J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 831-836 83 1 Pulse Radiolysis of Iodate in Aqueous Solution Stephen P. Mezyk* Research Chemistry Branch, AECL Research, Whiteshell Laboratories, Pina wa ,Manitoba, Canada ROE 1LO A. John Elliot System Chemistry and Corrosion, AECL Research, Chalk River Laboratories, Chalk River, Ontario, Canada KOJ 1PO The reactions of primary water radiolysis radicals with 10; have been re-investigated using electron pulse radiolysis and absorption spectroscopy. Rate constants for iodate reaction with e,,, C(5.45 f0.45) x lo9 dm3 mol-' s-'1, 0'-C(1.82 f0.04) x lo9 dm3 mol-' s-'1 and 'OH (<lo5 dm3 mol-' s-') have been measured and compared with previously determined values. The observed iodate concentration dependence of the two tran- sient absorptions produced by its reaction with the hydrated electron and the oxide radical were found to be due only to spur scavenging effects, and thus the dimerization equilibria proposed by previous workers is not neces- sary.The previously reported rate constants for the reaction of iodate with hydroxyl radicals have been shown to be due to just iodate reaction with the small concentration of 0'-present in equilibrium with the hydroxyl radical. The release of radioactive iodine to the environment follow- ing a nuclear reactor accident has been a concern in recent years.'-3 Owing to the large quantities of water also expected to be released in most accidents from water-cooled reactors, most of the iodine is assumed to be in solution within the reactor containment.As such, a good knowledge of aqueous iodine chemistry is necessary to model its behaviour accu-rately under these conditions. The formation of volatile iodine species is mediated by the aqueous chemistry that occurs. The reaction of volatile I, with water gives HO14 I, + H,OeHOI +I-+ H+ which further reacts viu the overall reaction 3HOIeIO; + 21-+ 3H+ to form the non-volatile iodate species in neutral or basic media. However, the presence of high radiation fields, which are also expected following an accident, can cause significant changes in the chemistry of the system. It has recently been shown, for example, that the radiolysis of iodate in the pres- ence of organics leads to the formation of CH,I.' Such low- molecular-weight iodoalkanes are more volatile and harder to contain than inorganic forms of iodine.A good knowledge of the aqueous radiation chemistry of iodate is needed in order to understand the mechanisms of radiolytic reactions that lead to volatile products. Unfor- tunately, the rate constants and mechanisms reported in the literature to date6-" are not in good agreement. This paper describes our investigation of the radiolytically induced oxidation and reduction reactions of iodate in aqueous solution, which resolves some of these previous dis- crepancies. Experimental Solutions of sodium iodate (Fisher Scientific, Certified) or potassium iodate (Aldrich, A.C.S. reagent) were prepared using triply distilled or Millipore water. The pH adjustments were made by using NaOH (Aldrich 99.99% semiconductor grade), KOH (Mallinckrodt Volumetric Solution, 1.0 mol dm-3), or HClO, (Baker, Analyzed), and by the addition of sodium phosphate (monobasic, Anachemica A.C.S.reagent) or sodium borate (Anachemica, A.C.S. reagent) buffers. Fisher certified sodium formate, potassium hydrogen-carbonate, potassium hexacyanoferrate(I1) and potassium thiocynate were all used as received. The electron pulse radiolysis facilities, at AECL Research, Chalk River Laboratories and the Radiation Laboratory, University of Notre Dame, were used for these experiments. Both these systems have been described in detail else-where.",12 Dosimetry was carried out using either 0,-(A = 475 nm, GE= 2.39 x lo4) or N,O-saturated (A= 472 nm, GE= 4.92 x lo4) mol dm-3 SCN- solutions, and corrections for the electron density of solutions were applied when