Nature and Reactivity of the Primary Reducing Species in the Radiolysis of Aqueous Solutions BY G. SCHOLES, M. SIMIC AND J. J. WEISS Laboratory of Radiation Chemistry, School of Chemistry, The University, Newcastle-upon-Tyne Received 24th June, 1963 A study has been made of aqueous solutions of various organic solutes irradiated with C060 prays in the presence of N20 and C02, which can act as scavengers for radiation-produced reducing species. The yields of the radiation products have been determined as a function of solute con- centration and of pH. The results clearly indicate the high specific reactivity of N20 and of C02 towards negative polarons (H20)-, and the conversion of these polarons into oxidizing species by reaction with N2O. The results further support the independent formation of two reducing species, i.e., of the negative polarons and of a species reacting as hydrogen atoms.The reactivities of both these species with different organic and inorganic solutes have been investigated. Chemical evidence 1-6 has shown that radiation-produced electrons constitute the major fraction of the reducing species in aqueous media, a conclusion conkmed by physical measurements.7 These species should be regarded as negative polarons,8 represented here by (H20)-. Man and Scholes 3 showed that, in addition to the negative polaron, another species is formed independently, in a yield G-0-6, pre- sumed to be hydrogen atoms and formed from excited water molecules according to H20*-,H+O€€. The existence of this latter species was confirmed by the work of Rabani and Stein? These hydrogen atoms are designated here by Ha. EXPERIMENTAL Irradiations were carried out with C060 y-rays, the dose-rate ( 2 .4 ~ 1017 eV ml-1 min-1) being determined by the Fricke dosimeter, taking G(FelI1) = 15.5. All solutions were prepared with triply distilled water. Medical-grade N20 was purified by several distillations on a vacuum line, prior to introduction into the radiation vessel containing the degassed solution. C-14-labelled sodium bicarbonate solutions were prepared by the addition of C-14-labelled sodium carbonate, of known activity, to solutions containing an excess of sodium bicarbonate. Under these experimental conditions, the activity was essentially present as C14-bicarbonate. Gas-analyses were carried out mass spectrometrically after trapping out all condensable gases by liquid nitrogen.The nitrogen and hydrogen yields are estimated to be accurate to f 3 %. ~ The yield of labelled oxalate was determined as follows. A known excess of oxalic acid was- added and total oxalate was then precipitated as the calcium salt in the presence of acetic acid+acetate buffer. The precipitate was filtered, dried and weighed, and the activity measured by standard counting techniques. RESULTS AND DISCUSSION EXPERIMENTS WITH N20 Dainton and Peterson 4 have investigated the y-radiolysis of aqueous N20 solutions and shown that this solute has a relatively high reactivity towards the 214G . SCHOLES, M. SIMIC AND J . J . WEISS 21 5 negative polaron, leading to the formation of nitrogen, viz., N2O+(H20)-+N;!+OH-+OH. (1) Hydrogen atoms, on the other hand, react only very slowly with N20.This system therefore is a useful one for confirmation of the presence of these two reducing species. Solutions of N20 (1-6 x 10-2 M) containing various concentrations of aliphatic alcohols were irradiated with C060 prays, and the yields of hydrogen and nitrogen were determined from the linear yield against dose plots. Fig. 1 shows the results obtained with isopropanol. The yields of hydrogen vary from G = 0-94 to G = 1.22 as the isopropanol concentration is varied from 10-3 to 1 M and are clearly in excess of the molecular hydrogen yield (G;,). If it is as- sumed that all of the negative polarons react with N20 according to reaction (1) and only Ha reacts with isopropanol to give molecular hydrogen, the measured hydrogen yield corresponds to the sum: