RADIOLYSIS OF AQUICOUS PERCHLORIC ACIDSOLUTIONS
reactions can account for the production of stable molecules of mass lower than the molecular weight of the parent molecule in the radiolysis of hydrocarbons.
3107
Acknowledgment. We are very grateful to Mr. W. C. Gieger for performing these experiments with his accustomed competence.
The RadiolysiEi of Aqueous Perchloric Acid Solutions’
by D. Katakis2 and A. 0.Allen Chemistry Department, Brookhaven National Laboratory, Upton, N e w York
(Receive8 November 6 , 1963)
The radiolysis of aqueous perchloric acid solutions was studied as a function of concentratlon in the absence and presence of radical scavengers. Free radicals from water radiolysis appear not to react with perchlorate ion. Perchlorate decomposes, as reported by others, by the direct action of radiation, with a yield increasing directly with its concentration, while the yields of Hz and other water decomposition products decrease. The major product of perchlorate ion decomposition is chlorate; most of the chloride formed arises from secondary decomposition of the chlorate ions. Excited perchlorate ions appear mainly to decompose, not to electrons and Clod radicals, but to chlorate ions and oxygen atoms.
Most studies in the radiation chemistry of aqueous solutions have been concerned with relatively dilute solutions, in which the energy of the radiation is essentially all given up to the water, and solutes are affected only by remon of their reactions with active species formed by decomposition of the water. In such systems the reaction yields are independent of the solute concentration, or nearly so. At higher concentration ranges, an apprecialble fraction of the radiation energy will appear as excitation of the solute molecules; the resulting reactions should appear with a yield approximately proportional to the solute concentration. Such a reaction is often called a ‘ldirect effect,” as distinguished from the “indirect effect” due to reactions of active radiolysis products of the solvent. The direct effect in the radiolysis of perchloric acid in water solutions was shown in 1952 by Milling, Stein, and Weiss3 and later a more detailed study was published by Cottin.4 Perchlorate E;olutions are particularly useful in studying the radiation chemistry of metal ions in solution, because the perchlorate ions do not form complexes with the metal ions and because free radicals
generated in the water appear to act less readily with perchlorate than with other common anions such as sulfate, nitrate, or chloride. To use perchloric acid in this way, however, the direct effect of radiation on perchlorate must be understood and taken into account. In the present work, the earlier studies are extended and the reaction mechanism of the direct effect is discussed. Experimental Dosimetry. yIrradiations were made in a cylindrical Corn source a t a dose rate of about 3.2 X 1020 e.v./l. min. Doses were determined with the Fricke dosimeter assuming G(Fe+a) = 15.5 and an extinction coefficient of €806 = 2180 at 24’. The energy deposition in solutions was assumed to be proportional to the electron density. For solutions where both H2SOe (1) Research performed under the auspices of the C . S. Atomic Energy Commission. (2) Nuclear Research Center Democritus, Aghia Paraskevi, Attikis, Greece. (3) B. Milling, G. Stein, and J. Weiss, Nature, 170, 710 (1952). (4) M . Cottin, J . chkm. phys., 53, 903 (1956).