appropriate. Throughout this paper, G is defined as the number of species produced or destroyed per 100 eV, and E is in units of dm3 mol -'cm -'.Care was taken with pH measurements throughout these experiments, with values only being recorded of solutions immediately before pulsing. It was found that in non-buffered solutions, pH changes of over one unit occurred during He or N20 bubbling, which significantly changed the chemistry that occurred. Solutions above pH 11 were made by quanti- tative dilution of 1.0 mol dm-, KOH, and their pHs calcu- lated based on the assumption that the concentration of hydroxide ions in the stock solution was 1.0 mol drn-,.All measurements were done at room temperature (21 f2°C). Results and Discussion Reaction of e& and C0;-The radiolysis of water produces the free radicals e,,,, 'H and 'OH, and the ensuing chemistry observed for iodate can be accounted for in terms of the reactions of these initial entities. By the addition of suitable scavengers, the reactions of specific species can be studied in isolation. The reduction of 10, by e;,, and C0;- may be under-stood in terms of the following mechanism: 10; + e,,, -+ IOj2-(1) 10, + c0;--b 10i2-+ CO, (2) I0i2-+ H+ =HOIO',- (34 I0i2-+ H,O=HOIO;-+ -OH (3b) There have been two assignments proposed for the absorb- ing species produced by the reduction of iodate.Buxton and Sellers'* postulated that the 425 nm species observed at higher pH was due to the species 1Oi2-, formed by the reac- tions (1) and (2), and that at lower pHs, the 490 nm absorp- tion was due to HOIO',-, formed by reaction (34. Support for this belief was obtained from pulse electron irradiated conductivity experiments, which found no conductivity change following the reduction of iodate by the hydrated electron, and also by an analysis of the pH dependence of the observed optical decays of iodate reduction by formate. This is in contrast to the findings of flash photolysis studies7*' performed in near neutral solutions. Based on a comparative study of aerated aqueous ClO, ,BrO, and 10, solutions,' the transient absorption produced for iodate photolysis was attributed to the YO2 radical, formed by primary decomposition of the iodate ion, 10, + hv+'I02 + 0-To reconcile these experimental observations, we believe that an additional reaction occurs for the photolytically pro- duced '102, '102 + H20 +HOIO',-+ H' (4) which gives the exact species created by pulse radiolysis.The rate constant for reaction (1) was measured in this study by observing the change in the decay of the hydrated electron at 600 nm as a function of iodate concentration over the range (1-10) x mol dm-3. The values obtained over the pH range 4.0-12.8 have an average value of (5.45 & 0.45) x lo9 dm3 mol-' s-'. This rate constant is about a factor of three lower than its diffusion-controlled value and, over the pH range studied, there does not appear to be any significant ionic strength effect on k,.We have no explanation for this observation; however, it is consistent with the behaviour of the self-reaction of the hydrated elec- tron.14 The individual data are shown in contrast to the pre- viously measured values in Table l,l59l6 and are seen to be about 20% lower. Evidence for pK, can be seen in Fig. 1 which shows the pH dependence of the spectra obtained from pulse irradiating helium-saturated, 10-mol dm -iodate solutions containing 1.0 mol dm-3 tert-butyl alcohol (Bu'OH) to scavenge the hydroxyl radical. These spectra show a single broad peak at Table 1 Comparison of the rate constants of this study with liter- ature values for radiolysis product reaction with iodate in aqueous soh tion reactive this work other values species pH k/dm3 mol-' s-' k/dm3 mol-' s-l (I 4.0 (5.02f0.08) x lo9 6.0 (5.03 k 0.15) x lo9 7.1 x lo9 (15) 7.0 (5.67 f0.08)x 109 7.7 x 109 (16) - 10.0 (5.70f0.15) x lo9 11.0 8.3 x lo9 (16) 12.0 (5.83 f0.25) x lo9 12.8 (5.46 f0.15) x lo9 14.0 9.6 x lo9 (16) c0;- 8.8 (8.1 f0.2) x 107 (1.3 f0.1) x lo8 (10) 'H - 9.5 x 107 (5) 0.