G(H2) = G;";,+G(Ha).The value of Gg2 in 1.6 x 10-2 M N20 solutions has been determined by irradiation in the presence of cupric ion (copper sulphate). Under these conditions, a yield of hydrogen of G(H2) = 0.35 was observed, which was independent of cupric ion concentration over the range 2-5 x 10-4-4.75 x 10-3 M. Thus, on the above assumption, the yield of Ha varies from G = 0.59 to 0-87 under the conditions of the experiments given in fig. 1. Part of this increase at high isopropanol concentration may only be apparent, since the alcohol may also scavenge hydrogen atoms which may otherwise contribute to the molecular yield. Under the above experimental conditions, the presence of N20 leads to a reduc- tion in GE, from 0.45 to 0.35.Since its reaction with hydrogen atoms is generally considered to be slow, this observation is also consistent with the view that the negative polarons contribute to the molecular yield. It has, in fact, been previously suggested 1 that molecular yield hydrogen is formed according to (H20)-+ (H20)-+H2 +20H- (2) and this has been recently confirmed by Dorfman and Taub.10 The yields of nitrogen recorded in fig. 1 are a composite function of the scavenging by N20 of negative polarons present in the bulk of the solution as well as some of those leading to molecular hydrogen, and possibly also some normally undergoing primary recombination, according to : (H20)- + (H20)++2H20 (3) (H20)- + OH+ OH- + H20.(4) The contribution to the observed yield of nitrogen by scavenging of some of the molecular yield precursors is equivalent to a G(N2) = 0.20. With regard to the recombination reactions (3) and (4), G(N2) in N20 + isopropanol solution appears to increase as the alcohol concentration is increased. The marked dependence of the yields of nitrogen on N20 concentration in pure N20 solutions,4 also probably indicates scavenging of some negative polarons normally recombining with the oxidizing species. The N20 + isopropanol system, therefore, clearly demonstrates the existence and reactions of the two reducing species. Addition of another acceptor for negative polarons to this system should lead to a simple competition between the acceptor and N20 for (H20)-, and hence to a decrease in the yield of nitrogen. Likewise, addition of another acceptor for Ha should lead to a competition reaction with isopropanol, and, if the gdded acceptor reacts with HG so as not to give molecular216 RADIOLYSIS OF AQUEOUS SOLUTIONS hydrogen, there should be a decrease in the hydrogen yield.It has been found that all these conditions can be realized using cupric ions as the additional solute. 3- 2- E z! Q) x *.+ E 2 2 # h .d I 0 - 10 -3 lG2 10 -I I [isopropanol] (mole/l.) FIG. 1.-Effect of isopropanol concentration on the yields of nitrogen and hydrogen in the y- radiolysis of aqueous solutions of N20 (1.6 x 10-2 M) at pH-6. 17, nitrogen ; 0, hydrogen. I I I 1 - I 2 3 4 5 OL C [Cu2+] (rnolej.) x lo3 FIG. 2.Effect of cupric ions on the yields of nitrogen and hydrogen in the 7-radiolysis of aqueous solutions of isopropanol(10-1 M) and N20 ( 1 .6 ~ 10-2 M); pH-6; 0, nitrogen; 0, hydrogen. Fig. 2 shows the yields of nitrogen and of hydrogen on irradiation of N20 (1.6 x 10-2 M) +isopropanol (10-1 M) solutions containing varying amounts of cupric ions.G . SCHOLES, M. SIMIC AND J . J . WEISS 21 7 The simple competition kinetics between reaction (1) and the reaction, C U ~ + + (HzO)--+Cu+ + H20. ( 5 ) can be expressed by the relationship : The experimental results satisfy these conditions as is evident from fig. 3, and lead to a value of k~/,k1 = 4-7. 0 0 L-.; 2 I 3 4 I J 5 [Cu2+] (mole/l.) x 103 FIG. 3.