition but the quantity decomposed per rontgen falls off at first rapidly and later for doses above about 10 r more slowly. Over the range of doses studied it never becomes constant as it should if at any stage the quantity decomposed were proportional to the total dose. The initial parts of the curves correspond to the decomposition of several hundred molecules of indole for each ion-pair formed in the solution.This result has been confirmed by direct measurements of the change in intensity of the absorption band of indole at 2700 8 during irradiation. The mean of three spectro-photometric determinations of the decomposition in a solution containing 130 mg/l. caused by 2-8 delivered in 1 min was 6.2 % as compared with 7.2 % estimated absorptio-metrically with Ehrlich’s reagent. The difference is less than the experimental error in the spectrophotometric determination. EFFECT OF DOSAGE RATE.-The effect of dosage rate is shown in fig. 2 in which the quantity of indole decomposed by a dose of 1 r is plotted against dosage rate in r per min for a solution containing 30 mg/l. of indole. The decomposition is greatest at the lowest dosage rates.It decreases by about 80 % between rates of 2 and 40 r/min but then appears to be approaching a constant value. EFFECT OF CONCENTRATION.-NO simple quantitative relationship has been deduced between the decomposition observed at the smallest dose measured and the concentration of indole although the graph relating them is a smooth curve. In fig. 3 it is plotted for convenience on logarithmic scales. The maximum decomposition of indole per rontge C . B . ALLSOPP AND MISS J. WILSON 301 increases very rapidly with the concentration in the curve shown by a factor of 100 over the concentration range studied. EFFECT OF OXYGEN.-In the experiments described so far no attempt was made to exclude oxygen from the solutions.Experiments have however been made in solutions in which the pressure of oxygen was reduced. For this purpose the manipulations were FIG. 2. - Variation with dosage-rate of the quantity of indole changed by one rontgen indole concentra-tion = 30 mg/l. ; total dose = 1 r. .?.5 x 10-5 I FIG. 3.-Variation with concentration of the quantity of indole changed per rontgen dosage-rate = 2-8 r/niin ; total dose = 2.8 r. carried out by remote control under a sealed bell-jar in an atmosphere of nitrogen. The indole was previously weighed into a graduated flask and placed under the bell-jar so that air could be swept out by a current of nitrogen before the oxygen-free water was brought to it. Nitrogen was bubbled through the solution in the atmosphere of nitrogen until all the indole was dissolved which required about four days.1 rnl portions were trans-ferred to irradiation tubes which were stoppered all without any exposure to air. The nitrogen-filled solutions were irradiated simultaneously with ordinary solutions and the irradiated specimens were analyzed side by side. K 302 SOLUTIONS OF INDOLE Typical results are shown in fig. 4. For very small doses the quantity of indole de-composed was always a few per cent higher at the reduced oxygen pressure at higher doses there was little significant difference. The nature of this difference did not seem to FIG. 4.-Effect of excluding oxygen on X-ray decomposition of indole : indole concentration = 30 mg/l. Full line = oxygen present ; broken line = irradiation in nitrogen ; dosage-rate (a) 70 r/min ; (6) 25 r/min.fild dose punto absorbed I iQ0 ,200 xi0 IS FIG. 5.-Photochemical decomposition of indole : indole concentration = 30 mg/l. quanta absorbed per second under each cm2 of exposed surface : (a) 1.0 x 1014; (b) 4.0 x 1014. justify any more elaborate measures to exclude oxygen. It is most probably due to in-creased stability of the unirradiated indole solution in the absence of oxygen indole being known to undergo slow auto-oxidation. No significant difference was observed between the effects produced when the indole solutions were irradiated simultaneously in Pyrex and fused silica tubes respectively C. R. ALLSOPP AND MISS J . WILSON 303 PHOTOCHEMISTRY OF INDOLE.-The solutions were exposed to various intensities of ultra-violet radiation X = 2537A in shallow layers in open Petri dishes placed under a neon-sensitized mercury lamp 4 under conditions described elsewhere.5 Incident light intensities in quanta/cm2 sec were measured by the uranyl oxalate actinometer method of Leighton and Forbes.6 The results from two experiments are shown in fig.5 where for comparison with fig. 1 the decomposition is plotted as milligrams of indole decomposed per 1020 quanta absorbed against the total number of quanta absorbed this latter number being deduced from the incident intensity the molecular extinction coefficient E = 1000, at X = 2537 A,7 and the geometry of the irradiated solution. The general form of the curves is similar to those of fig 1 and they have the same feature that the greater decom-position is caused at the lower intensity.No smooth curve corresponding to fig. 2 has, however been obtained in the photochemical experiments. DISCUSSION In interpreting these results the following considerations are relevant : (i) Ehrlich’s reagent is not specific for indole but is a general reagent for pyrrole groups; the almost steady concentration which is recorded at the higher X-ray doses thus indicates that the final product of the chemical reactions contains sub-stantially unchanged pyrrole. (ii) At the concentrations which have been used, the action of the X-rays must be indirect and the chemical reactions are most probably initiated through the production of hydroxyl and hydrogen radicals from the water ; in the presence of oxygen other oxidizing radicals may also be present.(iii) In irradiated solutions the effect of oxidizing radicals usually pre-dominates 8 and the reactions now observed may be expected to be oxidative in character. (iv) The very large ionic yields in the early stages of the reactions suggest that some form of chain mechanism is involved. Indole can be oxidized either on the pyrrole ring e.g. at the 2- or 3-positions, or on the phenyl ring at the 5- or 6-positions. An investigation of the first alter-native was made possible by a gift of materials from I.C.I. Ltd. Indole (I) and indoxyl (11) gave red colours with Ehrlich’s reagent. Dioxindole (111) prepared from isatin by reduction with alkaline sodium hydrosulphite,9 and isatin (IV) did not. The ability to produce the characteristic red colour with Ehrlichs reagent disappears with the double bond between the 2- and 3-carbon atoms.Indoxyl is extremely unstable and its formation could not possibly account for the apparent stability of indole to larger radiation doses. The oxidation deriva-tives of indole in the 2- and 3-positions readily oxidize to indigo; but no blue colour has been observed in any irradiated solution. It seems improbable therefore that this type of oxidation could account for our observations. (I) indole (11) indoxyl (111) dioxindole (IV) isatin Indole derivatives in which oxidation has taken place at the 5- or 6-positions, as in the quinone (V) occur during the enzymatic oxidation of tyrosine,lo and there is spectroscopic evidence that they are also present among the products of photochemical oxidation of tyrosine.7 Harley-Mason 11 has described reactions between quinones and indoles to form compounds such as 4 3’-indolyl-1 2-benzoquinone (VI) the condensation occurring at the 3-position.He has put (V) indole 5 6-quinone (VI) 4 3’4ndolyl-1 2-benzoquinon 304 SOLUTIONS OF INDOLE forward strong evidence that a melanin may be produced by polymerization of indole-5 6-quinone (V) by repeated oxidative condensation involving the 3-position of one molecule and the 4- or 7-positions of another as in (VII). Tt is significant that such a polymerization would not destroy the pyrrole rings and so reaction with Ehrlich's reagent would still be possible although we have not as yet of the in our To been able to test this.As the size of the molecule increases the nature Ehrlich colour would be slightly modified such changes were described previous paper.' (VII) suggested polyrncrization of indole-5 6-quinone (Harley -Mason) explain our present results therefore we suggest that oxidation of indole to 5 6-dihydroxy-indole or to indole-5 6-quinone might perhaps be the first stage in these radiation-induced reactions and that this is followed by polymer-ization. The largest effect is produced at lowest intensities of X-rays and at highest concentrations. A low intensity of X-rays would produce a low initial concentra-tion of hydroxyl radicals and the oxidized molecules would be formed relatively far apart from one another. The most likely polymerization since indole seems to disappear most at higher concentrations would then be an oxidative condensa-tion of the quinone with unchanged indole which could be propagated as in (VII), although condensation of quinone with quinone is not excluded.The suggestion is a tentative one and much further work will be necessary before this mechanism can be confirmed but as a working hypothesis it does not appear to be incon-sistent with the observations so far recorded. From the biological viewpoint the explanation is attractive in that it suggests that the pigmentation resulting from exposure to ionizing radiations may originate in the chemical degeneration of protein leading ultimately to melanin formation. Our study of the effects of X-rays on other indole compounds may throw light on this. Our results also suggest that it might be of value to re-examine with low intensities and low doses of ionizing radiation some of the substances of biological interest which appear to be " radiation resistant " when subjected to large intense doses. These experiments form part of a programme of research financed by the British Empire Cancer Campaign to which body the authors express their thanks. They are also indebted to I.C.I. Ltd. for a gift of materials and to Dr. M. A. T. Rogers and to Dr. F. H. Brain for advice on the preparation of indoxyl and dioxindole. 1 Allsopp Medical R4.r. Corrricil Reports (Metlical U.vcs of Rarlirrni) 1937 no. 2.32 12 ; 2 Allsopp and Wilson f. Cfiiin. Phys. 1951 48 195. 3 Allsopp Biocfiem. f. 1941 35 965. 4 Melville Trans. Faraday Soc. 1936 32 1525. 5 Allsopp and Szigeti Cancer Res. 1946 6 14. 1938 no. 236 1 I C . B . ALLSOPP AND MISS J . WILSON 6 Leighton and Forbes J . Ameu. Chern. SOC. 1930 52 3139. 7 Allsopp unpublished work. 8 see for instance Haissinsky and Lefort Cmnpt. rend. 1950 230 1156. 9 Maschalk Rer. 1912 45 582. 10 see Raper J . Chem. SOC. 1938 1952. 11 see Bu’Lock and Harley-Mason J. C h n . Suc. 1951 703. 30