- 14.0 (1.80f0.04)x 109 1.6 x 109 (8) 13.7 (1.85 f 0.04)x 109 3.0 x 109 (7) 2.0 x lo9 (6) 1.1 x lo7(8) 'OH < 105 <5 x lo7 (7) 9.2 x lo8 (6) ~~ a Reference number in parentheses. J.CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 4.0 -3.0 -u m-I 0 .-2.0 -1.0 - t J v' 0.01 I350 ' 400 " 450 500 " 550 ' I600 A/nm Fig. 1 End of pulse transient absorption spectra produced by the pulse electron irradiation (ca. 10 Gy) of He-saturated, mol dmW310; solutions, containing 1.0 mol dm-j Bu'OH at pH 14.0 (B),13.5 (V),13.0 (a),12.0(a),11.0 (A)and 7.0 (0) 490 nm for pH values lower than 12, whereas at higher pH values, the spectrum shifts to a peak centred at 425 nm. The dependence on pH of the measured intensity at various wavelengths across the spectrum was determined, with typical values shown in Fig. 2. The plotted E values were calculated from the measured GEvalues by assuming that the initial hydrated electron yield was enhanced by the competi- tive contribution from the hydrogen-atom reaction with the hydroxide ion at the different pHs, l7'H + -OH +e,,, + H20; k = 2.2 x lo7 dm3 mol-' s-' relative to its other scavenging reactions, l7'H + Bu'OH +products; k = 1.7 x lo5 dm3 mol-' s-' 'H + 10,+products; k = 5.9 x lo7 dm3 mol-' s-l The initial e,,, yield was taken as 2.86, based on the *OH spur scavenging effects of the 1.0 mol dm-3 Bu'OH.'' An initial 'H yield of 0.6 was assumed.The solid lines in Fig. 2 are calculated pK, curves, with an average value of 12.6 f0.1. An earlier study by Buxton and Sellers" reported 1200 -c'E 1000 -0 c -I 800 -0 600 -E'E 400 -200 -lll'lllt 11.0 . 11.5 . 12.0 . 12.5 ' 13.0 .13.5 . 14.0 I PH Fig. 2 pH dependence of the peak transient absorption coefficients observed in mol dm-3 10; ,He-saturated solutions at 375 (B), 400 (a)and 425 (A) nm. Solid lines are fitted intensity curves, corre- sponding to pK, values of 12.5, 12.6 and 12.7,respectively. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 a value for this pK, as 13.3; however, in their study, insuE- cient formate was present to scavenge all the 0'-produced at the highest pHs studied. This would allow the formation of the transient IOi2-(see later), a species that has an intense absorption peak at 360 nm, resulting in artificially high absorbances being obtained and thus a higher pK, value. The variation of the intensity of the 490 nm peak absorp- tion with iodate concentration has previously been reported to be due to the formation of a dimeric species' '10, + 10, 41,OJ-with a calculated equilibrium constant of ca.10 dm3 mol-'. Fig. 3 shows the yields determined at pH 5.2 in this study, over the concentration range 10-4-2.5 x lo-' mol drn-,. The fitted line corresponds to the increased yield expected from scavenging effects within spurs.' These theoretical values have been calculated from the equation where G(S)is the calculated scavenged electron yield, G,,, = 2.55 is the yield of electrons that escape spur recombination, Go = 4.80 is the initial electron yield and a is a constant that is related to the rate constant for iodate scavenging of spur electrons. Based on experimental observations of the reaction of hydrated electrons with CH,Cl," this constant is described by the expression a = 9.06 x lo-'' x k(e& + 10;) The calculated yields were normalized to the experimental GE data at low4mol dm-, iodate and the excellent agree- ment seen over the concentration range studied demonstrates that no such dimerization equilibrium needs to be considered.The reduction of iodate by the formate radical has also been investigated in this study. Formate acts as a scavenger of both 'H and 'OH radicals through the reactions 'H/'OH + HCO, -+H,/H,O + C0;-(5) and reduction of iodate then proceeds via reaction (2). The addition of 1.0 mol dm-3 HCO, to an N,O-saturated solu-4.5 I-=-I 251""1 ' ' "***" ' ' """' ' ' """' ' ''I 