-Competition plot for the nitrogen yields in the cupric ionfN20 + isopropanol system. Representing the reactions of Ha as those of hydrogen atoms, viz., H + (CH&CHOH-,H2 + (CH3)zCOH (6) H+CU~+-,CU++H+, (7) one obtains the equation : 1 1 k,[Cu2+] =- +- 1 G(H2)- G& G(H") G(H") k,[isopr.]' where G(H2) is the observed hydrogen yield.Taking GZ, = 0.35, the experimental hydrogen values obey this relationship (fig. 4), the plot giving an intercept corres- ponding to G(Ha) = 0.66 and a slope corresponding to a value of k,/k6 = 12.4. The effects of pH on the yield of the gaseous products in the N20 +isopropanol system have also been investigated.11 Increase of hydrogen ion concentration leads to a decrease in G(N2) and a corresponding increase in G(H2). This is the result of the reaction (H20)-+H30f+2H20+H (8) competing with reaction (1). From these data, a value of k&l = 1.7 has been obtained, in agreement with other workers.4, 12 In alkaline solution, the N20 + isopropanol system exhibits a radiation-induced chain reaction leading to high yields of acetone and of nitrogen. For example,21 8 RADIOLYSIS OF AQUEOUS SOLUTIONS at pH 13, a value of G(N2)-50 was obtained.It appears that the isopropanol radical-ion can interact with nitrous oxide giving an oxidizing radical, viz., 0- 0- 1 I CH3C CH3 + OH-CH34iCH3 + H20 (9) H 0- I CH3C CH3 + N20 + H20+CH3COCH3 + N2 + OH+ OH- with a chain-breaking process, e.g., the disproportionation of two alcohol radicals giving acetone and isopropanol. Solutions of N20 containing one of several other aliphatic alcohols have also been investigated. As with isopropanol + N20 solutions, where the negative polarons react essentially with N20, yields of nitrogen in the range G(N2) = 3.0-3.3 (10) I 0 I 2 3 4 5 [Cu2+](moIe/l.) x 103 FIG.4.-Competition plot for the hydrogen yields in the cupric ion+NZO+isopropanol system. were observed. The yields of hydrogen, however, vary, depending upon the reactivity of the organic substance towards He. This is evident from the data of table 1 where hydrogen yields from aqueous solutions of methanol, isopropanol and t-butanol at various concentrations are given. TABLE 1.-YIELDS OF HYDROGEN IN THE 7-RADIOLYSIS OF AQUEOUS SOLUTIONS OF N20 (1.6 x 10-2 M) AND ALIPHATIC ALCOHOLS : pH- 6 methanol t-butanol isopropanol yield aIcohol yield alcohol yield (moles/l. ) G(Hd (moles/I.) G(Hd (moles/I.) G(Hd 10-3 0 . 6 1 10-3 0.42 10-3 0.94 alcohol 10-2 0.94 10-2 0.54 10-2 1 -02 10-1 1 -05 10-1 0.88 10-1 1.10 1 1 -22 The suggestion 4 that the product of the reaction of negative polarons with N20 is an oxidizing species has been confirmed by an examination of the product yieldsG .SCHOLES, M. SIMIC AND J . J . WEISS 219 from methanol + N20 solutions. The following are the yields observed on irradi- ation of 10-1 M methanol-1.6 x 10-2 M N20 solutions : G(N2) = 3.10 ; G(H2) = 1.05 ; G(H202) = 0.45 ; G (formaldehyde) = 0.13 ; G (glycol) = 3.3+_0.2 (deter- mined with periodate 11). For the mechanism : OH + CH30H+*CH2OH Ha+ CH30H+*CH2OH + H2 2*CH2OH-+HCHQ + CH30H 2CH2OH- (CHZOH)~ rhere the oxidizing radicals reacting with methanol come both from the and from reaction (l), it follows that 2G(glycol) + 2G(formaldehyde) = G(0H) + G(Ha) = 7.2 & 0.4.Assuming that for every (H20)- reacting with N20 a corresponding number of oxidizing species from the water reacts with methanol and also that an additional number of OH radicals equivalent to Ha reacts with the alcohol, it can be shown that G(OH) = 6.7. Thus, G(OH)+G(Ha) = 7.4 agrees well with that from the organic products yield found above. Solutions of N20 at neutral pH are useful for the measurement of the reactivities of various compounds towards the negative polarons. Substances which compete efficiently with N20 for (H2O)- lower G(N2) and the relative rates compared to N20 can thus be obtained, as for example, with acetone and chloroacetate. In this way, it has been found that the anions of carboxylic' acids, e.g., formic acid and acetic acid, cannot compete with 1.6 x 10-2 M N20 and therefore must have relatively low reativities towards the negative polarons.EXPERIMENTS WITH c02 The influence of COz in the radiolysis of aqueous solutions has been ascribed to the reaction : 3, 139 14 (H,O)-+CO,+COT +H,O. Production of the COT