ISSN:0366-9033    

DOI:10.1039/DF9521200299

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

305 C . B . ALLSOPP AND MISS J . WILSON RADIATION CHEMISTRY OF ORGANIC SOLUTIONS BY W. MINDER AND H. HEYDRICH Radium Institute and X-Ray Dept. of the University of Berne Switzerland Received 21st January 1952 From halogenated hydrocarbons irradiated in organic solutions (alcohol acetone) halogen acids can be extracted with water. The amount of irradiation products rises proportionally to the dose of radiation and is dependent on (i) the concentration of the solution (ii) the number of halogen atoms in the compound and (iii) the type of binding The yields are about & to & that found in aqueous solutions but are not influenced at all by the addition of large amounts of water. The relation between yield and concentration is exponential in form rising to a saturation value.In compounds containing many halogen atoms a proportional increase is also found. These results are discussed and compared with theoretical conceptions. As has been previously demonstrated by many workers the biological actions of ionizing radiations are very complicated and difficult to understand ; because of this the radiation-chemical changes in aqueous systems of all types and range of concentrations are of fundamental interest. Almost all radiation-chemical investigations connected with medical or biological problems therefore started from this rather restricted aspect. It seems quite well established that the radiation- chemical behaviour of aqueous solutions can be understood in terms of the radical theory of Weiss.17~ 18 This theory is able to explain qualitatively not only the so-called dilution,s.16 protection 39 15 and saturation effects 2 but allows quantitative predictions about all these fundamental effects in aqueous systems.l.2.18 The theory however is probably restricted to aqueous solutions as it is unlikely that radiation effects in non-aqueous systems can be brought about qualitatively and quantitatively by radicals in the same manner as in aqueous systems. The radical theory has as yet not been confirmed by conclusive experiments and it is difficult to do so because it is necessary to obtain quantitative estimations of the number of radicals primarily formed their spatial distributions life-times and also quantitative agreement for all of the energy changes from the primary photon or particle to the final stable products.Nevertheless this theory is the best starting point for all radiation-chemical work and there are no known experi- mental facts which cast serious doubt on its validity.9~14 It seemed of interest to perform irradiation experiments with non-aqueous systems and to compare the results with those obtained with aqueous solutions. During the past 7 years we have studied the formation of halogen acids by irradi- ation of aqueous solutions of halogenated hydrocarbons. These experiments have given some results of general interest which we have explained in terms of existing or new theoretical conceptions.4- 6 7 9 9 10,129 15 It was found that the formation of halogen acids from aqueous solutions of halogenated hydro- carbons is a so-called indirect irradiation effect which is independent of con- centration over a rather wide range,49 12 and which shows the protection effect 12915 ORGANIC SOLUTIONS 306 when a new component is added and which is almost completely suppressed in frozen solutions.** 12914 The results have been expressed in terms of the ionic yield M/N which is the number of molecules of the measured compound formed by the radiation energy of 32.5 eV necessary for one ionization in air.We think that this figure is preferable to the figure G (corresponding to 100 eV) because the latter has to be calculated from the former which is more directly obtained in radiation dose measurements. We choose these halogenated compounds for our experiments first because it is not difficult to measure the most important effect the formation of halogen acids.with sufficient accuracy in very small amounts using the increase of the ionic solute. measurements is the high purity of the solvent (water) and the requirement solutions. specific yield conductivity of M/N - of 1 the radiation irradiated effects of 1000 The r can only be determined With with of such an an accuracy of at least 10 %. Unfortunately radiation-chemical products other than electrolytes are not detected by this method. Therefore our general results have to be restricted to these products and do not allow us to formulate the complete chemical reaction within our systems. Nevertheless these same general results (dilution effect protection effect suppression of radiation effects in solid state) were found with different solutions.It was only after the war that we had access to the results of other workers in the field of radiation chemistry. Consequently we did not explain our results on the radical theory but tried to formulate them in terms of general reaction kinetics,73 9 without regard to the detailed chemical reaction phenomena and it was possible to show that all the slopes of the reaction curves with respect to the radiation energy could be calculated from simple kinetic conceptions. The radical theory of the effects of radiations on aqueous solutions involves the reaction of free radicals primarily formed from water with the solute.We thought it interesting to perform irradiation experiments with our substances dissolved in solvents other than water. Many simple halogenated hydrocarbons are easily soluble in alcohol. Very pure alcoholic solutions of our compounds were irradiated with X-rays (31.5 kV beryllium window tube) with doses up to 106 r integrated over the whole sample. After irradiation the samples were diluted with very pure water and the irradiation product the halogen acid was estimated quantitatively from the specific conductivity of this solution and determined qualitatively by precipitation as the silver salt. Such experiments were made with 3 D.D.T. products viz. p-methyldiphenyl- trichlorethane (CH3 . C6H4)2 . CH . CCl3 1 2 dimethyldiphenyltrichlorethane [(CH3)2C6H3]2 .CH . CC13 p-chlordiphenylchlorethylene (p-C1 . C6H4)2 . C= CHCI y-hexachlorocyclohexane y-C6H&16 p-dichlorbenzene p-C6H&12 hexachlor- ethane C2C16 and chloroform CHC13 at different known concentrations in the range of M/5-M/10 down to M/1000.11*12~13 In every case the increase of conductivity in the aqueous extract was due to the formation of electrolytes by irradiation the greatest part of these being halogen acids. Other experiments were made with acetone solutions. In this solvent the formation of electrolytes has also been found but in smaller amounts than in alcohol. In fig. 1 the results with hexachlorethane in alcohol and acetone are shown. that The other investigations show the same general behaviour. It is clear (i) The amount of the irradiation product (hydrochloric acid) increases pro- portionally with the amount of radiation at all concentrations.(ii) The yield of the radiation-chemical product depends on the concentra- tion of the solute but a simple proportionality is found only with very small concentrations. (iii) In acetone as solvent the yield is not proportional to the concentration. Taking other experiments 11’ 12913 into account W . MINDER A N D H . HEYDRICH (iv) If water is added to the solvent in concentrations corresponding to 12.5 or 25 vol. % no change in the yield is found. FIG. 1.-Formation of HCl from C2Cl6 by irradiation of alcoholic solutions at differen concentrations. Dotted lines c2c16 diluted in acetone. Indications in % (g/lOO ml).I i f i 6 r 10 I t5 FIG. 2.-Formation of electrolytes by irradiation of pure alcohol and alcohol containing water. (1) Pure alcohol and alcohol containing up to 50 % H20. (2) Alcohol diluted in 90 vol. % H20. Dotted line 1 pure acetone. (v) In pure alcohol and pure acetone electrolytes are formed in small quanti- ties by irradiation. The yield is about 20 % smaller in acetone. (vi) Relatively large concentrations of water up to 50 vol. % do not change the yield of electrolytes in irradiated alcohol. Only at water concentrations greater than 75 % can a fall in yield be detected this fall being only about 30 % at water concentrations of 90 vol. % (fig. 2). 307 p [Z,] = ORGANIC SOLUTIONS cc13 concentrations and slowly at higher concentrations.It has been shown that the slope of all these curves can be expressed with a high precision by the following equation 11- 13 2 = Zo(1 - e-OC) + kc where 2 is the ionic yield and 20 u and k are constants having the following dimensions molecules of reaction product 32-5 eV [kl = [ cm3 x molecules of reaction product- I molecules of solute x 32.5 eV - ’ The above equation contains two terms one rising exponentially with con- Therefore and the other increasing linearly. 308 (vii) The ionic yield of all solutions investigated depends on concentration but is found to be between 0.2 and 2 in concentrations of about M/lO. The results of the irradiations of the above compounds dissolved in alcohol have been plotted in fig.3. This graph shows the ionic yield ( i s . the number of HCl molecules formed in the solution by a radiation energy of 32.5 eV equal to one ionization in air) as a function of the concentrations c of the solute. All the curves shown are of the same general form. The yield rises sharply at small FIG. 3.-Relation between ionic yield M/N and concentration of different compounds irradiated in alcoholic solutioc. centration to a constant value &) 309 TABLE 1 .-CALCULATION 0.008 0.180 0.327 0.545 0.794 0.957 0.990 ( y ~ w ~ l c 25.0 25.6 20.8 15.5 10.1 7.1 5.1 3% 0.20 0.42 0.67 1.00 1-30 1.80 2.60 0.016 0.032 0.064 0.128 0.255 0.510 W . MINDER AND H. HEYDRICH the yield against concentration curve can be represented by summing an ex- ponential curve and a straight line.We have suggested in earlier publications that the exponentially rising part may be caused by an indirect irradiation effect while the linear part which is only significant at higher concentrations is due to the direct action of radiation on the solute. In table 1 the calculated values of the ionic yield of C2C16 in alcohol are compared with the experimental results. OF THE IONIC YIELD FOR DIFFERENT CONCENTRATIONS OF c2c16 1N ALCOHOLIC SOLUTION ionic yield ionic yield kC 0.025 0.050 0.099 0.199 0.398 0.797 1-592 calc. Z = MIN 0-205 0.377 0.644 0.993 1-355 1.786 2.592 This table was calculated using the following values 1-00 20 = 1-00 HC1/32.5 eV k = 3-13 X 10-20 cm3 HCI/C2C&j .32.5 ev. cr = 25 x 10-20 cm3/C2C16 The agreement between calculation and experiment is as good as with other substances which have been investigated in the same manner. We therefore believe that the formulation is a possible manner of representing the behaviour of our compounds when irradiated in alcoholic solutions. What therefore is the physical significance of the constants used above? The constant 20 is the maximum number of molecules of the reaction product which can be formed by indirect action by the energy of 32.5 eV compared with one ionization in air and 2 is this number (the ionic yield) at different concentra- tions. On the other hand k is the factor by which the yield of the direct irradi- ation effect increases with concentration.It has the same dimension as Z/c indicated in table 1 i.e. the number of molecules of the reaction product formed by the energy of 32.5 eV per one molecule of the solute in 1 cm3 (ionic yield per unit concentration). For the direct irradiation effect k is constant while the corresponding term a(& - 2) decreases with increasing concentration for the indirect irradiation effect (maximum value 020). In fig. 4 the ionic yields 2 = M/N and the figures (M/N)/c are plotted against concentration of solute on a logarithmic scale. It can be seen from the graph that for small concentrations (M/N)/c is constant indicating that the yield is proportional to concentration. Between the concentration values 0.016 and 0.1 28 (M/N)/c falls exponentially.At higher concentrations the decrease is smaller because the direct irradiation effect is significant. Within a concentration range of 1-32 (M/N)/c decreases by 1-5. The terms [(M/N)/c] E [oZO] f [k] representing the mean number of molecules of the reaction product formed by the energy of 32.5 eV per one molecule of the solute in 1 cm3 of the irradiated solution and also the mean volume per molecule of solute to which the energy of 32.5 eV must be added for the production of 1 molecule of the reaction product. If such volumes are thought to have any reality (cp. absorption phenomena) they have molecular dimensions corresponding to about 2600 alcohol molecules at the lowest to about 500 alcohol molecules at the highest concentrations in- vestigated.The dimension of CT is that of a reciprocal concentration i.e. the volume per molecule of solute for an ionic yield of Z = M/N = 1 caused by the indirect irradiation effect. ORGANIC SOLUTIONS 3 10 It may be of interest to compare the results of experiments plotted in fig. 3 with each other. It can be seen that the yields of the substances investigated are different. Within the concentration range of the experiments a definite direct effect is found only with C2C16 and yC6H6C16 ; with all other compounds such an effect is not proved with certainty. The values of Zo and cr are very close to the product numbers 2 3 4 on the one hand and 6 and 7 on the other. If the yields are compared with each other it is necessary to do so at the same molec- ular concentrations of 0-2 x 1020 or 0.1 x 1020 molecules/cm3 for example.Table 2 shows the corresponding results. zot 2-0 yield/Cl 0.27 0.33 0.30 0.27 4 3 LO.008 0.016 0.032 0-064 O-QB 0255 05/0 yield 1 *oo 1.61 0.90 FIG. 4.