104 10-3 1o-2 10-1 [IO;]/mol dm-3 Fig.3 Iodate concentration dependence of the 490 nm absorbance in He-saturated solutions containing 5.0 x lop3 mol dmp3 NaH,PO, (pH 5.2) and 1.0 mol dm-3 Bu'OH. The solid line is the theoretical prediction of increased yield due to radiation spur scav- enging, based on the model of LaVerne and Pimblott." These two yields have been normalized at an iodate concentration of mol dm-'. tion containing mol dmF3 10, gave the same spectrum as that observed for e& reduction of iodate, with about twice the yield. The rate constant for reduction, k,, as estimated from the rate of formation of the transient absorption at 490 nm for this solution, was (8.1 f0.2) x lo7 mol dm-3 s-l, which is a little lower than previously reported" for a 0.20 mol dm-, formate solution (see Table 1).At lower formate concentrations, the observed rate of reaction was slightly slower; however, this decrease is readily explained by the expected ionic strength dependence of this reaction of two, single negatively charged, species. In this study, at pH 8.8 and at a constant iodate concentra- tion of 0.10 mol dmP3, little change in the observed decay rate at 490 nm was found when the formate concentration was changed from 1.0 mol dm-, (2k/~= 4.1 x lo6 cm s-l) to mol dm-3 (2k/c = 3.9 x lo6 cm s-'). These rate con- stants are in excellent agreement with the flash photolysis value reported previously7 (2kl.5= 4.0 & 0.7 x lo6 cm s-'). The Buxton and Sellers" analysis of the pH dependence of the observed decays of the species produced upon iodate reduction by formate was based on a pK, value that is too high.A re-analysis of these data is thus necessary. Although these data would have some interference from IOi2-forma-tion, from the incomplete formate scavenging of OD-,at worst only ca. 10%of the radicals produced would be in this form. This small contribution has been neglected in the fol- lowing kinetic treatment. The measured 2k/c values of Buxton and Sellers," at various pHs, are given in Table 2. From our experimental peak GEvalues shown in Fig. 1, absorption coefficients can be calculated, and thus these values converted to 2k decay rate constants. Under these experimental conditions, the absorp- tion decay is comprised of only two species in equilibrium, each reacting with itself and the other.Based on the Buxton and Sellers mechanism," this would be HOI0;-+ HOI0;-+products (6) IOi2-+ HOI0;-+products (7) 1Oi2-+ IOi2--+ products (8) An analysis of this decay mechanism gives the observed bimolecular rate constant as lo Refitting this equation to the newly calculated 2k data, using linear regression techniques and the value of K, deter-mined in this study, gives the values 2k, = (5.99 & 0.11) Table 2 Rate constants data taken from ref. 10 for decay of '10, at 480 nm in Ar-saturated 2.0 x mol dm-3 10; and 2.0 x lo-' mol dm-3 HCO; ~____ ~ 2k/109 dm3 mol-' s-' (WE)PH /lo6 cm spl experimental" calculatedb 14.0 2.1 f0.1 2.6 2.6 13.0 3.3 f0.1 4.1 4.1 12.0 5.1 0.1 5.8 5.8 11.0 8.7 5.1 f0.1 5.3 * 0.1 5.9 6.0 6.0 6.0 3.0 5.8 f 0.2 6.1 6.0 'Values of 2k calculated using absorption coefficient values from this work.Calculated values using the model described in the text. 834 x lo9 dm3 mol-' s-', k, = (1.19 & 0.06) x 10" dm3 mol-' s-' and 2k8 = (2.33 & 0.02) x lo9 dm3 mol-' s-'. The pH dependence of the calculated decay rate constants using this model is also shown in Table 2 and is seen to be in very good agreement with the experimental data. At both neutral and basic pHs, replacing 1.0 mol dm-, formate by 1.0 mol dm-, Bu'OH gave an increase of ca. 35% in the observed decay rate. This is in agreement with previous observations," and thus we also attribute this change to the reactions of HOIOi- and IOj2- radicals with the organic radicals produced by the scavenging of 'OH radicals.Further corroboration of this effect was obtained by com- puter simulation of the