J. CHEM. SOC. FARADAY TRANS., 1994, 90(4), 591-595 591 Primary Yields of Water Radiolysis in Concentrated Nitric Acid Solutions Ryuji Nagaishi, Pei-Yun Jiang, Yosuke Katsumura" and Kenkichi lshigure Department of Quantum Engineering and Systems Science, Faculty of Engineering, University of Tokyo, 73-1 Hongo, Bunkyo-ku, Tokyo 113,Japan The primary yields of water radiolysis have been evaluated as a function of nitric acid or nitrate concentration in the 6oCo y-radiolysis of nitric acid and 0.4 rnol dm-3 sulfuric acid with nitric acid or sodium nitrate solutions containing Ce'" and Cell', in the presence and absence of TI+. The radiolytic decomposition of water is signifi- cantly enhanced in these systems, from g,( -H,O) = 0.60 pmol J-' for 0.4 rnol dm-3 sulfuric acid-sodium nitrate solutions tog,( -H,O) = 0.76 pmol J-' for nitric acid solutions at high solute concentrations.In order to understand the radiolysis of concentrated nitric acid solutions, we have studied the direct action of radiation of nitric acid, which has been shown to give rise to NO,+ e,& and 0 + NO,.'.2 Since nitrate anion is a strong scav- enger of pre-hydrated (e;) and hydrated (e,) the radiolysis of water is significantly influenced in nitric acid solutions, which is evident from the fact that the radiolytic reduction of the Ce" to Ce"' is remarkably enhanced in nitric acid and acidic nitrate sol~tions.~-'~ However, there seems to have been no estimation of the primary yields of water radiolysis in concentrated nitric acid and acidic nitrate solu- tions, which are indispensable for a fuller understanding of the radiolytic processes.The radiolytic reduction of Ce" to Ce"' in nitric acid, 0.4 mol dmP3 sulfuric acid-nitric acid and 0.4 mol dmP3 sulfuric acid-sodium nitrate solutions in the presence and absence of Tl+ has been studied in the present work with 6oCo y-radiation. The primary yields of water radiolysis have been evaluated as a function of nitric acid or nitrate concentration for these systems. Experimental Nitric acid solutions, sodium nitrate and other chemicals were of the highest available purity and were used as sup- plied. The water was purified by distillation followed by fil- tration through a millipore system. Aerated solutions were irradiated at room temperature with a 3 kCi 6oCo y-source, which provided dose rates in the range 0.02-0.14 Gy s-l as determined by the Fricke dosimeter with G(Fe3') = 1.62 pmol J-'.The concentrations of Ce" were measured spec- trophotometrically using molar absorption coefficients deter- mined in the present study. The energy deposition was assumed to be proportional to the electron density and was corrected accordingly. Results and Discussion Molar Absorption Coefficients (E) of Ce" Stock solutions of 4 mmol dm', Ce(S04), in 0.4 mol dm-, sulfuric acid were prepared and the concentration of Ce" as determined on the basis of &(Ce", 320 nm) = 561 m2 mol-' at 298 K in 0.4 mol dmP3 sulfuric acid solution^.'^ Solutions of 0.4 mol dm-3 sulfuric acid with nitric acid or sodium nitrate containing 0.2-0.4 mmol dm- Ce(SO,), were pre- pared by diluting the stock solutions and the concentrations of Ce" were calculated.The absorbances of the solutions were measured and the molar absorption coefficients of Ce" were evaluated at selected wavelengths in the range 350-420 nm. The decadic &(Ce", 370 nm) values obtained are shown in Table 1 for 0.4 mol dm-3 sulfuric acid-nitric acid solu- tions and 0.4 mol dm-3 sulfuric acid-sodium nitrate solu-tions. For nitric acid solutions, a weighed amount of anhydrous (NH,),Ce(NO,), was dissolved into the solutions and the concentrations of Ce" (0.2-0.6 mmol dm-3) were calculated. The decadic &(Ce", 370 nm) values obtained are also shown in Table 1.The determination of Ce" in irradi- ated solutions was carried out at several wavelengths in the range 350-390 nm where only Ce" absorbs. The measure- ment and the molar absorption coefficients of Ce" are not affected by the presence of Ce"' and Tl', at least in the wave- length range 350-390 nm. Yields of Radiolytic Reduction