-Ionic yield M/N and ionic yield per concentration (M/N)/c at different con- centrations for C2C16 irradiated in alcoholic solution. conc. 0.2 x 1020 no. - compound yield/Cl ~ 2 ~ 1 6 1 2 TABLE 2 conc. 0.1 x 1020 yield 1.20 0.70 0.64 0.20 0.23 0.21 (CH3C6H4)2CHCC13 CHClj ((CH3)2C6H3)2CHCC13 0.18 0.82 0.55 0.46 0.66 0.1 1 5 0.075 In the first four compounds the CI atoms are aliphatic in the last three aromatic in type.As far as the indirect radiation effect is concerned the yield per atom of C1 is practically constant for the aliphatic as well as for the aromatic binding. For the former these figures are about 3 times greater than for the latter. The yield therefore is dependent on the number of C1 atoms in the compound and the type of binding. If the curves of fig. 3 are plotted in terms of the concentrations of CI atoms they are close together for the aliphatic compounds on the one side and for the aromatic on the other. This rule is valid for the concentration range in which the indirect irradiation effect is far greater than the direct one. The compound (CI . C6H&C=CHCl merits special interest. It contains two aromatic C1 atoms and one aliphatic linked to a double-bonded C atom.compound 0.84 0-53 2-52 1.60 CHC13 CHBr3 As far as the dilution effect in aqueous solutions has been investigated i.e. at strictly comparable concentrations the yield from aliphatic compounds is 3 to 4 times greater than from aromatic compounds just as in alcoholic solutions. It is also about 3 to 4 times greater in aqueous solutions. We therefore believe that the reaction mechanisms cannot be fundamentally different in both cases. In table 4 the constants of the above theoretical equation for all substances investigated are calculated. compound no. TABLE 4 1020 20 molecules/ 325 eV 23.5 11.2 1.9 0.48 0.5 W . MINDER AND H. HEYDRICH The yield per Cl atom here is somewhat smaller in all concentrations than for the other aromatic compounds and on the basis of the aromatic C1 atoms only is somewhat greater.The aliphatic C1 atom is therefore bound somewhat more tightly than the aromatic and can only be detached in amounts which are only just detectable. The similarity of the yields per C1 atom of all aliphatic compounds on the one hand and the aromatic on the other cannot be fortuitous. It indicates that our measured irradiation effect (i.e. the increased specific conductivity of the aqueous extract of the irradiated alcoholic solutions) is almost entirely due to the formation of halogen acids if the effect on pure alcohol is subtracted. The yield of acid in more concentrated solutions is many times greater than the formation by irradiation of electrolytes in alcohol and acetone and it is improbable that the two simultaneous reactions are closely connected.The yield of acid is of the same order of magnitude as in aqueous solutions as shown by the values in table 3 which indicate the yields of some compounds in the same concentration range as in table 2. TABLE 3.-IONIC YIELDS OF ACID FORMED BY IRRADIATION OF HALOGEN COMPOUNDS IN WATER yield conc. MI100 yield/Cl 1.7 5 6 7 ?‘-c6H6c16 CI(C6H4)2C= CHCl P-C6H4C12 0.44 3.8 3.8 conc. M/1000 yield 1 *60 1.56 uzo diam. of uZo a;Cod& in A It may be seen from table 4 that the values of 20 are similar for all aliphatic and also for the aromatic compounds. The differences are about 1 2 to 1 3.The coefficients (volumes) cr of all compounds with the same number of C1 atoms are about the same (if only the two aromatically bound C1 atoms of compound 6 are taken into account). The product 020 indicating the increase of the ionic yield at very small concentrations with increasing concentration is more different from one compound to another because the maximum yields are also different. 59.8 33.2 32.0 yield/<=] 0.53 0.52 molecules 175 31 1 1150 196 GENERAL DISCUSSION c - t o ; z-to; -=az,; c r = - 1 I d Z d Z dc 312 The definition of 020 is given in the following expression for very small con- centrations 10 Minder Radiol. Clin. 1947 16 339. 11 Minder Radiol. Clin. 1950 19 277. 12 Minder Brit.J. Rad. 1951 24 435. 13 Minder J. Chim. Phys. 1951 48 423. 14 Minder and Liechti Experentia 1946 2 410. 16 Risse Ergebn. Physiol. 1930 42 228. 17 Weiss Nature 1944 153 748. 18 Weiss Nature 1946 157 584. 20 d c ' The dimension of OZO (see above) is the same as that of (M/N)/c and k and can be considered as a volume. For all substances investigated these virtual or perhaps real volumes have about the same linear dimensions between 30 and 80A and contain between 175 to 2600 molecules of alcohol as indicated in table 4. Their linear dimensions may be considered as the mean distances of " travel " of energy for the observed irradiation effect. With increasing concentration these dimensions decrease as the radiation energy is used for other effects which cannot be detected by the measurement of only one irradiation product.Therefore it is an urgent problem to perform irradiation experiments with the possibility of quantitative detection of all reaction products and with substances of which all molecular properties are quantitatively known; only then do we think that it will be possible to give a general theory of the phenomena governing the actions of ionizing radiations on all kinds of liquid systems. The results above were obtained by means of a special X-ray tube which was acquired through financial assistance given by the Foundation for Research at the University of Berne to which we are greatly obliged. 1 Dainton J. Physic. Chem. 1948 52 490. ZDale Phil. Trans. Roy. SOC. A 1949 242 33.3 Dale Meredith and Tweedie Nature 1943 151 295. 4 Feller Minder and Liechti Radiol. Clin. 1948 17 156. 5 Fricke and Morse Amer. J . Rontgen. 1927 28 426 430. 6 Meister and Minder Radiol. Clin. 1950 19 238. 7 Minder Radiol. Clin. 1946 15 30. 8 Minder Radiol. Clin. 1946 suppl. vol. 15 81. 9 Minder Radiol. Clin. 1947 16 73. 15 Mullis Minder Liechti and Wegmuller Radiol. Clin. 1946 15 295. C . B . ALLSOPP AND MISS J . WILSON 305 RADIATION CHEMISTRY OF ORGANIC SOLUTIONS BY W. MINDER AND H. HEYDRICH Radium Institute and X-Ray Dept. of the University of Berne Switzerland Received 21st January 1952 From halogenated hydrocarbons irradiated in organic solutions (alcohol acetone) halogen acids can be extracted with water. The amount of irradiation products rises proportionally to the dose of radiation and is dependent on (i) the concentration of the solution (ii) the number of halogen atoms in the compound and (iii) the type of binding The yields are about & to & that found in aqueous solutions but are not influenced at all by the addition of large amounts of water.The relation between yield and concentration is exponential in form rising to a saturation value. In compounds containing many halogen atoms a proportional increase is also found. These results are discussed and compared with theoretical conceptions. As has been previously demonstrated by many workers the biological actions of ionizing radiations are very complicated and difficult to understand ; because of this the radiation-chemical changes in aqueous systems of all types and range of concentrations are of fundamental interest.Almost all radiation-chemical investigations connected with medical or biological problems therefore started from this rather restricted aspect. It seems quite well established that the radiation-chemical behaviour of aqueous solutions can be understood in terms of the radical theory of Weiss.17~ 18 This theory is able to explain qualitatively not only the so-called dilution,s. 16 protection 39 15 and saturation effects 2 but allows quantitative predictions about all these fundamental effects in aqueous systems.l.2.18 The theory however is probably restricted to aqueous solutions, as it is unlikely that radiation effects in non-aqueous systems can be brought about qualitatively and quantitatively by radicals in the same manner as in aqueous systems.The radical theory has as yet not been confirmed by conclusive experiments, and it is difficult to do so because it is necessary to obtain quantitative estimations of the number of radicals primarily formed their spatial distributions life-times and also quantitative agreement for all of the energy changes from the primary photon or particle to the final stable products. Nevertheless this theory is the best starting point for all radiation-chemical work and there are no known experi-mental facts which cast serious doubt on its validity.9~14 It seemed of interest to perform irradiation experiments with non-aqueous systems and to compare the results with those obtained with aqueous solutions.During the past 7 years we have studied the formation of halogen acids by irradi-ation of aqueous solutions of halogenated hydrocarbons. These experiments have given some results of general interest which we have explained in terms of existing or new theoretical conceptions.4- 6 7 9 9 10,129 15 It was found that the formation of halogen acids from aqueous solutions of halogenated hydro-carbons is a so-called indirect irradiation effect which is independent of con-centration over a rather wide range,49 12 and which shows the protection effect 1291 306 ORGANIC SOLUTIONS when a new component is added and which is almost completely suppressed in frozen solutions.** 12914 The results have been expressed in terms of the ionic yield M/N which is the number of molecules of the measured compound formed by the radiation energy of 32.5 eV necessary for one ionization in air.We think that this figure is preferable to the figure G (corresponding to 100 eV), because the latter has to be calculated from the former which is more directly obtained in radiation dose measurements. We choose these halogenated compounds for our experiments first because it is not difficult to measure the most important effect the formation of halogen acids. with sufficient accuracy in very small amounts using the increase of the specific conductivity of the irradiated solutions. The only requirement of such measurements is the high purity of the solvent (water) and the solute. With an ionic yield of M/N - 1 radiation effects of 1000 r can be determined with an accuracy of at least 10 %.Unfortunately radiation-chemical products other than electrolytes are not detected by this method. Therefore our general results have to be restricted to these products and do not allow us to formulate the complete chemical reaction within our systems. Nevertheless these same general results (dilution effect protection effect suppression of radiation effects in solid state) were found with different solutions. It was only after the war that we had access to the results of other workers in the field of radiation chemistry. Consequently we did not explain our results on the radical theory but tried to formulate them in terms of general reaction kinetics,73 9 without regard to the detailed chemical reaction phenomena and it was possible to show that all the slopes of the reaction curves with respect to the radiation energy could be calculated from simple kinetic conceptions.The radical theory of the effects of radiations on aqueous solutions involves the reaction of free radicals primarily formed from water with the solute. We thought it interesting to perform irradiation experiments with our substances dissolved in solvents other than water. Many simple halogenated hydrocarbons are easily soluble in alcohol. Very pure alcoholic solutions of our compounds were irradiated with X-rays (31.5 kV beryllium window tube) with doses up to 106 r integrated over the whole sample. After irradiation the samples were diluted with very pure water and the irradiation product the halogen acid, was estimated quantitatively from the specific conductivity of this solution and determined qualitatively by precipitation as the silver salt.Such experiments were made with 3 D.D.T. products viz. p-methyldiphenyl-trichlorethane (CH3 . C6H4)2 . CH . CCl3 1 2 dimethyldiphenyltrichlorethane [(CH3)2C6H3]2 . CH . CC13 p-chlordiphenylchlorethylene (p-C1 . C6H4)2 . C= CHCI, y-hexachlorocyclohexane y-C6H&16 p-dichlorbenzene p-C6H&12 hexachlor-ethane C2C16 and chloroform CHC13 at different known concentrations in the range of M/5-M/10 down to M/1000.11*12~13 In every case the increase of conductivity in the aqueous extract was due to the formation of electrolytes by irradiation the greatest part of these being halogen acids. Other experiments were made with acetone solutions.In this solvent the formation of electrolytes has also been found but in smaller amounts than in alcohol. In fig. 1 the results with hexachlorethane in alcohol and acetone are shown. It is clear that : (i) The amount of the irradiation product (hydrochloric acid) increases pro-portionally with the amount of radiation at all concentrations. (ii) The yield of the radiation-chemical product depends on the concentra-tion of the solute but a simple proportionality is found only with very small concentrations. (iii) In acetone as solvent the yield is not proportional to the concentration. Taking other experiments 11’ 12913 into account : The other investigations show the same general behaviour W . MINDER A N D H . HEYDRICH 307 (iv) If water is added to the solvent in concentrations corresponding to 12.5 or 25 vol.