observed kinetic decays across the pH range 8.7-14. The decay of the reduced species to form non- absorbing products was found not to be purely second order. These curves were modelled by the following reaction scheme: radiation .H2O ' OH, e,,,, '€4 e(;,, + 10, -,IOj2-; k, = 5.45 x lo9 dm3 mol-' s-' H20$Hf + -OH; K,= 1.0 x IOj2-+ H+=HOIO',-; K, = 2.51 x lo-', dm3 mol-' HOI0;-+ HOI0;---* products; 2k6 = 5.99 x lo9 dm3 mol-' s-' HOI0;-+ 1Oi2--+products; k, = 1.19 x 10" dm3 mol-' s-' IOj2-+ IOj2-4products; 2k8 = 2.33 x lo9 dm3 mol-' s-' 'OH + (CH,),COH -,'CH2C(CH3),0H; l7k = 6.0 x lo8 dm3 mol-' s-' HOIO',-+ *CH,C(CH,),OH +products; k, (9) 1Oi2-+ 'CH,C(CH,),OH -,products; k,, (10) using the FACSIMILE2' modelling program and calculated absorption coefficients for the two absorbing species IOi2-and HOIOi-.The two equilibrium constants were incorpor- ated in the model by assuming rate constants for their forward and backward components; values for K, of k, = 1.11 x lop3s-' and k, = 1.11 x 10'' dm3 mol-' s-',~' and for K, of k, = 2.0 x 10" dm3 mo1-' s-' and k-, = 8.0 x lo8 s-I, were chosen. Varying the last values by an order of magnitude had no effect on the results obtained. The two unknown values, k, and klo, were optimized for each pH, and average values of k, = (2.45 f0.35) x lo8 dm3 mol-' s-' and klo = (1.59 & 0.28) x lo8 dm3 mol-' s-' were obtained.An example of the calculated fit to the experi- mental data at pH 12.7 is shown in Fig. 4. Reaction of 'H with 10, The reaction of hydrogen atoms with iodate has been pre- viously investigated by steady-state competition technique^,^ and a reaction rate constant determined as 5.9 x lo7 dm3 mol-' s-'. However, the pulse radiolysis of 0.10 mol dm-, 10, and 1.0 rnol dmV3 Bu'OH in pH 1-2 solution did not give any change in absorption; thus, no value of the reaction rate constant could be determined in this study. At present time-resolved EPR studies are underway to determine this J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3.0 -1 2.5 2.0 G7 1.5 z 1.o 0.5 0.0 1 I 1 1 1 I-0.5 I 0.0 20.0 40.0 60.0 80.0 f/P Fig. 4 Comparison of the experimental and calculated decay at 490 nm for a pulse electron irradiated (80.7 Gy), N,-saturated solution containing mol dm-3 10; and 1.0 mol dm-3 Bu'OH at pH 12.7 Reaction of 0'-with 10, At basic pHs, the 'OH radical deprotonates (pK = 11.9)22 to give O'-. This radical also reacts with iodate, according to7 0.-+ 10; -,10i2-(1 1) to produce an intense transient with a peak at 360 nm. The spectrum obtained in this study [Fig. 5(a)] agrees well with that previously determined.' By varying the 10, concentra-tion over the range (1-10) x mol drn-,, and observing the growth kinetics of the transient absorption in N20-saturated solutions, a reaction rate constant of (1.82 & 0.03) x lo9 dm3 mol-' s-' was obtained, [Fig.5(b)], in general agreement with previous determinations6-8 (see Table 1). The 360 nm transient absorption yield dependence on iodate concentration was also determined in this study and the data are shown in Fig. 6 for helium- or nitrogen-saturated solutions at pH 14. Under these experimental conditions the electron-reduced species, HOI0;-and 10;' -, have a signifi- cant contribution to the observed absorbance. Based on the spectra presented in Fig. 1, an absorption coefficient of E = 830 dm3 mol-' cm-l at 360 nm for the e;,, reduced species is calculated (it was assumed that no spur scavenging 1'1'1'1 -20 15 -$ 10-0 [10,]/10-3 rnol dm-3 5-I 0-1 200 300 400 500 600 700 800 A/nm Fig.5 (a) End of pulse transient absorption spectrum produced in N,O-saturated, rnol dm-3 10; solution at pH 14.0. (b) Deter-(.) mination of the 0'-reaction rate constant at 360 nm at pH 14 value. and 13.7 (A). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 12.0 t -1 6.01 1 t 1 104 i 0-3 10-2 lo-' [IO;]/mol drr~-~ Fig. 6 Iodate concentration dependence of the 360 nm absorbance produced in He- or N,-saturated solutions at pH 14.0 at end of pulse (I)in comparison to the calculated values for e;,, (-), 0'-(---) and total (. .) yields due to spur scavenging occurs for lop3 mol dm-3 10;).For the data in Fig. 6, this = 2780 at mol dm-3 10,.corresponds to GE,,~ A similar analysis can be performed to determine the 10;' -yield. Based on the measured23 total yield of oxidizing species, in N,O-saturated solutions at pH 14, of G = 7.65, an absorption coefficient at 360 nm of E = 2680 dm3 mol-' cm-' is determined from the data given in Fig. 5. For de- oxygenated solutions, the yield of oxidizing species has been determined as G = 3.87, which gives GE,, = 10380 for an iodate concentration of mol dm-3. The sum of these two values is GE,, = 13 160, ca. 3% higher than the experimental value of GE= 12770. To account for this small difference, each of the individual GE values was multiplied by 0.97, to give a normalized yield at mol dmP3 iodate.The effects of spur scavenging at higher iodate concentra- tions for each component were calculated using the method of Laverne and Pimbl~tt.'~ The initial yield and escape probability of 0'-have been assumed to be the same as for 'OH. This approximation was made since no direct param- eters for 0'-scavenging could be found in the literature. These values are shown in Fig. 6 and the excellent agreement between the total scavenged yield and the experimental values shows that only this process is occurring. At higher doses, the decay of the absorption at 360 nm was found to be second order to zero intensity. The rate constant for an N,O-saturated, lop3mol dm-3 10, solution at pH 14 was 2k,k = 1.37 x lo5 cm s-'. From the measured absorption coefficient of 10;' -at this wavelength, this corre- sponds to a value of 2k = 3.67 x 10' dm3 mol-' s-', in fair agreement with the previous determinationg of 1.8 x lo8 dm3 mol-' s-'.Owing to the high initial pH, a full ionic strength dependence on this decay was not performed. Reaction of 'OH + 10, The largest variation in the previously reported rate constant for iodate reaction is that for the 'OH radical. Previous deter- minations have given values in the range (1-100) x lo7 dm3 mo1-l s-' (see Table l),and although a transient peak with a maximum at 360 nm has been identified, little agreement on its intensity has been obtained. Our initial experiments, with only iodate in water, showed a weak transient spectrum with a maximum at 360 nm.The observed growth rate at this wavelength became faster at 835 higher iodate concentrations, over the range (5.0-20) x lo-, mol dm-3 and, although the data were very scattered, an apparent second-order rate constant of (1.0 0.4) x lo6 dm3 mol-' s-' was obtained. The spectrum shape obtained under these experimental conditions was essentially identical to that produced by the 0'-transient. This suggested that the reaction sequence that occurred was 'OH + -0HeO'-+ H,O (12) 0.-+ 10, +10;2-with the observed growth rate being limited by the forward component of the first reaction. This hypothesis was checked as follows. When the experiment was repeated using N,O-saturated, lo-, mol dm-3 iodate solutions, containing 5.0 x mol dm-3 phosphate buffer to ensure a constant pH of 5.2, no peak was observed.This finding is in agreement with the experiments of Buxton and Sellers." A very weak absorption rising at shorter wavelengths was seen, but there was essentially zero absorbance at 360 nm. As the reaction of 'OH with 10, did not appear to give any product species absorbance, the measurement of its rate constant by Fe(CN):- and HCO, competition kinetics was attempted. A solution containing loW4mol dm-3 Fe(CN):- and 0.10 mol dm-3 10; gave the same intensity change at 420 nm as only mol dm-3 Fe(CN):- and similarly, the transient absorbance at 600 nm in a solution of 0.10 rnol dm-3 HCO; did not change upon addition of 0.10 mol dm-3 10;. Based on the known rate constants