of Ce'" to Ce"' Aerated nitric acid and 0.4 mol dm-3 sulfuric acid with nitric acid or sodium nitrate solutions containing Ce'" and Ce"' with or without T1' were irradiated and analysed for CeIV. The concentrations of Ce" were 0.4-0.6 mmol dmP3 before irradiation and the ratios of [Ce"']/[Ce"'] were kept between Table 1 Molar absorption coefficients (370 nm) of Ce" (m2mol-') 0 253 0.25 250 0.25 258 0.14 40.5 0.50 249 0.50 262 0.60 63.5 0.75 250 0.75 266 0.8 1 75.3 1.o 253 1.o 269 1.1 89.0 2.0 266 2.0 278 2.2 135 3.0 283 3.0 283 3.1 178 4.0 304 4.0 286 4.2 219 5.0 329 5.0 288 5.1 303 6.0 357 6.0 289 6.1 390 7.0 387 - 7.1 445 8.0 417 8.1 470 2.0 I 1 I 1.5 c I 7-0 --.E, h 1.0 20" I Y Q 0.50 0.0 0.0 2.0 4.0 6.0 8.0 [HNO, or NaNO,]/mol dm-, Fig.1 Yields of radiolytic reduction of Ce'" to Ce"' in the presence and absence of T1+ in nitric acid and 0.4 rnol dm-3 sulfuric acid with nitric acid or sodium nitrate solutions. Nitric acid: (0)without Tl';(a)5.0 mmol dm -TI 0.4 mol dm- ' sulfuric acid-nitric acid : (A)+ .without T1+; (A)1.0 mmol dm-j TI+.0.4 mol dm-3 sulfuric acid- sodium nitrate: (0)without Tl+;(.)1.0 mmol dm-3 TI+. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 was neglected in the present study because of the small elec- tron fraction for sulfuric acid. H,O- e,;, H, H, , OH, H,O,, H' (1) gw(-H,O) = gw(ea; + H) + 2gdH2) = g,(OH) + 2gw(H,O,) (1) NO;(HNO,)-.,NO, + eJe,; + H') (2) gSl(-nitric acid) = gs1(N03)= gSl(ea;) (11) NO;-.,O+NO, (3) gs,(-NOi) = Ssz(N02)= gs20 (111) HNO, -., 0 + HNO, (4) d2(-HN03) = d2(HN02) = !&2(O) (IV) 0 + NO, + 0, + NO, (5) In the solutions containing Ce" and Ce"' without Tl', Ce"' is oxidized by OH and NO, and Ce" reduced by HNO, and H,02 as well as by e;/e,; and H uia formation of HNO, by the following reactions [( 1 1)-( 15)I.l' 6-' Assuming that 9' H, and H,O, are only formed from the radiolysis of water, the yields of G(-Ce") in the absence of TI', G,(-CeIV), can be expressed by eqn.(V) and the yields of oxygen G(0,) by eqn. (VIa) and (VIb), wheref, andf, are the electron fractions of water and nitric acid or nitrate anion calculated from their total (inner-sphere and valence) electrons, respectively, with f, +fw = 1, and a is the degree of dissociation of nitric acid. 3 and 3 during irradiation, with Ce"' being added prior to irradiation or introduced by pre-irradiation. The initial con- centration of T1' was 1.0 mmol dm-, for 0.4 mol dm-, sul- furic acid with nitric acid or sodium nitrate solutions and 5.0 mmol dmP3 for nitric acid solutions.The dose rates were ca. 0.02-0.075 Gy s-l in the presence of T1' and 0.06-0.14 Gy s-' in its absence. The yields of the reduction of CeW to Ce,"' G( -Ce"), were calculated from the [Ce'v]-dose curves with good linearity in the dose range studied, i.e. less than 330 Gy. The results are shown in Fig. 1. The radiolytic reduction of CetV to Ce'" in 0.4 mol dm-, sulfuric acid has been studied extensively and established as a standard dosimeter. l4 In nitric acid and 0.4 mol dm-, sulfuric acid with nitric acid or sodium nitrate solutions, the yields, G( -Cew), are remark- ably enhanced. Addition of T1' nearly doubles G( -Ce"'). Our results in 0.4 mol dm-, sulfuric acid with nitric acid or sodium nitrate solutions are in good agreement with those reported previously7-' whereas the present results in nitric acid solutions are appreciably higher than those reported by Bugaenko and Roschektaer" and Vladimirova et ~1.'~As shown in Fig.1, the three systems are slightly different from each other. Reaction Mechanism of Radiolytic Reduction of Ce'" to Ce"' The radiolysis of water generates e;, H, H,, OH, H,O, and H' as primary products and the radiolysis of nitric acid gives rise to NO, + e,; and 0 + Material balances were assumed for these processes. The yield, g(e,;), designates the sum of pre-hydrated and hydrated electrons. The oxygen atom formed in reactions (3) and (4) has been shown to be probably in the triplet state, O(,P), which reacts with nitrate anion to yield 0, + NO, by reaction (5)., The added metal ions, Ce", Ce"' and Tl', cannot