% no change in the yield is found. FIG. 1.-Formation of HCl from C2Cl6 by irradiation of alcoholic solutions at differen concentrations. Dotted lines c2c16 diluted in acetone. Indications in % (g/lOO ml). I f i 6 i r 10 I , t5 p FIG. 2.-Formation of electrolytes by irradiation of pure alcohol and alcohol containing water. (2) Alcohol diluted (v) In pure alcohol and pure acetone electrolytes are formed in small quanti-ties by irradiation. The yield is about 20 % smaller in acetone. (vi) Relatively large concentrations of water up to 50 vol. % do not change the yield of electrolytes in irradiated alcohol. Only at water concentrations greater than 75 % can a fall in yield be detected this fall being only about 30 % at water concentrations of 90 vol.% (fig. 2). (1) Pure alcohol and alcohol containing up to 50 % H20. in 90 vol. % H20. Dotted line 1 pure acetone 308 ORGANIC SOLUTIONS (vii) The ionic yield of all solutions investigated depends on concentration, but is found to be between 0.2 and 2 in concentrations of about M/lO. The results of the irradiations of the above compounds dissolved in alcohol, have been plotted in fig. 3. This graph shows the ionic yield ( i s . the number of HCl molecules formed in the solution by a radiation energy of 32.5 eV equal to one ionization in air) as a function of the concentrations c of the solute. All the curves shown are of the same general form. The yield rises sharply at small FIG. 3.-Relation between ionic yield M/N and concentration of different compounds irradiated in alcoholic solutioc.cc13 concentrations and slowly at higher concentrations. It has been shown that the slope of all these curves can be expressed with a high precision by the following equation 11- 13 where 2 is the ionic yield and 20 u and k are constants having the following dimensions : 2 = Zo(1 - e-OC) + kc, molecules of reaction product 32-5 eV [Z,] = I cm3 x molecules of reaction product-[kl = [ molecules of solute x 32.5 eV - ’ The above equation contains two terms one rising exponentially with con-Therefore centration to a constant value &) and the other increasing linearly W . MINDER AND H. HEYDRICH 309 the yield against concentration curve can be represented by summing an ex-ponential curve and a straight line.We have suggested in earlier publications that the exponentially rising part may be caused by an indirect irradiation effect, while the linear part which is only significant at higher concentrations is due to the direct action of radiation on the solute. In table 1 the calculated values of the ionic yield of C2C16 in alcohol are compared with the experimental results. TABLE 1 .-CALCULATION OF THE IONIC YIELD FOR DIFFERENT CONCENTRATIONS OF c2c16 1N ALCOHOLIC SOLUTION kC ( y ~ w ~ l c ionic yield ionic yield calc. Z = MIN 3% 0.008 0.180 0.025 0-205 0.20 25.0 0.016 0.327 0.050 0.377 0.42 25.6 0.032 0.545 0.099 0.644 0.67 20.8 0.064 0.794 0.199 0.993 1.00 15.5 0.128 0.957 0.398 1-355 1-30 10.1 0.255 0.990 0.797 1.786 1.80 7.1 0.510 1-00 1-592 2.592 2.60 5.1 This table was calculated using the following values : 20 = 1-00 HC1/32.5 eV, cr = 25 x 10-20 cm3/C2C16, k = 3-13 X 10-20 cm3 HCI/C2C&j .32.5 ev. The agreement between calculation and experiment is as good as with other substances which have been investigated in the same manner. We therefore believe that the formulation is a possible manner of representing the behaviour of our compounds when irradiated in alcoholic solutions. What therefore is the physical significance of the constants used above? The constant 20 is the maximum number of molecules of the reaction product, which can be formed by indirect action by the energy of 32.5 eV compared with one ionization in air and 2 is this number (the ionic yield) at different concentra-tions.On the other hand k is the factor by which the yield of the direct irradi-ation effect increases with concentration. It has the same dimension as Z/c indicated in table 1 i.e. the number of molecules of the reaction product formed by the energy of 32.5 eV per one molecule of the solute in 1 cm3 (ionic yield per unit concentration). For the direct irradiation effect k is constant while the corresponding term a(& - 2) decreases with increasing concentration for the indirect irradiation effect (maximum value 020). In fig. 4 the ionic yields 2 = M/N and the figures (M/N)/c are plotted against concentration of solute on a logarithmic scale. It can be seen from the graph that for small concentrations (M/N)/c is constant indicating that the yield is proportional to concentration.Between the concentration values 0.016 and 0.1 28 (M/N)/c falls exponentially. At higher concentrations the decrease is smaller because the direct irradiation effect is significant. Within a concentration range of 1-32 (M/N)/c decreases by 1-5. The terms representing the mean number of molecules of the reaction product formed by the energy of 32.5 eV per one molecule of the solute in 1 cm3 of the irradiated solution and also the mean volume per molecule of solute to which the energy of 32.5 eV must be added for the production of 1 molecule of the reaction product. If such volumes are thought to have any reality (cp. absorption phenomena), they have molecular dimensions corresponding to about 2600 alcohol molecules at the lowest to about 500 alcohol molecules at the highest concentrations in-vestigated.The dimension of CT is that of a reciprocal concentration i.e. the volume per molecule of solute for an ionic yield of Z = M/N = 1 caused by the indirect irradiation effect. [(M/N)/c] E [oZO] f [k 3 10 ORGANIC SOLUTIONS It may be of interest to compare the results of experiments plotted in fig. 3 with each other. It can be seen that the yields of the substances investigated are different. Within the concentration range of the experiments a definite direct effect is found only with C2C16 and yC6H6C16 ; with all other compounds such an effect is not proved with certainty. The values of Zo and cr are very close to the product numbers 2 3 4 on the one hand and 6 and 7 on the other.If the yields are compared with each other it is necessary to do so at the same molec-ular concentrations of 0-2 x 1020 or 0.1 x 1020 molecules/cm3 for example. Table 2 shows the corresponding results. zot 2-0 LO.008 0.016 0.032 0-064 O-QB 0255 05/0 FIG. 4.-Ionic yield M/N and ionic yield per concentration (M/N)/c at different con-centrations for C2C16 irradiated in alcoholic solution. TABLE 2 conc. 0.1 x 1020 conc. 0.2 x 1020 no. compound -1 ~ 2 ~ 1 6 1.20 0.20 1.61 0.27 2 (CH3C6H4)2CHCC13 0.70 0.23 1 *oo 0.33 3 CHClj 0.64 0.21 0.90 0.30 4 ((CH3)2C6H3)2CHCC13 0.55 0.18 0.82 0.27 yield yield/Cl yield yield/Cl 5 0.46 0.075 0.66 0.1 1 In the first four compounds the CI atoms are aliphatic in the last three aromatic in type.As far as the indirect radiation effect is concerned the yield per atom of C1 is practically constant for the aliphatic as well as for the aromatic binding. For the former these figures are about 3 times greater than for the latter. The yield therefore is dependent on the number of C1 atoms in the compound and the type of binding. If the curves of fig. 3 are plotted in terms of the concentrations of CI atoms they are close together for the aliphatic compounds on the one side and for the aromatic on the other. This rule is valid for the concentration range in which the indirect irradiation effect is far greater than the direct one. The compound (CI . C6H&C=CHCl merits special interest. It contains two aromatic C1 atoms and one aliphatic linked to a double-bonded C atom W .MINDER AND H. HEYDRICH 31 1 The yield per Cl atom here is somewhat smaller in all concentrations than for the other aromatic compounds and on the basis of the aromatic C1 atoms only is somewhat greater. The aliphatic C1 atom is therefore bound somewhat more tightly than the aromatic and can only be detached in amounts which are only just detectable. The similarity of the yields per C1 atom of all aliphatic compounds on the one hand and the aromatic on the other cannot be fortuitous. It indicates that our measured irradiation effect (i.e. the increased specific conductivity of the aqueous extract of the irradiated alcoholic solutions) is almost entirely due to the formation of halogen acids if the effect on pure alcohol is subtracted. The yield of acid in more concentrated solutions is many times greater than the formation by irradiation of electrolytes in alcohol and acetone and it is improbable that the two simultaneous reactions are closely connected.The yield of acid is of the same order of magnitude as in aqueous solutions as shown by the values in table 3, which indicate the yields of some compounds in the same concentration range as in table 2. TABLE 3.-IONIC YIELDS OF ACID FORMED BY IRRADIATION OF HALOGEN COMPOUNDS IN WATER conc. MI100 conc. M/1000 compound yield/<=] yield yield/Cl yield CHC13 2-52 0.84 1 *60 0.53 CHBr3 1.60 0-53 1.56 0.52 As far as the dilution effect in aqueous solutions has been investigated i.e. at strictly comparable concentrations the yield from aliphatic compounds is 3 to 4 times greater than from aromatic compounds just as in alcoholic solutions.It is also about 3 to 4 times greater in aqueous solutions. We therefore believe that the reaction mechanisms cannot be fundamentally different in both cases. In table 4 the constants of the above theoretical equation for all substances investigated are calculated. TABLE 4 no. compound 20 molecules/ 1020 uzo diam. of uZo a;Cod& 325 eV in A molecules 5 ?‘-c6H6c16 0.48 23.5 11.2 59.8 1150 6 CI(C6H4)2C= CHCl 0.5 3.8 1.9 33.2 196 7 P-C6H4C12 0.44 3.8 1.7 32.0 175 It may be seen from table 4 that the values of 20 are similar for all aliphatic and also for the aromatic compounds. The differences are about 1 2 to 1 3. The coefficients (volumes) cr of all compounds with the same number of C1 atoms are about the same (if only the two aromatically bound C1 atoms of compound 6 are taken into account).The product 020 indicating the increase of the ionic yield at very small concentrations with increasing concentration is more different from one compound to another because the maximum yields are also different 312 GENERAL DISCUSSION The definition of 020 is given in the following expression for very small con-centrations : d Z 1 d Z c - t o ; z-to; -=az,; c r = - I dc 20 d c ' The dimension of OZO (see above) is the same as that of (M/N)/c and k and can be considered as a volume. For all substances investigated these virtual or perhaps real volumes have about the same linear dimensions between 30 and 80A and contain between 175 to 2600 molecules of alcohol as indicated in table 4.Their linear dimensions may be considered as the mean distances of " travel " of energy for the observed irradiation effect. With increasing concentration these dimensions decrease as the radiation energy is used for other effects which cannot be detected by the measurement of only one irradiation product. Therefore it is an urgent problem to perform irradiation experiments with the possibility of quantitative detection of all reaction products and with substances of which all molecular properties are quantitatively known; only then do we think that it will be possible to give a general theory of the phenomena governing the actions of ionizing radiations on all kinds of liquid systems. The results above were obtained by means of a special X-ray tube which was acquired through financial assistance given by the Foundation for Research at the University of Berne to which we are greatly obliged. 1 Dainton J. Physic. Chem. 1948 52 490. ZDale Phil. Trans. Roy. SOC. A 1949 242 33. 3 Dale Meredith and Tweedie Nature 1943 151 295. 4 Feller Minder and Liechti Radiol. Clin. 1948 17 156. 5 Fricke and Morse Amer. J . Rontgen. 1927 28 426 430. 6 Meister and Minder Radiol. Clin. 1950 19 238. 7 Minder Radiol. Clin. 1946 15 30. 8 Minder Radiol. Clin. 1946 suppl. vol. 15 81. 9 Minder Radiol. Clin. 1947 16 73. 10 Minder Radiol. Clin. 1947 16 339. 11 Minder Radiol. Clin. 1950 19 277. 12 Minder Brit. J. Rad. 1951 24 435. 13 Minder J. Chim. Phys. 1951 48 423. 14 Minder and Liechti Experentia 1946 2 410. 15 Mullis Minder Liechti and Wegmuller Radiol. Clin. 1946 15 295. 16 Risse Ergebn. Physiol. 1930 42 228. 17 Weiss Nature 1944 153 748. 18 Weiss Nature 1946 157 584