volume 68, Number 11 Noaember; 1964
3108
and HC104 were present it was assumed that there is no volume change on mixing solutions of the two acids.5 Cyclotron irradiations employed the techniques of Schuler and Allen.6 Reagents. Baker Analyzed perchloric acid was diluted 1 :3, preirradiated to about 1 Mrad, and distilled in Vacuo a t 110'. The central cut of the distillate was used. Perchloric acid solutions were titrated with standard alkali using methyl red as indicator. The water used was triply distilled. Ce(HS04)d was purified by heating a t ca. 80' in a stream of oxygen and its solutions were left to age before use. All other reagents were analytical grade and used without further purification. A Welsbach ozonizer was used to produce ozonized oxygen. Preparation of Samples. The glassware was steamcleaned and preirradiated. For determination of gaseous products the samples were degassed in vacuo. For other determinations air was removed by passing helium successively through an activated charcoal trap immersed in liquid X2 and through the solution. Care was taken to have the irradiation vessels completely filled. Analytical Procedures. H 2 0 2 . The iodide method7 was used with NaI instead of KI to avoid precipitation of E(C104. Commercial S a 1 contains impurities which were destroying some of the H202and giving consistently low results. It was found necessary to add a little H202 to the buffered I- reagent, which gave a high blank but did not appreciably reduce the accuracy of the method. The change in the blank due to the oxidation of I- by air was followed as a function of time and taken into consideration. The acid solutions were neutralized with NaOH before mixing with the Ireagent. For zero dilution and a 1-cm. cell, 38.8 pmoles/l. of H202 correspond to one optical density unit. Chlorate was determined by difference. It does not react with I- in neutral solution. HzOz was determined in an aliquot by I-, then in another aliquot the sum of H202and CIOa- was found, using Fe+2in 0.7 M Hc104and determining Fe+3at 240 mp (E240 = 4185 a t 23'). No change in €240 was found by changing HC10, from 0.2 to 3 M . H202 oxidizes two ferrous ions, while C103- oxidizes six. To accelerate the reaction of Fe+2and c103-,1drop of 0.1% cadmium sulfate solution was added and the mixture heated to 80' for about 30 min., as recommended by C o t t h 4 An unirradiated aliquot was treated in the same way and the resulting small blank subtracted. The results were reproducible and consistent to within *2%. Chlorate in the presence of Ce" was determined in a similar way The Journal of Physical Chemistry
D. KATAKIS AND A. 0. ALLEN
by first determining CeIVand then the amount of Fe+z oxidized by both C103- and Ce". CeIVwas in 0.8 N HzS04 and was determined a t 320 mp using €320 = 5565.8 Changing the perchloric acid concentration from 0.2 to 3 M does not have any observable effect on €320 for Ce". The Ce'" yields were determined by measuring the decrease in Ce". T1+ does not interfere with the analysis. Fe+3 was determined spectrophotometrically as described above. For C1-, the method described by Hayon and Allen9 was used. The C1- concentration was calculated from a calibration curve. For O2 and H2, the low-pressure combustion method was used. l o Under the conditions of our experiments the only other possible products that are stable are Clz and CIOz. We found neither of them up to 4 M HC10,. The test consisted in determining, in one aliquot, H202by the I- method, then purging another aliquot with helium and analyzing again by the I- method. KO difference was observed. A small amount of oxidizing gas (presumably Clz) was found in ceric solutions containing chloride ion after standing several weeks, probably beIt was detercause of the oxidation of C1- by Ce". mined by passing helium through successive traps containing I- reagent. In the experiments reported this gas was removed before irradiation. The reaction was much too slow to produce any significant autoreduction of CeIV during the irradiation period.
Results A. Dilute Perchloric Acid Solutions-y-Rays. I. Air-Saturated. Figure 1 gives the H202 production in air-saturated perchloric acid solutions M in HC104) without anything else added. H202 is produced initially with a G value of 1.25, the same as in pure water,'l and then reaches a steady state at 590 pM. After 14.5 hr. (point not shown in the figure), the concentration of HzOarose slightly to 611 p M . 2. Air-Free. In the absence of air, 0.01 M HClO4 showed little decomposition and was very sensitive to impurities in a way reminiscent in many respects (H2, COz yields) of the behavior of pure water.12 (5) M. Uaanovich, T. Sumarokova, and V. Udovenko, Acta Physicochim. U R S S , 11, 606 (1939). (6) R. Sohuler and A. 0. Allen, Rev. Sci. Instr., 26, 1128 (1955). (7) C. J. Hoohanadel, J . Phys. Chem., 56, 587 (1952). (8) C . M. Henderson and N. Miller, Radiation Res., 13, 641 (1960). (9) E. Hayon and A. 0. Allen, J . Phys. Chem., 65, 2181 (1961). (10) E. R. Johnson and A. 0. Allen, J . Am. Chem. Soc., 74, 4147 (1962). (11) A. 0. Allen and R. A. Holroyd, ibid., 77, 5852 (1955). (12) H. Fricke, E. J. Hart, and H. P. Smith, J . Chem. Phys., 6 , 229 (1938).