compete for the oxygen atom owing to their low concentrations.The direct action of radiation on sulfuric acid gives rise to SO, + e,; ,15 which Ce"' + OH + H' + CetV+ H,O (6) OH + HNO, + H,O + NO, (7) Ce"' + NO, + CetV+ NO; (8) 2CetV+ HNO, + H,O + 2Ce"' + NO, + 3H+ (9) 2Ce" + H,O, + 2Ce"' + 0, + 2H' (10) e,; + H++ H (1 1) e,g(ep) + NO, + H,O + NO, + 20H-(12) H + NO, +NO, + OH-(13) NO, + NO2 = N204 (14) N,04 + H20 + HNO, + H' + NO, (15) G,(-Ce'V) =fwCs,(e,, + H) -s,(OH) + 2gw(H20,)1 +f,[ssl(e,,) -gs,(NO,)l + 4f,Cag,,(-NO;) + (1 -a)g;2(-HNO,)I (V) G(02) =fwgw(H202) +f,C.s,,(-NO,) + (1 -4s:2(-HNO3)I (VI4 G(02) = CG,(-CetV) + 2fwgw(H2)1/4 (VIb) In the solutions containing Ce" and Ce"' with Tl', the following reactions (16)-(18) occur additionally.The yields of G(-CerV) in the presence of Tl', G,(-Ce"), can be expressed by eqn. (VII) and the yields of oxygen G(0,) still by eqn. (VIa). Thus eqn. (VII1)-(XIII) can be derived from eqn. (V) and (VII) and the material balance eqn. (I)-(IV) giving the primary yields of water radiolysis obtainable experimentally on the basis of G,(-Ce"), G2(-Ce"), G(H2),gsl,gs2,and g:, , where G(H,) is the observed yield of H, in the solutions. Primary Yields of Water Radiolysis The yields of G(H2) in nitric acid and neutral sodium nitrate solutions are identical up to at least 8 mol dm-3.9 For the three systems investigated in the present study, the yields of G(H2) were assumed to be the same as those in nitric acid and sodium nitrate solutions.The yields of gsl, gs2and g12 have been evaluated as 0.50,0.16 and 0.21 pmol J-' for nitric acid solutions1i2 and were assumed to be the same for the three systems of the present study. The concentrations of molecular HNO, were neglected for 0.4 mol dmP3 sulfuric 0.80 0.60 r I -7 f 0.40 -3 Q 0.60 0.40 0.20 t 0.0 0.0 2.0 4.0 6.0 8.0 [HNO,]/mol dm-, Fig. 3 Primary yields of water radiolysis in 0.4 mol dm-3 sulfuric acid-nitric acid solutions. gw(-H20) (O),gw(ea; + H) (A), g,(OH) (B)9gw(H,O,) (a)and gw(H2) (0). acid-sodium nitrate solutions and were assumed to be the same as those in nitric acid solutions2' for 0.4 mol dmP3 sulfuric acid-nitric acid solutions.In the calculation of the electron fractions, the contribution of sulfuric acid was neglected and for 0.4 mol dm-, sulfuric acid-sodium nitrate solutions, the electron fractions were modified as f, = f(NO,)/Cf(H,O) +f(NO,)I and f, = 1 -A 3 where f(H2O) and f(N0;) are the real electron fractions of water and nitrate anion, which means that the energy absorbed by Na+, by hypothesis, was shared by water and nitrate anion pro- portionally to their electron fractions. Thus we can calculate the primary yields of water radiolysis by eqn. (1X)-(XIII).The results are shown in Fig. 2-4. The decomposition of water Om80r---0.60 c I 7 0.40 f 0.20 0.0 0.0 2.0 4.0 6.0 8.0 [NaN03] /mol dm-Fig.4 Primary yields of water radiolysis in 0.4 mol dm-3 sulfuric acid-nitrate solutions. gw(-H,O) (O),gw(e, + H) (A), g,(OH) (B), gw(H20,)(a)and gw(H,) (0). 594 0.30 0.20 I 7-0 -..5. h N E! tl Q 0.10 0.0 0.0 2.0 4.0 6.0 8.0 [HNO, or NaNO,]/mol drr3 Fig. 5 The yields of (40,)in nitric acid and 0.4 mol dm-3 sulfuric acid-sodium nitrate solutions containing CeIV and Ce"'. Predicted G(0,) by eqn. (VIa) in nitric acid: (0)dose rate 0.02-0.14 Gy s-', this work and 0.4 mol dmP3 sulfuric acid-sodium nitrate (0):dose rate 0.02-0.14 Gy s-', this work. Calculated (40,)as [G,(-Ce'") + 2fwgw(H,)]/4in nitric acid: (A) dose rate 0.0016 Gy s-'; (0)dose rate 1.6 Gy s-', ref.13 and 0.4 rnol dm-3 sulfuric acid-sodium nitrate (A), ref. 10. Measured G(0,) in nitric acid: (B) dose rate 0.0016 Gy s-'; (+) dose rate 1.6 Gy s-', ref. 13 and 0.4 mol dm-3 sulfuric acid +sodium nitrate (O),ref. 10. increases rapidly with nitric acid or nitrate concentration at lower concentrations whereas it increases slowly or reaches a constant value at higher concentrations with gw(-H,O) values as high as 0.76 pmol J-' in higher than ca. 1 mol dm-, nitric acid solutions. Apparently the three systems differ from each other, presumably, owing to the influence of acidity on