ISSN:0366-9033    

DOI:10.1039/DF9521200305

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

312 GENERAL DISCUSSION Dr. H. A. Dewhurst (Edinburgh Utziversity) (cotnniimicated) With regard to GENERAL DISCUSSION Dr. Dale’s interpretation of enhanced radiation effects an alternative mechanism has been proposed to account for the enhanced oxidation of ferrous sulphate produced by the addition of aliphatic alcohols l o aerated ferrous sulphate solutions.1 Here the enhanced yield has been attributed to a competitive reaction between the Fe2+ ion and the organic solute for the OH radicals with the production of an organic free radical. This organic free radical reacts with dissolved oxygen to form an organic peroxide which ultimately brings about an enhanced yield. A I Deuhur\t .I. Cheiji. t ’ / ~ ~ ‘ i i c ~ 1951. 19 1329. GENERAL DISCUSSION hit. Physiol.1951 59 442. 10 Bacq rind I-lerve Bull. Acari. Roj,. Me'd Belg. 1952 18 13. 313 similar type of mechanism has been postulated by Hart 2 to explain the enhanced oxidation of ferrous sulphate in the presence of formic acid. This peroxide mechanism could also account for the increased reduction yield found by Fricke and Brownscombe 3 for aerated dichromate solutions containing aliphatic acids ; and also for the increased ceric ion reduction yield produced by various organic solutes which was reported by Clark and Coe.4 IL may well be that these results are of biological significance because it is well known that many radiobiological reactions are greatly enhanced by the presence of oxygen. In this connection ii is noteworthy that a peroxidic mechanism has been indicated to account for the post irradiation effect observed with aerated solu- tions of deoxyribonucleic acid.5 Dr.W. M. Dale (Christie Hospitul Manclzestw) (communicated) Dr. Dewhurst's suggestion that an organic peroxide formed from an aliphatic alcohol is the cause of an enhanced radiation effect i s very plausible and could be valid in some cases. It is however known that alcohol in oxygenated systems can also suppress radiation effects i.e. can act as a protective substance.6 7 8 It seems therefore relevant to ask why alcohol does not always react as an oxidative agent via a peroxide. Prof. 2. M. Bacq (Liigc Belgium) said The biologists are much interested in this phenomenon of chemical protection against ionizing radiations studied by Dale and his collaborators.Recently we found that many amines (simple ali- phatic or aromatic amines) have a remarkable protective action. But the best seems to be P-mercaptoethylamine (cysteinamine) HS-CH2-CH2-NH2.91 dose of 700 r kills all the mice of my pure stock. If one injects 3 mg of p-mer- captoethylamine to 20 g mice before irradiation 95 out of 100 animals survive after 700 r. Dr. Philpot at Harwell has been kind enough to repeat those experiments and he has confirmed my results (6 mice weighing40 g received 6 mg of the amine all survived to a dose of 950 r). 6-mercaptoethylamine is a non- toxic substance very suitable for human use. The oxidized -S-S-derivative is unexpectedly as active as the reduced form. Both the thiol and the disulphide 10 A are completely inactive when injected immediately after irradiation.A possible explanation is that these amines inhibit the action of free radicals on living matter. Amines protect the oxidation of ferrous ions to ferric. The disulphide seems more active than the reduced compound in low concentration and less active in higher concentrations. In our experimental conditions (3 ml of 10-3 M ferrous ammonium sulphate in 0-8 N sulphuric acid saturated with air) a constant dose of 20,000 r (1,000 r in 35 sec 50 kV focal distance 4 cm) oxidizes 25 ;{ of the ferrous ions. The presence of /%mercaptoethylamine 5 x 10-4 M rcduces the percentage of oxidized ferrous to 20 %. The same Concentration of S-S/2 reduces the oxidation to 10 %. If one raises the concentration of cysteinamine to 2-5 >.10-3 M the oxidation is only 2.5 % (= 90 :( protection); in order to obtain the same protection with the corresponding disulphide one has to reach a concentration of 8 " 10 -3 M/2. 2 Hart J . Amer. Chenr. Soc. 1952 (in press). 3 Fricke and Brownscornbe J. Amer. Chem. Soc. 1933 55 2355. 4 Clark and Coe J . Ciiem. Physics 1937 5 97. 5 Butler and Conway J . Chem. Soc. 1950 3418 ; 1952 834. 6 Fricke Hart and Smith J . Ciiern. Physics 1938 6 229. 7 Hollaender Stapleton and Burnett Isotopes in Biochemistry a Ciba Foundatioii Conference (Churchill Ltd. London 1951) p. 96. 8 McDonald Aim. Report (Dept. Genetics Carnegie Institution of Washington Cold Spring Harbour N.Y. 1949-50). 9 Bacq Herve Lecomte Fischer Blavier Dechamps Le Bihan and Rayet Arch.GENERAL DISCUSSION 314 The -SH substance is oxidized to S-S (up to 6 x 10-4 M) by irradiation in the same conditions (fig. 1 ) ; it reduces 50 % of the ferric ions at a concentration of 10-2 M (fig. 2a). If one adds 0.25 M ferric + 0.75 M ferrous solution to the same irradiated solution of mercaptoethylamine no reduction of ferric ions takes place (fig. 26). Thus the action of ,B-mercaptoethylamine cannot be explained by a simple reduction of the ferric ions formed during irradiation. El SH /f3 / 2 3 4 5 6 ;7 8 9 FIG. 1 .-Oxidation of p-mercaptoethylamine at different concentrations by a constant dose of X-rays (20,000 r ; 50 kW ; focal distance 4 cm ; 1,000 r/35 sec) ; acidity of solution 0.8 N HzSO4.ordinates concentration of S-S/2 (cystinamine) ; abcissae concentration of /3-mercaptoethylamine (SH). FIG. 2.-Ferric ions reduction of a solution containing the same ferrous-ferric ions concentration as an irradiated ferrous solution (0.25 x 10-3 M ferric; 0.75 x 10-3 M ferrous ; 0.8 N H2S04). Dosage by o-phenanthroline or- dinates % reduction abscissae concentration of P-mercapto- ethylamine. (a) F-mercapto-ethylamine (b) irradiated (20,000 r) 8- mercap to-ethylamine. GENERAL DISCUSSION 315 Dr. A. J. Swallow (Birmingham University) said Dr. Bacq has mentioned that certain substances will protect animals against lethal doses of radiation. Cysteine is quite a good protective substance and I have irradiated pure solutions of cysteine in order LO investigate the reactions occurring.11 Cysteine is oxidized to cystine and in concentrated solutions in the presence of oxygen there is a yield of 24 molecules of cysteine lost for 32.5 eV absorbed.This is a rather large yield and is probably due to a chain reaction. I have suggested the following mechanism RSH + OH -+ RS + H2O RS + RSH -+ RSSR + H H + 0 2 -+ HO2 11 Swallow J. Chem. SOC. 1952 1334. H02 + RSH -+ RS + €3202. Other workers have detected hydrogen peroxide in the system.12 In the absence of oxygen the yield for concentrated aqueous solutions is about 3. Dr. Dale said that he had also irradiated cysteine and he confirmed that large yields were obtained. In his experiments the yield fell rapidly when more than 10 % of the cysteine was oxidized.The ionic yields and the reactions given above are for the early part of the reaction where yield is still proportional to dose. Dr. George wondered whether the reaction RS + RSH -+ RSSR + H was not endothermic. This reacrion is endothermic but even so may possibly occur. The irradiations were at a low dose rate and a high yield was obtained only with concentrated solutions of cysteine. If the reaction RSH + OH 2- RS + H20 occurs then the thiol radical so produced will have many collisions with cysteine molecules before meeting any other possible reactants. I have not yet investigated the effect of temperature on the reaction. Dr. J. Weiss (Durham University Newcastle) (communicated) It would be interesting to know how much cystine is actually formed in this reaction because it is very likely that in the presence of oxygen there are also some oxygen-containing products formed.If that is the case the actual chain mechanism will be rather different from the one suggested by Dr. Swallow which also meets with some difficulties from a thermodynamic point of view. Dr. W. M. Dale (Manchester) said With regard to Dr. Stein’s remarks on methylene blue we can in general confirm his conclusions. However the problem is rather more complicated than would appear at first because while the rate of bleaching of methylene blue in water is the same in oxygenated as in evacuated solution in gelatine as pointed out by Stein the evacuated system is bleached more quickly. Furthermore if you add thiourea to the aqueous solution it acts protectively on the dye in both oxygen-containing and oxygen-free solution while in gelatine the protective power is more doubtful.Experimental circumstances make a clear- cut decision between anaerobic and aerobic conditions difficult because of the induction period in the presence of oxygen and the decrease in diffusion rate of oxygen into the gel may give rise to a condition of partial lack of oxygen in the interior of the gel. I don’t know whether Dr. George was under the impression that we had worked with catalase. However that may be carboxypeptidase is insensitive to H202 and does not contain iron. Dr. M. Magat (Paris) (partly communicated) I think the results of Dr. Minder can be kinetically interpreted along the lines developed by Dainton and Miller 13 12 Rotheram Todd and Whitcher AECD UCLA-119 195 1.13 Proc. Znt. Congr. Pure Appl. Chem. (London 1947). 316 and Chapiro,l4 i.e. as a competition between the recombination of the free radicals and the attack of one of them on the organic chlorine compound with extraction of chlorine e.g. If 1 GENERAL DISCUSSION RCl -1- H - R* -i- HCl. ( 1 ) Since such a reaction would require a relatively high activation energy a rather large concentration of RCI would be necessary to catch all the free radicals produced and so obtain a yield independent of the RCl concentration and of the nature of the compound used. As was correctly pointed out by Dr. Minder the concen- trations would be so high that direct effects as well as secondary reactions would become important.The kinetics is so involved that it seems hardly possible to use this type of reaction for determining the G value of the solvent. However at low concentration the initial slope of the curve should be proportional to the concentration of C-C1 bonds and to the rate constant kl. For two different compounds in the same solvent and under identical irradiation conditions the ratio of the slopes is roughly given by K = 4 e x p (-) - RT AE* where n and n’ are the numbers of C-Cl bonds in each molecule and AE+ is the difference in activation energies which is about 0-3 AD where A D is the difference of the C-C1 bond dissociation energies. The compounds listed in fig. 3 of Dr. Minder’s papers belong to 4 different groups (i) c2cl(j (ii) RCC13 (iii) Y - C ~ H ~ C I ~ and (iv) P‘CfjH4C12.Comparing the slopes of groups (ii) and (iv) one gets AE* = 1 kcal and AD N 3 kcal a reasonable value for the difference between aliphatic and aromatic C-C1 bonds while the AE* between groups (i) and the (ii) is N 0.4 kcal. In general an increase in the bond energy when going from group (i) to group (iv) seems in keeping with the rest of our knowledge on the subject. Dr. W. Minder (Radium Institute Berne) said We compleLely agree with Dr. Magat that the curves fig. 1 and 2 of Dr. Maget’s and co-worker’s paper and fig. 3 of our paper are representing exactly the same things. Nevertheless the interpretations are somewhat difl‘erent. In our curves we plotted the yield of perhaps the most probable reaction of different solutes capable of the same reaction against concentration in the same solvent.Our yields are within experimental error proportional to the number of C1 atoms in the solute molecules both for the aliphatic and for the cyclic compoimds. The difference between the two yields per C1 atom is of course due to the different binding energies. Table 1 of Dr. Magat and co-worker’s paper shows the decomposition yields of one and the same solute (diphenyl picryhydrazil) in different solvents. If we were to try to interpret our results in a similar manner it would be neces- sary to assume the liberation of all the Cl atoms which are bound to one molecule of the solute by one radical produced from the solvent alcohol or the production of different numbers of radicals in the same solvent if different solutes are present.Furthermore it would be difficult to understand the fact that relatively large con- centrations of water in the solvent do not in any way influence the number of active radicals. Dr. W. Minder (Radium Institute Berne) said ; The total radiation dose within a spherical vessel containing a suitable radioisotope solution can be calculated exactly 15 if the decay properties of the isotope is known. We have compared such a calculation with the results for the oxidation by self-irradiation of a ferrous sulphate solution (0.005 N FeSO4 -1- 0-8 N H2S04) containing 0.1479 mc of Rb86 14 this Discussion. 