radical-ion in the irradiation of aquo-organic systems can lead to the formation of carboxylic acids.14 In general, the carboxylation process depends upon the formation of a free radical from the organic solute (RH), e.g., by the process, RH+OH+R*+H20, (16) R*+CO,+RCQ, (17) and association of the free radical with COT according to A variety of organic solutes have been carboxylated in this way. Many of the ex- periments were carried out with carbon dioxide or bicarbonate labelled with carbon- 14, this technique being used to facilitate identification and analysis of the carboxylated products. Irradiation of aqueous solutions of methanol (10-3 M) with C060 y-rays gave glycollic acid.Under similar conditions, irradiation of aqueous ethanol gave some lactic acid, acetic acid solutions some malonic acid, solutions of methane some acetic acid and from formic acid solutions containing C14-labelled CO2, labelled oxalic acid was obtained. In general, the extent of carboxylation is markedly220 RADIOLYSIS OF AQUEOUS SOLUTIONS pH-dependent, falling off rapidly in acid solution. This is not unexpected since reaction (8) competes with reaction (15). In preliminary studies 15 of the extent of carboxylation in methanol + C02 solutions the maximum yield was G(glycol1ic acid) = 2.3, obtained on irradiation of 10-1 M methanol solutions saturated with C02 and adjusted to pH4.5-5.5.This seemed indicative of a somewhat lower negative polaron yield than in alcohol solutions irradiated in the presence of N20. However, a re-investigation of this system has shown that the lower glycollic acid yield results from a loss of some of the COT radical-ions due to the process, COY +H202+C02+OH+OH-. (18) There is, in fact, a low stationary concentration of hydrogen peroxide in solutions irradiated in the neutral pH range. Correction for the extent of reaction (18) leads to a minimum value of G(C0;) of 2.7-2-8. In addition, as in the &O+methanol system, Ha is also observed in the presence of C02, reacting with methanol to form hydrogen.1 - 0 % to4 lo3 lo2 lo-’ t sodium bicarbonate (mole/l.) FIG. 5.-Irradiation with C060 y-rays of deaerated aqueous solutions of sodium formate in the presence of sodium bicarbonate. Dependence of the yield of C-14-labelled carboxyl group in oxalic acid on the concentration of C-14-labelled bicarbonate; 0, 10-3 M formate; 0, 10-1 M formate. With formate solutions, relatively high carboxylation yields have been obtained. Fig. 5 shows the results using HCOONafNaHC1403 solutions. The data are given in terms of C1402, incorporated into the radiation-produced oxalic acid. The ex- periments show that it is the COa (in equilibrium with the bicarbonate ion) rather than the bicarbonate ion itself which reacts with the negative polarons. Such a view is also supported by similar results obtained with methanol+HCO; solu- tions.16 In the formate system, oxalic acid is presumably produced by the process 17 2c0, -+(coo-)2.(19)G . SCHOLES, M. SIMIC AND J . J.. WEISS 221 Isotopically-labelled COT radical-ions result from reaction (1 5 ) whereas unlabelled ones are formed mainly according to OH + HCOO--+H20 + COO- (20) Hence both -0OC14 COO- and -OOC14 C14OO- wifl be formed. In 10-1 M formate solutions, values up to G(C14O;) 21 4.5 have been observed (fig. 5). Since formate has a relatively high reactivity towards the oxidizing species formed in the radiolysis of water,l8 these high yields may be due to an increased availability of negative polarons as a result of some competition by formate with the recombin- ation reactions (reactions (3) and (4)).In all the systems examined, no carboxyl- ation occurs in strongly alkaline solutions. This is understandable if the negative polarons can only react efficiently with C02. When irradiations are carried out in the presence of organic acceptors which can be dehydrogenated, the competition of reactions (8) and (15) manifests itself in a pH-dependent hydrogen yield.19 Allan