RADIOLYSIS OF AQUEOUS PERCHLORIC ACIDSOLUTIONS
3109
2. Cl- and clo3- Yields. The chlorate yields are given in Fig. 3 both with and without chloride added
600
1
I 2.4
-
I
1
4
5
1
1
I
I
1
1
0
I
-
2.2 -
200
2.0 I50
1.8
-
Figure 1. Hydrogen peroxide production in air-saturated low2 M HClOd solution. Intensity = 3.24 X 1 0 2 0 e.v. 1.-' min.-'.
1.6
-
60
90
I20
IRRADIATION TIME ( m i d
1.4G
B. HC10, Solutions M in NaCl-y-Rays. 1. O2 and H , Yields in Degassed Xolutions. NaCl was added to eliminate the effect of the impurities and protect the molecular hydrogen from radical attack. It was found necessary to maintain the chloride concentration higher than usual, namely at M , in order to obtain reproducible results, and concentration vs. dose curves having no intercept. The hydrogen and oxygen G values were determined in solutions up to 11.8 M HC104 (Fig. 2). The concentration us. dose curves were all linear up to the highest doses used (around1 2.5 X e.v./l.). The oxygen yields are not too reliable because at high acid concentrations they are affected by post-irradiation reactions. Up to 3 - 4 M HC10, no post-irradiation effect occurred over a period of time much longer than that required normally to carry out the analysis. At acid concentrations above 5 M , the analysis was made after the thermal reactions were practically over. The intermediate region is uncertain. These thermal reactions were not investigated; it seems likely, however, that they involve a reaction between C1- and c103-,which is strongly dependent on acid and gives products which rapidly oxidize HX),. The discrepancy, at high acid, between our oxygen yields and those reported by Cottin4 may be due to these thermal reactions. The hydrogen yields are not affected by the post-irradiation reactions; the analysis gave the same results at all acid concentrations whether performed at once or 3 days after the irradiation. Addition to a solution 4.43 M in HC1O4, M in NaC1, of Fe+2 (3 X M) and Fe+3(7 x loF4104) resulted in a hydrogen yield of 0.25. For this acid concentration the hydrogen yield, from Fig. 2 , is 0.235. The difference is only a little outside experimental error. The result indicates that the fall in G(Hz)with increasing HCIOl concentration is not due to attack on the hydrogen by free radicals.
1.2 -
1 .o
-
0.8 0.6 0.4 0.2 0 0
1
2
3
6 7 8 9 ( H CI Oa),
1
1
1
2
fi
Figure 2. Effect of HC10, concentration on product yields in deaerated solutions containing 10-2M NaCl: 0, G(H2); 0,G ( 0 1 ) ; A, G(H202). 1.4
1
I
1
I
/'
I
/
1.2 1.0 0.8 G 0.6
-
-
0.4
-
0.2
I
3.0
4.0
5.0
( H C I O,).M
Figure 3. Effect of HClOa concentration on chlorate and chloride yields in deaerated solutions: 0 , Cloy-; A > C1-; 0, A, results of Cottin (ref. 4 ) ; x, C10~-yield with M NaCl added initially. Dashed line is sum of curves 1 and 2.