15 Minder and H. Schindler Strahletttker 1952 86 602.GENERAL DISCUSSION 317 per cm3. The total radiation dose administered to the solution in such an ex- periment during the time t is D = KMG s’ exp (- At) dt = KMG -(1 - exP (- W) 0 x where K is the overall dosage rate (in r/mc hj M the total amount of the radio- isotope in mc X the decay constant and G the so-called geometrical factor which takes into account the form size and absorption and which is in every case a rather complicated mathematical expression. Comparing our irradiation results with the calculations (the irradiation was made in a closed vessel of 700 ml without adding 0 2 and the oxidation was deter- mined by titration with KMn04) we have found a total dose (t = a) of 193,000 r by calculation and 200,000 r by oxidation of Fe3+.This value was obtained with an ionic yield (Fe3+ per 32.5 eVj of 3. This yield is in good agreement with our earlier experiments with y-rays 16 and X-rays 17 and with the value gwen by Wright 18 for a 0.0025 N solution after oxygen removal. Dr. J. F. Suttle and Dr. J. W. Schulte (Los Alamos New Mexico) (communicated) For the past several months we have been irradiating dry pure chloroform solu- tions with gamma rays from a C060 source. The results to date indicate that good linearity is obtained up to 4 x 105 r when chloride ion in the irradiated solution is determined potentiometrically with silver nitrate. For 4 x 105 r approximately 0.18 mequiv. of chloride ion is found per ml of irradiated chloroform. However to obtain any significant decomposition by irradiation it is necessary that oxygen be present.We find it interesting that the authors of this paper make no mention that even traces of oxygen might have decided influence on the system. Prof. Milton Burton (University of Notre Dame Indiana) said In this Discus- sion we have witnessed two kinds of difficulty regarding expression of radiation yield. The first and more important is an experimental discrepancy in the G value for Fez+ -+ Fe 3+ in 0.8 N sulphuric acid solution dependent on the source of the information. Although the difference is presently unexplained we may confidently expect that an explanation will soon be forthcoming. The second type of difficulty results from the method of expression of yield. The custom persists of reporting M/N in cases where N is not and probably never can be experimentally known.The number of ion pairs is calculated on a variety of bases depending on logic prejudice admitted inadequacy of information or on purely formal advantage. Unfortunately readers who get only raw M/N values from summaries or abstracts sometimes hunt vainly through an article for details as to the basis of calculation. This latter difficulty is really an avoidable nuisance. Although occasionally an author can profitably include consideration of M/N in his reasoning in many cases nothing is contributed to our knowledge of mechanism by use of this nota- tion. Consequently irrespective of individual prejudices it is desirable that we join in a notation which has singularity of meaning.I have discussed this matter with several participants in the Discussion and we have jointly arrived at con- clusions which I have summarized as follows. G This notation (i.e. 100 eV yield) should be used whenever molecules pro- duced or converted and actual energy input are actually measured. Of course errors may occur in both determinations but such values are subject to check and the methods (if adequately described) are subject to criticism. G’ Sometimes energy input is not directly measured but is computed from other data e.g. from observations with a dosimeter. In such case the author should indicate both his methods and his conversion factors unambiguously and 16 Minder and Liechti Experientia 1946 2 410. 17 Minder Radiol.Clin. 1951 20 286 1951. 1 * Wright this Discussion. GENERAL DISCUSSION 318 use the symbol G’ to indicate that the 100eV yield is calculated rather than measured. G20 Yields may be measured in terms of another actinometer ; e.g. the highly favoured FeS04 actinometer. In a recent paper which I had the good fortune LO read prior to publication Johnson and Allen expressed yields in an aqueous system in terms of a comparison with Fezf -+ Fe3+ in such an actinometer. Since they explicitly indicated their method it will be possible at some fuwre time to establish their G’ values. Simple numerical values would have been useful if difficulty of interpretation could have been simultaneously avoided. It is sug- gested that in the future when the FeS04 + 0.8 N H2SO4 actinometer is used as a standard the conditions of use be explicitly stated G(Fe2+ -f Fe3+) be taken as 20 until an accepted value is established and the symbol G20 be used for expression of the calculated 100 eV yield.G (. . . ) For convenience of typography we suggest that the change to which G refers be indicated where necessary in parentheses after G ; e.g. G (H202 produced) or G (Fe2+ -z Fe3+). G 1.5 The papers of both Dr. Hardwick and Dr. Minder emphasize the signi- ficance of microscopic G value for molecules converted or produced per 100 eV in an increment of path where the energy of the effective particle is not significantly changed. Unlike the usual G values this is not an average value over the whole length of path. It tells of the chemical effect produced by a particle in a micro- scopic portion of its path when it has a very specific energy.We suggest use of subscripts such as shown where units of energy are in MeV. In any case we join in the urgent suggestion that explicit statements be made as to the quantities actually measured. Dr. N. Miller (Edinburgh Universify) (partly conmunicated) Prof. Burton’s suggestion that methods of expressing radiation yield be standardized is a timely one. I think that the moment is also appropriate for the standardization of units of dose and dose rate. Most workers in the field would agree that units of dose based on ionization such as the roentgen or rep are less satisfactory than absolute units such as eV/ml ergs/g. It should be emphasized however that full experi- mental details of all physical dose measurements should be included by authors on publication.With regard to the relative merits of the units eV/ml and ergs/g it may be pointed out that the former involves the use of very large numbers whereas the latter is currently receiving considerable support among medical physicists. Prof. Burton has made the further suggestion that yields observed in X- or y-ray induced reactions in aqueous media be expressed relative to that observed for ferrous sulphate oxidation under the same experimental conditions. Such yields could then readily be adjusted to conform with any future changes in the accepted yield value for the ferrous sulphate system. I think that such a procedure is quite satisfactory at low dose rates but that utitil more work is done on this method of dosimetry at high dose rates and general agreement on the yield value in this region is reached an upper limit should for the time being be set on the dose rate at which such a procedure is considered reliable.A figure of 1000 r/min would for the moment be a safe or even con- servative upper limit. Very probably this limit will be raised or even consider- ably raised when more becomes known about the general kinetics of ferrous sulphate oxidation at high dose rates. It seems almost certain however that the upper limit for the use of ferrous sulphate will prove to be lower than that for ceric sulphate or other systems which are not oxygen-dependent. 312 GENERAL DISCUSSION GENERAL DISCUSSION Dr.H. A. Dewhurst (Edinburgh Utziversity) (cotnniimicated) With regard to Dr. Dale’s interpretation of enhanced radiation effects an alternative mechanism has been proposed to account for the enhanced oxidation of ferrous sulphate produced by the addition of aliphatic alcohols l o aerated ferrous sulphate solutions.1 Here the enhanced yield has been attributed to a competitive reaction between the Fe2+ ion and the organic solute for the OH radicals with the production of an organic free radical. This organic free radical reacts with dissolved oxygen to form an organic peroxide which ultimately brings about an enhanced yield. A I Deuhur\t .I. Cheiji. t ’ / ~ ~ ‘ i i c ~ 1951. 19 1329 GENERAL DISCUSSION 313 similar type of mechanism has been postulated by Hart 2 to explain the enhanced oxidation of ferrous sulphate in the presence of formic acid.This peroxide mechanism could also account for the increased reduction yield found by Fricke and Brownscombe 3 for aerated dichromate solutions containing aliphatic acids ; and also for the increased ceric ion reduction yield produced by various organic solutes which was reported by Clark and Coe.4 IL may well be that these results are of biological significance because it is well known that many radiobiological reactions are greatly enhanced by the presence of oxygen. In this connection ii is noteworthy that a peroxidic mechanism has been indicated to account for the post irradiation effect observed with aerated solu-tions of deoxyribonucleic acid.5 Dr.W. M. Dale (Christie Hospitul Manclzestw) (communicated) Dr. Dewhurst's suggestion that an organic peroxide formed from an aliphatic alcohol is the cause of an enhanced radiation effect i s very plausible and could be valid in some cases. It is however known that alcohol in oxygenated systems can also suppress radiation effects i.e. can act as a protective substance.6 7 8 It seems therefore relevant to ask why alcohol does not always react as an oxidative agent via a peroxide. Prof. 2. M. Bacq (Liigc Belgium) said The biologists are much interested in this phenomenon of chemical protection against ionizing radiations studied by Dale and his collaborators. Recently we found that many amines (simple ali-phatic or aromatic amines) have a remarkable protective action.But the best seems to be P-mercaptoethylamine (cysteinamine) HS-CH2-CH2-NH2.91 10 A dose of 700 r kills all the mice of my pure stock. If one injects 3 mg of p-mer-captoethylamine to 20 g mice before irradiation 95 out of 100 animals survive after 700 r. Dr. Philpot at Harwell has been kind enough to repeat those experiments and he has confirmed my results (6 mice weighing40 g received 6 mg of the amine all survived to a dose of 950 r). 6-mercaptoethylamine is a non-toxic substance very suitable for human use. The oxidized -S-S-derivative is, unexpectedly as active as the reduced form. Both the thiol and the disulphide are completely inactive when injected immediately after irradiation. A possible explanation is that these amines inhibit the action of free radicals on living matter.Amines protect the oxidation of ferrous ions to ferric. The disulphide seems more active than the reduced compound in low concentration and less active in higher concentrations. In our experimental conditions (3 ml of 10-3 M ferrous ammonium sulphate in 0-8 N sulphuric acid saturated with air) a constant dose of 20,000 r (1,000 r in 35 sec 50 kV focal distance 4 cm) oxidizes 25 ;{ of the ferrous ions. The presence of /%mercaptoethylamine 5 x 10-4 M rcduces the percentage of oxidized ferrous to 20 %. The same Concentration of S-S/2 reduces the oxidation to 10 %. If one raises the concentration of cysteinamine to 2-5 >. 10-3 M the oxidation is only 2.5 % (= 90 :( protection); in order to obtain the same protection with the corresponding disulphide one has to reach a concentration of 8 " 10 -3 M/2.2 Hart J . Amer. Chenr. Soc. 1952 (in press). 3 Fricke and Brownscornbe J. Amer. Chem. Soc. 1933 55 2355. 4 Clark and Coe J . Ciiem. Physics 1937 5 97. 5 Butler and Conway J . Chem. Soc. 1950 3418 ; 1952 834. 6 Fricke Hart and Smith J . Ciiern. Physics 1938 6 229. 7 Hollaender Stapleton and Burnett Isotopes in Biochemistry a Ciba Foundatioii 8 McDonald Aim. Report (Dept. Genetics Carnegie Institution of Washington, 9 Bacq Herve Lecomte Fischer Blavier Dechamps Le Bihan and Rayet Arch. 10 Bacq rind I-lerve Bull. Acari. Roj,. Me'd Belg. 1952 18 13. Conference (Churchill Ltd. London 1951) p. 96. Cold Spring Harbour N.Y. 1949-50). hit. Physiol. 1951 59 442 314 GENERAL DISCUSSION The -SH substance is oxidized to S-S (up to 6 x 10-4 M) by irradiation in the same conditions (fig.1 ) ; it reduces 50 % of the ferric ions at a concentration of 10-2 M (fig. 2a). If one adds 0.25 M ferric + 0.75 M ferrous solution to the same irradiated solution of mercaptoethylamine no reduction of ferric ions takes place (fig. 26). Thus the action of ,B-mercaptoethylamine cannot be explained by a simple reduction of the ferric ions formed during irradiation. El SH /f3 / 2 3 4 5 6 ;7 8 9 , FIG. 1 .-Oxidation of p-mercaptoethylamine at different concentrations by a constant dose of X-rays (20,000 r ; 50 kW ; focal distance 4 cm ; 1,000 r/35 sec) ; acidity of solution 0.8 N HzSO4. ordinates concentration of S-S/2 (cystinamine) ; abcissae concentration of /3-mercaptoethylamine (SH).FIG. 2.