spur reactions, the nuances of which are rather difficult to understand quantitatively at present if possible. A rational description might be as follows: the scavenging of e;/ea; by NO; /HNO, leads to the increases in g,(ea; + H) and gw(-H,O) because of the lower reactivities of the NO, species formed and its precursors whereas the competition for ea; (but not e,) by H+ suppresses this effect due to the high reactivity of the H atom formed.The scavenging of OH by undissociated HNO, leads to increases in g,(OH) and g,( -H,O) owing to the lower reactivity of the NO, radical formed. Thus it is expected that the increases in g,(ea; + H), g,(OH) and g,( -H,O) should be greatest in pure nitric acid solutions and least in the 0.4 mol dm-3 sulfuric acid-sodium nitrate solutions, which is in general agreement with the experiment results. The formation of 0, has been investigated and the values of G(0,) reported for nitric acid and 0.4 mol dm-3 sulfuric acid-sodium nitrate solutions.'0~'3 The values of (30,)cal-culated from eqn.(VIa) on the basis of the primary yields evaluated in the present study are compared with the experi- mentally measured values of G(0,) as shown in Fig. 5, which are in good agreement for 0.4 mol dm-, sulfuric acid-sodium nitrate solutions" and for nitric acid solutions at lower dose rate under more or less similar experimental conditions to the present study.', The lower G(0,) values in nitric acid solu- tions at higher dose rate (10-100 times higher than those in the present study)', are presumably due to the reaction of H,O, with HNOz2' because of the higher dose rate and lower initial concentration of [Ce'"], = 0.1 mmol dm-3.The J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 values of G(0,) can also be obtained from eqn. (VIb) which is valid even when reaction (19) occurs. As shown also in Fig. 5, the values of G(0,) derived as [G,(-Ce") + 2f,gW(H,)]/4 are in good agreement with the experimental values of G(0,) under various conditions for nitric acid solutions' and 0.4 mol dmP3 sulfuric acid-sodium nitrate solutions" even when eqn. (VIa) is not applicable. These facts are in quantitive agreement with the present reaction mechanism and the derived primary yields. HNO, + H,O, + H,O + NO, + 3H+ (19) Attempts have been made to evaluate the yields of G(H,O,) in nitric acid solutions.".22 In 0.4 rnol dm-3 sul- furic acid-5.0 mol dm-3 sodium nitrate solutions, the value of G(H,O,) has been deduced as 0.12 pmol J-'," in reason- able agreement with the value of G(H,O,) =f,gw(H2O2) = 0.14 pmol J-' derived in the present study.In nitric acid solutions containing sulfanilamide as a HNO, scavenger, G(H,O,) has been obtained as 0.083 (0.05),0.084 (O.l), 0.11 (OS), and 0.114 (1.0 rnol dm-, nitric acid) pmol J-',,, increasing with nitric acid concentration, which is fairly con- sistent with the trend predicted in the present study. Since sulfanilamide is a scavenger of OH and NO,,' the system containing sulfanilaide is presumably different from that of the present study, which may give different G(H,O,) values. The mechanism of water radiolysis has been established and the primary yields are well documented. In 0.4 mol dm-, sulfuric acid solutions, g,(ea; + H) = 0.383, gw(H2)= 0.041, g,(OH) = 0.30, gw(H,O,) = 0.083 and g,( -H,O) = 0.466 pmol J- ' as measured by the Fricke dosimeter.In the pulse radiolysis studies, the initial value of g,(OH) was found to be 0.611 pmol J-' at 200 PS,,~ and that of g,(ea;) to be 0.50 pmol J-' at 30 ps24 and 0.48 pmol J-' at 100 PS.,~ These initial yields have been generally accepted in computer simu- lation of spur reactions.26 The yield of water decomposition in nitric acid solutions is as high as 0.76 pmol J-', which may give considerably higher initial yields for OH and ea;, presumably, corresponding to earlier times. The yield of water decomposition (initial or at 10-2-10-' s) has been obtained as 0.90,,' 0.71,,* ca.0.8,29and 0.70,' pmol J- ' by therotical calculation, which is supported by the present results. 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