Volume 68, Number 11
November, 1964
initially. Cottin's results a t perchloric acid coiicentration, 2.5 '21 arid above, arc higher than those obtaiiicd iri thew experinients arid do riot appwr in the figure. Air dors not affect the yield of C103-, and no difference was found at 0.313 d l EICIO, when C1- was replaced by 1%-. Addition of A4 NaCl increased the chlorate yield, and below 3 iz/ TTClO,, G(ClO:,-) bccanie equal to the sun1 of G(CI-) arid G(C10,-) dcterniined in thr absencc of added C1- (dotted curve). Above 3 M HClO,, G(CIOd-) fell below the dot tcd eurvr, perhaps because of thernial reaction bctween CIOJ- arid CI-. G(C1-) could not be dcterniiiicd in thc prcsencc of added KaC1. The iniplication is that most of the C1found in the radiolysis is a secondary product, resulting from attack on C103- by free radicals in the bulk of the solution, arid that added low2M Cl- protects Cl0,from this attack. To vcrify this idea, we irradiated ail air-free solution of 1rationus. dose curves up to the total input employed (ea. 1020 e.v.). The C103- yields are, within experimental error, the same in air-free and air-saturated solutions. The Hz
where a is in our case Z(Oz) R the ratio of the rate constants for the reaction of OH with Hz and HzOz, and K' the ratio for the rate constants of the reactions of H atoms with Hz02and 02. At pH 2 , the hydrogen atoms are mainly in the acidic form and K' to this p11,19 {ilI= :m,G~~~~= 2 . ~ 3ell, , = 0.42, and = 520, and froni Fig. 1 (H202)?? 000 p M , n~ find K = O.%j, esscritially t h r Sam(' as in neutral solution.'* The sanic conclusion was rrached by IIochanadr120from exprrimerits on peroxide photolysis. In sulfuric acid solutions, by contrast, 110 steady state is reached, becausc. the bisulfate ion reacts with 011 to give a radical that does not react with hydrogrn. Apparrrit ly, prrchlorate ions do riot react with H or 011 and thc drcoriiposition of perchlorate in aqueous solution must be Frit ircly duc to thc direct cffcct. The reaction products of the prrchlorate deconiposition as thcy emerge froin the spur include chlorate ion, with possibly a sinal1 amount of chloride fortning a t highcr pcvchloric acid concrntrations. The chloride appearing in pure prrchloric acid solutions a t niodcrat e concc.ntrations was shown to be inhibit cd by additional chloride and must result froni the action of OH radicals on the chlorate ions. T h e presurnptivc reaction is 011 CIOs11 + = H20 (;lo3, followed by decomposition or reduction of the radical C103 to yield ult iniatrly chloride ion. The accderat ion by radiation of the oxidation of E'e+*hy C103 - probably invo1vc.s reaction of Clod with IceA2initiating a serics of steps that lead ultiniat cly to forniation of C1- and swcn Ice t 3 To understand the systcrn coniplctcly would rcquirc a complete study of the radiation cheniistry of chloratr solutions, which we have riot undertaken. Even in the presrnce of chloride, sonic of the chlorate ions emerging froni the spurs may be attacked by radicals, arid our valurs of the chlorate yield rrpresent only a lower limit for the atiiount of direct action on the perchloratc ions. The oxygen formrd together with chlorate in the perchlorate deconipositiori might be expected to emerge froin the spur in a nuniber of differcnt chemical forms: 0 2 or 11202 nioleculcs, H 0 2 or OH radicals, or 0 atonis. Because of 1he complications involving the sccondary react ions with chlorate, the oxygen-peroxide systeni is not suitable for drawing quantit ativc conclusions in perchloric acid, and we turned to the use of ions of iron, cerium, and thallium as scavengers. In aerated ferrous ion solutions, each oxygen atom would be expcwtcd to oxidize two equivalents of iron. In addition, as in the usual ferrous sulfate reaction, each H or 1102 should oxidize three equivalcnts, each HZO, two, and each 011 one equivalent E:ach chlorat,e ion formed will oxidize six quivalent s. Thus
+
+
+
GcL,p(Fe+y - W(CIO3) - =
8GH
+
2GlI202
+
GO11
+ 2Go + :3Giro,
In air-frec solutions in the presence of excess ferric ion