-Ferric ions reduction of a solution containing the same ferrous-ferric ions concentration as an irradiated ferrous solution (0.25 x 10-3 M ferric; 0.75 x 10-3 M ferrous ; 0.8 N H2S04). Dosage by o-phenanthroline or-dinates % reduction abscissae, concentration of P-mercapto-ethylamine. (a) F-mercapto-ethylamine, (b) irradiated (20,000 r) 8-mercap to-ethylamine GENERAL DISCUSSION 315 Dr. A. J. Swallow (Birmingham University) said Dr. Bacq has mentioned that certain substances will protect animals against lethal doses of radiation. Cysteine is quite a good protective substance and I have irradiated pure solutions of cysteine in order LO investigate the reactions occurring.11 Cysteine is oxidized to cystine and in concentrated solutions in the presence of oxygen there is a yield of 24 molecules of cysteine lost for 32.5 eV absorbed.This is a rather large yield, and is probably due to a chain reaction. I have suggested the following mechanism : RSH + OH -+ RS + H2O RS + RSH -+ RSSR + H H + 0 2 -+ HO2 H02 + RSH -+ RS + €3202. Other workers have detected hydrogen peroxide in the system.12 In the absence of oxygen the yield for concentrated aqueous solutions is about 3. Dr. Dale said that he had also irradiated cysteine and he confirmed that large yields were obtained. In his experiments the yield fell rapidly when more than 10 % of the cysteine was oxidized. The ionic yields and the reactions given above are for the early part of the reaction where yield is still proportional to dose.Dr. George wondered whether the reaction RS + RSH -+ RSSR + H was not endothermic. This reacrion is endothermic but even so may possibly occur. The irradiations were at a low dose rate and a high yield was obtained only with concentrated solutions of cysteine. If the reaction RSH + OH 2- RS + H20 occurs then the thiol radical so produced will have many collisions with cysteine molecules before meeting any other possible reactants. I have not yet investigated the effect of temperature on the reaction. Dr. J. Weiss (Durham University Newcastle) (communicated) It would be interesting to know how much cystine is actually formed in this reaction because it is very likely that in the presence of oxygen there are also some oxygen-containing products formed.If that is the case the actual chain mechanism will be rather different from the one suggested by Dr. Swallow which also meets with some difficulties from a thermodynamic point of view. Dr. W. M. Dale (Manchester) said With regard to Dr. Stein’s remarks on methylene blue we can in general confirm his conclusions. However the problem is rather more complicated than would appear at first because while the rate of bleaching of methylene blue in water is the same in oxygenated as in evacuated solution in gelatine as pointed out by Stein the evacuated system is bleached more quickly. Furthermore if you add thiourea to the aqueous solution it acts protectively on the dye in both oxygen-containing and oxygen-free solution while in gelatine the protective power is more doubtful.Experimental circumstances make a clear-cut decision between anaerobic and aerobic conditions difficult because of the induction period in the presence of oxygen and the decrease in diffusion rate of oxygen into the gel may give rise to a condition of partial lack of oxygen in the interior of the gel. I don’t know whether Dr. George was under the impression that we had worked with catalase. However that may be carboxypeptidase is insensitive to H202 and does not contain iron. Dr. M. Magat (Paris) (partly communicated) I think the results of Dr. Minder can be kinetically interpreted along the lines developed by Dainton and Miller 13 11 Swallow J. Chem. SOC. 1952 1334. 12 Rotheram Todd and Whitcher AECD UCLA-119 195 1. 13 Proc. Znt. Congr. Pure Appl.Chem. (London 1947) 316 GENERAL DISCUSSION and Chapiro,l4 i.e. as a competition between the recombination of the free radicals and the attack of one of them on the organic chlorine compound with extraction of chlorine e.g. If 1 RCl -1- H - R* -i- HCl. ( 1 ) Since such a reaction would require a relatively high activation energy a rather large concentration of RCI would be necessary to catch all the free radicals produced and so obtain a yield independent of the RCl concentration and of the nature of the compound used. As was correctly pointed out by Dr. Minder the concen-trations would be so high that direct effects as well as secondary reactions would become important. The kinetics is so involved that it seems hardly possible to use this type of reaction for determining the G value of the solvent.However at low concentration the initial slope of the curve should be proportional to the concentration of C-C1 bonds and to the rate constant kl. For two different compounds in the same solvent and under identical irradiation conditions the ratio of the slopes is roughly given by - AE* K = 4 e x p (-) RT where n and n’ are the numbers of C-Cl bonds in each molecule and AE+ is the difference in activation energies which is about 0-3 AD where A D is the difference of the C-C1 bond dissociation energies. The compounds listed in fig. 3 of Dr. Minder’s papers belong to 4 different groups (i) c2cl(j (ii) RCC13 (iii) Y - C ~ H ~ C I ~ , and (iv) P‘CfjH4C12. Comparing the slopes of groups (ii) and (iv) one gets AE* = 1 kcal and AD N 3 kcal a reasonable value for the difference between aliphatic and aromatic C-C1 bonds while the AE* between groups (i) and the (ii) is N 0.4 kcal.In general an increase in the bond energy when going from group (i) to group (iv) seems in keeping with the rest of our knowledge on the subject. Dr. W. Minder (Radium Institute Berne) said We compleLely agree with Dr. Magat that the curves fig. 1 and 2 of Dr. Maget’s and co-worker’s paper and fig. 3 of our paper are representing exactly the same things. Nevertheless the interpretations are somewhat difl‘erent. In our curves we plotted the yield of perhaps the most probable reaction of different solutes capable of the same reaction against concentration in the same solvent. Our yields are within experimental error proportional to the number of C1 atoms in the solute molecules both for the aliphatic and for the cyclic compoimds.The difference between the two yields per C1 atom is of course due to the different binding energies. Table 1 of Dr. Magat and co-worker’s paper shows the decomposition yields of one and the same solute (diphenyl picryhydrazil) in different solvents. If we were to try to interpret our results in a similar manner it would be neces-sary to assume the liberation of all the Cl atoms which are bound to one molecule of the solute by one radical produced from the solvent alcohol or the production of different numbers of radicals in the same solvent if different solutes are present. Furthermore it would be difficult to understand the fact that relatively large con-centrations of water in the solvent do not in any way influence the number of active radicals.Dr. W. Minder (Radium Institute Berne) said ; The total radiation dose within a spherical vessel containing a suitable radioisotope solution can be calculated exactly 15 if the decay properties of the isotope is known. We have compared such a calculation with the results for the oxidation by self-irradiation of a ferrous sulphate solution (0.005 N FeSO4 -1- 0-8 N H2S04) containing 0.1479 mc of Rb86 14 this Discussion. 15 Minder and H. Schindler Strahletttker 1952 86 602 GENERAL DISCUSSION per cm3. The total radiation dose administered to periment during the time t is KMG D = KMG s’ exp (- At) dt = -(1 0 x the solution in - exP (- W), 317 such an ex-where K is the overall dosage rate (in r/mc hj M the total amount of the radio-isotope in mc X the decay constant and G the so-called geometrical factor which takes into account the form size and absorption and which is in every case a rather complicated mathematical expression.Comparing our irradiation results with the calculations (the irradiation was made in a closed vessel of 700 ml without adding 0 2 and the oxidation was deter-mined by titration with KMn04) we have found a total dose (t = a) of 193,000 r by calculation and 200,000 r by oxidation of Fe3+. This value was obtained with an ionic yield (Fe3+ per 32.5 eVj of 3. This yield is in good agreement with our earlier experiments with y-rays 16 and X-rays 17 and with the value gwen by Wright 18 for a 0.0025 N solution after oxygen removal.Dr. J. F. Suttle and Dr. J. W. Schulte (Los Alamos New Mexico) (communicated) : For the past several months we have been irradiating dry pure chloroform solu-tions with gamma rays from a C060 source. The results to date indicate that good linearity is obtained up to 4 x 105 r when chloride ion in the irradiated solution is determined potentiometrically with silver nitrate. For 4 x 105 r approximately 0.18 mequiv. of chloride ion is found per ml of irradiated chloroform. However, to obtain any significant decomposition by irradiation it is necessary that oxygen be present. We find it interesting that the authors of this paper make no mention that even traces of oxygen might have decided influence on the system. Prof.Milton Burton (University of Notre Dame Indiana) said In this Discus-sion we have witnessed two kinds of difficulty regarding expression of radiation yield. The first and more important is an experimental discrepancy in the G value for Fez+ -+ Fe 3+ in 0.8 N sulphuric acid solution dependent on the source of the information. Although the difference is presently unexplained we may confidently expect that an explanation will soon be forthcoming. The second type of difficulty results from the method of expression of yield. The custom persists of reporting M/N in cases where N is not and probably never can be, experimentally known. The number of ion pairs is calculated on a variety of bases depending on logic prejudice admitted inadequacy of information or on purely formal advantage.Unfortunately readers who get only raw M/N values from summaries or abstracts sometimes hunt vainly through an article for details as to the basis of calculation. This latter difficulty is really an avoidable nuisance. Although occasionally an author can profitably include consideration of M/N in his reasoning in many cases nothing is contributed to our knowledge of mechanism by use of this nota-tion. Consequently irrespective of individual prejudices it is desirable that we join in a notation which has singularity of meaning. I have discussed this matter with several participants in the Discussion and we have jointly arrived at con-clusions which I have summarized as follows. G This notation (i.e. 100 eV yield) should be used whenever molecules pro-duced or converted and actual energy input are actually measured.Of course, errors may occur in both determinations but such values are subject to check and the methods (if adequately described) are subject to criticism. Sometimes energy input is not directly measured but is computed from other data e.g. from observations with a dosimeter. In such case the author should indicate both his methods and his conversion factors unambiguously and G’ 16 Minder and Liechti Experientia 1946 2 410. 17 Minder Radiol. Clin. 1951 20 286 1951. 1 * Wright this Discussion 318 GENERAL DISCUSSION use the symbol G’ to indicate that the 100eV yield is calculated rather than measured. G20 Yields may be measured in terms of another actinometer ; e.g. the highly favoured FeS04 actinometer.In a recent paper which I had the good fortune LO read prior to publication Johnson and Allen expressed yields in an aqueous system in terms of a comparison with Fezf -+ Fe3+ in such an actinometer. Since they explicitly indicated their method it will be possible at some fuwre time to establish their G’ values. Simple numerical values would have been useful if difficulty of interpretation could have been simultaneously avoided. It is sug-gested that in the future when the FeS04 + 0.8 N H2SO4 actinometer is used as a standard the conditions of use be explicitly stated G(Fe2+ -f Fe3+) be taken as 20 until an accepted value is established and the symbol G20 be used for expression of the calculated 100 eV yield. G (. . . ) For convenience of typography we suggest that the change to which G refers be indicated where necessary in parentheses after G ; e.g.G (H202 produced) or G (Fe2+ -z Fe3+). G 1.5 The papers of both Dr. Hardwick and Dr. Minder emphasize the signi-ficance of microscopic G value for molecules converted or produced per 100 eV in an increment of path where the energy of the effective particle is not significantly changed. Unlike the usual G values this is not an average value over the whole length of path. It tells of the chemical effect produced by a particle in a micro-scopic portion of its path when it has a very specific energy. We suggest use of subscripts such as shown where units of energy are in MeV. In any case we join in the urgent suggestion that explicit statements be made as to the quantities actually measured.Dr. N. Miller (Edinburgh Universify) (partly conmunicated) Prof. Burton’s suggestion that methods of expressing radiation yield be standardized is a timely one. I think that the moment is also appropriate for the standardization of units of dose and dose rate. Most workers in the field would agree that units of dose based on ionization such as the roentgen or rep are less satisfactory than absolute units such as eV/ml ergs/g. It should be emphasized however that full experi-mental details of all physical dose measurements should be included by authors on publication. With regard to the relative merits of the units eV/ml and ergs/g, it may be pointed out that the former involves the use of very large numbers, whereas the latter is currently receiving considerable support among medical physicists. Prof. Burton has made the further suggestion that yields observed in X- or y-ray induced reactions in aqueous media be expressed relative to that observed for ferrous sulphate oxidation under the same experimental conditions. Such yields could then readily be adjusted to conform with any future changes in the accepted yield value for the ferrous sulphate system. I think that such a procedure is quite satisfactory at low dose rates but that utitil more work is done on this method of dosimetry at high dose rates and general agreement on the yield value in this region is reached an upper limit should for the time being be set on the dose rate at which such a procedure is considered reliable. A figure of 1000 r/min would for the moment be a safe or even con-servative upper limit. Very probably this limit will be raised or even consider-ably raised when more becomes known about the general kinetics of ferrous sulphate oxidation at high dose rates. It seems almost certain however that the upper limit for the use of ferrous sulphate will prove to be lower than that for ceric sulphate or other systems which are not oxygen-dependent