et aZ.,19 from studies of the yields of hydrogen from irradiated ethanol+ COz solutions, reported that k8/k15 1: 3. There- fore, kl/kls N 1.8. Dr. A. Appleby of this department has recently measured the yields of nitrogen on irradiation of solutions containing various amounts of N20 and COa, obtaining directly a value of kl/k15=2-2, which is in good agreement with the above ratio.CONCLUSION The systems described above can be readily interpreted on the basis of the presence of two reducing species. The fact that Ha reacts with Cu2+ ions, as well as with ferricyanide,g means that the appearance of this species is not specifically associated with the presence of organic compounds as such. The identification of the dehydrogenating species as a hydrogen atom appears to be supported by kinetic evidence 9 as well as by its reactions with OH- ions.20. 21 These hydrogen atoms could be formed either, as originally suggested,3 from excited water molecules, H20* +H + OH, or from the reaction, (H20)- + H30++H+ 2H20, occurring in the spurs.22 The existence of Ha in systems containing oxygen has been questioned; Czapski and Allen23 concluded that in the y-radiolysis of solutions of oxygen and hydrogen peroxide, the reducing radicals behave as if they were all (H20)-.On the other hand, Hummel and Allen24 in an investigation of the yields of hydrogen on y- irradiation of aqueous ethanol solutions containing oxygen, concluded that there was a competition between ethanol and oxygen for a reducing species, but were unable to decide whether this was the negative polaron or a hydrogen atom. This dehydrogenation of an organic solute in the presence of oxygen has recently been investigated using formate solutions,25 the latter solute being chosen since its reaction with the negative polaron does not lead to the formation of hydrogen? The results indicated that it was Ha which was competing between formate and oxygen.Some questions still remain concerning the yields of the reducing species in solutions containing oxygen and investigations along these lines should lead to information which may help to elucidate the problem of the exact nature and distribution of these hydrogen atoms. We acknowledge with thanks the financial support of this investigation by the U.S. Department of the Army, through Contract No. DA-9 1-591-EUC-2750.222 RADIOLYSIS OF AQUEOUS SOLUTIONS 1 Hayon and Weiss, Proc. U.N. Int. Conf. Peaceful Uses Atomic Energy (Geneva, 1958), 29, 80. 2 Baxendale and Hughes, Z. Physik. Chem., 1958, 14, 306, 323. 3 Allan and Scholes, Nature, 1960, 187, 218. 4 Dainton and Peterson, Nature, 1960,186,878 ; Proc. Roy. SOC. A, 1962,267,443. 5 Hayon and Allen, J. Physic. Chem., 1961, 65, 2181. Czapski and Schwarz, J. Physic. Chenz., 1962, 66, 471. 7 Hart and Boag, J. Amer. Chem. SOC., 1962, 84, 4090. Keene, Nature, 1963, 197, 47. 8 Weiss, Nature, 1960, 186, 751. 9 Rabani, J. Amer. Chem. SOC., 1962, 84, 868. Rabani and Stein, J. Chem. Physics, 1962, 37, 1865. 10 Dorfman and Taub, J. Amer. Chem. SOC., 1963, 85, 2370. 11 Munday, unpublished resdts. 12 Jortner, Ottolenghi and Stein, J. Physic. Chem., 1962, 66, 2037. 13 Getoff, Scholes and Weiss, Tetrahedron Letters, 1960, 18, 17. 14 Simic, Scholes and Weiss, Nature, 1960, 188, 1019. 15 Appleby, Holian, Scholes, Simic and Weiss, Proc. 2nd Int. Congr. Radiation Res. (1962). 16 Appleby, Ph.D. Xhesis (University of Durham, 1963). 17 Czapski, Rabani and Stein, Trans. Paraday SOC., 1962,58,2160. 18 Rabani and Stein, Trans. Faraday Soc., 1962,58,2150. 19 Allan, 6etoff, Lehmann, Nixon, Scholes and Simic, J. Inorg. Nucl. Chem., 1961, 19, 204. 20 Allan, Robinson and Scholes, Proc. Chem. SOC., 1962, 381. 21 Rabani, personal communication. 22Lifshit2, Can. J. Chem., 1962, 40, 1903. 23 Czapski and Allen, J. Physic. Chem., 1962,66,471. 24 Hummel and Allen, Radiation Res., 1962, 17, 302. 25 Scholes and Simic, Nature, 1963, 199, 276. 26 Smithies and Hart, J. Amer. Chem. SOC., 1960, 82, 4775.