for the reac- tion of 'OH with Fe(CN):- (k = 1.05 x 10" dm3 mol-' sW1)l7and HCO, (k = 8.5 x lo6 dm3 mol-' s-'),~ and the assumption that a 10% change in the observed intensity could be detected, we believe that the rate constant for the reaction 'OH + 10, + products has to be <lo5 dm3 mol-' SKI.This value is significantly slower than the three reported values in the In these experiments it was found that the pH of unbuffered, air-saturated, 10, solutions was ca.7.7, but this rose to >8.7 upon bubbling with He or N,O. At these more basic pHs, a greater fraction of the 'OH radical exists in its base form, Om-,which can react with iodate ions. Even though the equilibrium concentration of 0'-would typically only be 0.1% of 'OH, the only other loss of 'OH radicals is by the relatively slow process of H,O, production; thus, iodate scavenging of 0'-could severely perturb this system.The observed absorbance change at 350 nm in an N,O- saturated solution containing 5.1 x mol dm-3 10; and 1.1 x mol dm-3 sodium tetraborate is shown in Fig. 7. To test our hypothesis, this curve was modelled by the fol- lowing simplified reaction scheme (rate constants not from this study taken from ref. 17) 'OH + -OH + 0'-+ H,O; k,, = 1.3 x 10" dm3 mol-ls-' 0'-+ H,O +'OH + -OH; k-12 = 1.86 x lo6 dm3 mol-ls-' 0'-+ 10; +IOi2-; k,, = 1.84 x lo9 dm3 mol-' s-' e& + N,O +O*-+ N,; k = 8.9 x lo9 dm3 mol-' s-* 'OH + 'OH + H,O,; k = 4.5 x lo9 dm3 mol-' s-' 836 3.5 I I I I I I 113.0 -I 2.5 2.0 (5 m 1.5 0 F 1.o 0.5 0.0 1 V." 0.0 20.0 40.0 60.0 80.0 tlW Fig.7 Comparison of the experimental and calculated kinetic data at 350 nm for a pulse electron irradiated (4.90 Gy), N,O-saturated solution containing 5.1 x mol dm-3 10; and 1.1 x mol dm-' sodium tetraborate at pH 9.2 e,,, + 10, + H+ HOI0;-; k, = 5.45 x lo9 dm3 mol-' s-' HOIOi-+ HOI0;-+products; 2k6 = 5.99 x lo9 dm3 mol-' s-' 10:: -+ IOi2--+ products; 2k = 3.67 x lo8 dm3 mol-' s-l 10;' -+products ; k 13 (14) again using the FACSIMILE2' program and known absorp- tion coefficients for all the absorbing species. The k13 rate constant was fitted to the experimental data by the program, with an optimized value of 5.9 x lo3s-' obtained.This first- order decay of the IOi2-species had to be included to simu- late the experimental decay timescales. The agreement of this value to the previously measured value of 3.3 x lo3 s-' gives further support to this model. The calculated fit for this curve is also shown in Fig. 7 and is seen to be in fair agree- men t. Conclusion The major conclusion of this work is that the rate constant for 'OH reaction with iodate is much slower (<lo5 dm3 mo1-l s-') than previously The reaction inter- mediate observed at 360 nm under these conditions has been shown to be due to the formation of IOk2-by reaction of iodate with Ow-,where this species is produced by the reac- tion 'OH + -OH --+ H20 + 0'-J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 The iodate concentration dependence of the yield of the transients produced by e,,, and 0'-reaction with 10, has been shown to be due only to spur scavenging, and thus no additional dimerization equilibria need be considered. We thank Dr. R. H. Schuler and the staff of the Radiation Laboratory for the use of their facilities in this study, and Drs. N. Sagert and G. V. Buxton for helpful discussions throughout the course of this work. We also thank a referee who pointed out some inconsistencies in an earlier version of this manuscript. References 1 Proc. 1st CSNI Workshop on Iodine Chemistry in Reactor Safety, ed. A. M. Deane and P. E. Potter, Harwell Research Report, AERE-R 11974,1986. 2 Proc. 2nd CSNI Workshop on Iodine Chemistry in Reactor Safety, ed.A. C. Vikis, Atomic Energy of Canada Ltd. Research Report, AECL-9923,1989. 3 Proc. 3rd CSNI Workshop on Iodine Chemistry in Reactor Safety, ed. K. Ishigure, M. Saeki, K. Soda and J. 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