ISSN:0366-9033    

DOI:10.1039/DF9521200312

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

Allen A. D. 79 114 123 247. Allsopp C. B. 299. Alper T. 234 266 291. Amphlett C. B. i20 144 247 254 255 Luft N. W. 120 122 258 266 271. 257 272. Bacq Z. M. 313. Barb W. G. 127 261 262 272. Bartindale G. W. R. 246 276. Baxendale J. H. 253 256 287. Butler J. A. V. 291. Boag J. W. 189. Bonet-Maury P. 72. Miller N. 46 50 118 318. Burton,M.,88 112 114 117 124 130 317. Minder W. 305 316. Chapiro A, 98 1 15 129 276. Coleby B. 125. Richards E. W. T. 45 48. Collinson E. 49 121 125 212 251 277 Rigg T. 119. 285. Conway B. E. 250 291. 245 251 264 266 277. cousin c. 98. Dainton F. S. 9 44 120 121 126 212 Spiers F. W. 13 49. Stein G. 227 243 280 283 285 286 287 288. Suttle J. F. 317. Sutton H. C. 121 266 281. Swallow J. A. 315. Uri N.118 243 248 259 277 283. Valentine L. 13 1. Waters W. A. 284. Weiss J. 48 118 161 250 255 258 260 261 263 266 272 287 288 292 315. Wild W. 111 113 127. Wright J. 60 114 116 126 247. Rollefson G. K. 155 260. Rowbottom J. 264. Schulte J. W. 317. Dale W. A, 293 313 315. Day M. J. 280. Dewhurst H. A, 255 312. George P. 1 3 I. Gray L. H. 266. Ebert M. 189 266 271. Garrison W. M. 155 260. Gordon S. 88. Haissinsky M. 113 123 133 248 259 264 282 Hardwick T. J. 112 203. Hart E. J. 111 112 169 266. Heydrich H. 305. * The references in heavy type indicate papers submitted for discussion. AUTHOR INDEX * Jenkins A. D. 275. Lefort M. 117 122 263 266 272. Maddock A. G. 118 124 271 288. Landler Y. 98. Magat M.48 98 129 244 274 290 315. Magee J. L. 33 287 288 290. Massey H. S. W. 24. Matheson M. S. 169 270. Mund W. 262. Prevo$t-BCrnas A. 98. Wilkinson J. 50. Wilson Miss J. 299. 319 AUTHOR Allen A. D. 79 114 123 247. Allsopp C. B. 299. Alper T. 234 266 291. Amphlett C. B. i20 144 247 254 255, 257 272. Bacq Z. M. 313. Barb W. G. 127 261 262 272. Bartindale G. W. R. 246 276. Baxendale J. H. 253 256 287. Boag J. W. 189. Bonet-Maury P. 72. Burton,M.,88 112 114 117 124 130 317. Butler J. A. V. 291. Chapiro A, 98 1 15 129 276. Coleby B. 125. Collinson E. 49 121 125 212 251 277, Conway B. E. 250 291. cousin c. 98. Dainton F. S. 9 44 120 121 126 212, 245 251 264 266 277. Dale W. A, 293 313 315. Day M. J. 280. Dewhurst H.A, 255 312. Ebert M. 189 266 271. Garrison W. M. 155 260. George P. 1 3 I. Gordon S. 88. Gray L. H. 266. Haissinsky M. 113 123 133 248 259, 264 282, Hardwick T. J. 112 203. Hart E. J. 111 112 169 266. Heydrich H. 305. 285. INDEX * Jenkins A. D. 275. Landler Y. 98. Lefort M. 117 122 263 266 272. Luft N. W. 120 122 258 266 271. Maddock A. G. 118 124 271 288. Magat M. 48 98 129 244 274 290, Magee J. L. 33 287 288 290. Massey H. S. W. 24. Matheson M. S. 169 270. Miller N. 46 50 118 318. Minder W. 305 316. Mund W. 262. Prevo$t-BCrnas A. 98. Richards E. W. T. 45 48. Rigg T. 119. Rollefson G. K. 155 260. Rowbottom J. 264. Schulte J. W. 317. Spiers F. W. 13 49. Stein G. 227 243 280 283 285 286 287, Suttle J. F. 317. Sutton H. C. 121 266 281. Swallow J. A. 315. Uri N. 118 243 248 259 277 283. Valentine L. 13 1. Waters W. A. 284. Weiss J. 48 118 161 250 255 258 260, 261 263 266 272 287 288 292 315. Wild W. 111 113 127. Wilkinson J. 50. Wilson Miss J. 299. Wright J. 60 114 116 126 247. 315. 288. * The references in heavy type indicate papers submitted for discussion. 31

ISSN:0366-9033    

DOI:10.1039/DF9521200319

出版商:RSC    

年代:1952

数据来源: RSC