and of chloride ion, the hydrogen atoms react with ferric ion to reduce it in prcftwncc to oxidizing ferrous ion. Any free oxygen foriiied mill bc attacked by H atonis to forni H 0 2 which results in oxidation of three equivalcnts of ferrous ion. ?'he combined effect of the yiclds of O2 and H then will br ati oxidation to the extent of 3Go,, less a reductioti by the balance of the €1 atoms, GI,- GO,, so the net oxidizing yield will be 4Go, - GI[. The yield of iron oxidized under these conditions will therefore be given by ~ 4 t ? c + 3-) w(cioJ-)= -GI! ~GII,o, Gori
+
+
+ 2Go + ~ G H o+, 4G02
In the absence of ferric ion, each O2 fornicd will again pick up an I1 atoni to form HOz and oxidize three equivalcrits of ferrous ion, but the balance of the H atonis, GIr - Go,, will then oxidize one equivalent of ferrous iron. The yield of oxidat ion resulting froni the combined action of H and 0, should than be 2G02 GH. Thus the initial yields should be given by
+
G,(lCed3)- 6G(ClO3-) = GII 2Gii101 Coir
+
+
+ 2Go + ~ G I I O+, 2Go,
We see then that the initial air-free yield should always lie just half-way between the final air-free yield and the yield obtained in the prcscnce of air. Figure 6 shows that this appears to be thc case, within the rather poor precision with which the initial yield in air-free solutions could bc detcrniincd. The sigriificancc of the yields of ceric sulfate rrductiori in the presence and absericc of thallous ion is soniewhat ambiguous as far as the behavior of the 0 atom is concerned. The cwic reduction yield in the absence of thallous ion is somewhat less than twice the yield of forniation of peroxide in aerated perchloric acid of the sanie concentration, but not containing any sulfate. In the absence of cerium, the oxygcri atom might be expected tn destroy pcroxide by thc reaction 0 H202 = 0 2 € 1 2 0 . Since the ceric reduction yield is not grcater than twicc the yicld of € 1 2 0 2 fornicd in the absence of cerium, it sc'enis likcly that if any oxygen a t o m are present they niust either oxidize trivalent cerium to the tetravalent form h i this systeni, or destroy H 2 0 2before the peroxide has time to react with Ce". 'l'hcri
+
+
G(Ce"')
=
CII
+ ~ G H , o-, Got, - 2G0 + GHO2
In the prescncc of thallous ion we do not know if the 0 atom would oxidizc t halliuni to thc divalent state which (19) A . 0. Alleii, " T h e Iklintion Chemistry of Water arid Aqueous Solutions." I). Vim Nostwnd Co., Princeton, N. J.. 1962. (20) C:. .I. Horli:mndel, Radialion Re,.?., 17, 286 (1'362).
Volicme 68. N74mber 1 I
November. 1964
D. KATAKIS AND A. 0. ALLEN
3114
Table I11 : Effect of HClOd on Some Yield Relationships WOE
(HC10d
2M
~QH - Go,
-0.05 -0.11
3M
+ Go + Go,
GHO*
+ Go
-0.30 -0.45
0.45 0.66
~GH,o, - 00
-0.19 -0.28
~GH,
-0.1 -0.14
GClO3-
0.57 0.81
Gci-
... 0.04
all the water decomposition products appears in fact to be decreased by perchloric acid, as expected if a considerable fraction of the energy is going to excitation of the perchlorate ions. G(Ce"')Ti+ = GH f ~GH,o, -k GOH G H O ~ Since the yields all vary nearly linearly with C104concentration, in both y-ray and helium ion results, the The experiments with scavengers thus give us for excited perchlorate ions do not react with one another any particular concentration of perchloric acid a series or with unexcited perchlorate; they rather react uniof relationships between the changes in yields of H , molecularly. One might expect the perchlorate to OH, and H202produced by the perchloric acid and the ionize to a C104 radical and an electron, which would yields of 02,0, and H02which may arise from the acid. give rise to an H atom in acid solutions, but the results If we let 6G represent the yield of a water radiolysis make it look very unlikely that an important fraction product in the presence of any concentration of perof the excited perchlorate reacts in this way. The yield chloric acid minus its value extrapolated to zero perof H atoms certainly seems to decrease with increasing chloric acid, the results may be summarized as perchlorate concentration. Thus the ratio G(02)/G 8G0,r(Fe+3)= (HzOz)shown in Fig. 2 and 8 depends on the ratio of 3 6 G ~-I- ~ ~ G H ,fo ,~ G O -IH 2Go 4- ~ G H o , oxidizing radicals (which oxidize H202 to 0 2 ) to H atoms (which reduce O2 to H202); this ratio increases sharply 6Gf(Fe+a)= with perchlorate concentration, while the yield of H2, - ~ G H 26G~,o, ~ G O H 2Go ~ G H of, 4Go, which arises from intraspur reactions of H, declines. 6G(Ce111) = ~ G H 26G~,o,- ~ G O H 2Go 4-CHO, Thus the usual fate of the excited perchlorate ions must be to decompose into ClO3- ions and 0 atoms. Most GG(Ce'")TI+ = ~ G H f 26G~,o,4- ~ G O -kH GHO? of the 0 atoms must be formed in the ground state, where the values of G(Fe+3)have been corrected for the since Taube" has shown that 0 atoms formed in the exchlorate yield. cited la state react readily with water, forming HzOz, Since there are six unknowns and only four equations, while GHzOz seems to decrease rather than increase with the system is not entirely determined. However, some perchlorate concentration. The 0 atoms may emerge interesting combinations nf the unknowns may be obfrom the spurs as such, or react in the spurs with H to tained form OH, or with OH to form HO2. The higher yields of c103- and Cl- in the helium 6GoH Go = 1/2[6G(Ce111)T1+- 6G(Ce1")] ion tracks are unexpected. Again, the approximate ~GH - Go, = '/4[6Gair(Fef3) - 6 G ~ ( F e + ~ ) l linearity of the yields with perchlorate concentration s h o w that the track density effect is not due to an GHO, f Go Go, = 1/4[6G,i,(Fe+3) 6Gf(Fe+3)- 26G(Ce1'')~~+1 interaction with one another of excited perchlorate ions or radicals derived from them. The enhanced yields sG(H202) - Go = probably arise rather from interaction of perchlorate [6G(Ce1") - 6G,i,(Fe+3) 26G(Ce'")~l+ 1 with the higher density of excited water molecules and free radicals. There must be many events in which an Values of these quantities for 2 and 3 M HC104 are excited perchlorate ion loses its energy without deshown in Table I11 together with yields of chlorate and composition or decomposes to C103- and 0 only t o have chloride and change in the yield of hydrogen. The these products recombine to C104- in the solvent cage. sum of GO,, GO, and GHO,is seen to be somewhat less If, however, a neighboring water molecule is excited than G(C108-). Some of the oxygen from the peror has decomposed to H or OH, it can enter into the chlorate ion thus probably appears as H202 or OH. reaction and bring about generally more extensive deHowever, the sum GoH GHIoP is seen to be smaller in composition than would have occurred had the water perchloric acid solutions than in water. The yield of
reduces cerium, or directly to the trivalent state. I n the latter case the yield of cerium reduction in the presence of thallous ion is
+
+
+
+
+
+
+
+
+
+
+
The Journal of Physical Chemistry
TRITIcnf
6-RADIATIOS-INDUCED ISOTOPIC EXCHANGE WITH WATER VAPOR
excitation been locat,ed farther from the perchlorate excitation. The higher chloride yield may also arise
3115
from secondary reactions with free radicals suffered by C103- as it diffuses out of the track.
Tritium p-Radiation-Induced Isotopic Exchange with Water Vapor
by John Y. Yang and L. H. Gevantman U . S . Naval Eadiological Defense Laboratory, S a n Francisco, California
(Received February 24, 1964)
The rate of tritium 6-decay-induced isotopic exchange between tritium gas and water vapor is found to be constant within a fourfold variation in the water vapor density, but to increase as a second-order function of the tritium concentration. Inert gases, present in large excess, appeared to function purely as moderating media for the tritium 6-energy. With helium gas as the moderator and tritium concentration in the range from 0.05 to 0.7 c./l., the reaction rate in mc./l.’/day at the ambient temperature of 22 =t2’ is observed as d(HTO)/dt = 3.6 X 10-5(T2)2,where (Tz) is the initial tritium concentration in mc./l. This exchange reaction is strongly inhibited by the presence of nitric oxide in the reaction mixture. The results are consistent with a reaction mechanism involving radical intermediates.
Introduction Ever since the dmcoveryl of tritium incorporation in organic compounds by exposure to tritium gas, a number of investigations devoted to detailed studies of the isotopic exchange in tritium-hydrocarbon systems have appeared in the literature.2-10 Although the over-all reaction mechanism appears to vary consid&ably with each individual system, it is generally agreed that the tritium /3-radiation-induced intermediates play a major ‘Or tritium in the Organic substrate. We Chose to stud:y the tritium-water vapor system because radiolytic processes in aqueous systems are reasonably well un
