3909
COMMUN
TIONS TO THE EDITOR
y Radiolysis of Xenon Trioxide in Aqueous Solution Pubkation costs assisted by the U. S. Atomic Energy Commission
Sir: Aqueous solutions of XeO3 were irradiated with y rays (1100 Ci) either at pH 6.0 or 1.25 (solutions acidified with H2S04) and saturated with either air or 99.999% argon. Diose rates of 5.6, 0.50, and 0.13 X l0ls eV g--l hr-I were used, and the effect 5f solute concentration was studied at the highest dose rate. It has been shown in previous work,l by gas analysis an$ iodometry, that XeO3 is decomposed irito Xe and 0 2 by y radiolysis of its aqueous solutions. In the present work, we have measured yields of the disappearance of XeOa by iodometry following the method of Appelm.an and M a l m 2 At pH 1.25 the initial G, molecules of XeO3 decomposed per 100 eV absorbed, is the same in aerated and argonsaturated solutions. The amount of decomposition increases linearly with dose up to a t least 20% decomposition of the solute. G increases with increase in solute concentration and with decrease in dose rate, I . Results for a 1.2 X M sodution are presented in Table I. At pH 6 in aerated solutions the initial G is high and increases with increase in
TABLE I: Effect ot Dose Rate, I , on G(-Xe03) for Aerated and Argon-Saturated Solutions of 1.2 X 10 -? M XeO3 at pH 1.25 la 6 , aerated G , argon
0.13
6.13 6.09
0.50 5.2 5.12
5.6 4.1 4.1
salute concentration as shown in Table I1 for I = 5.6 X l0ls e\T 8-l h r - 1 ~With decrease in dose rate to aerated solutions M solution, G a t pH 6, G increases; e.g., for a n 8.5 X increases from 10.5 t o 44 with decrease in I from 5.6 to 0.13 X 10l8 eV g-l hr-l. In solutions saturated with argon a t p1-i 6, G decreases slightly with increase in solute concen-
tration and becomes almost constant a t approximately 7.5 for concentrations greater than M ,The G =: 7.5 at high concentrations is independent of the dose rate while G at small concentrations increases with decrease in 1. It has been noted3 that more than 1 mol of hydrogen peroxide is consumed for 1 mol of XeO3 in the thermal reaction; however, the ratio HzOz/XeOz has not been determined, A systematic study of the thermal reaction has been undertaken; preliminary results indicate that the reaction is rather complex. The ratio for reactants consumed, HzO2/ Xe03, varies with pH and with the ratio of initial concentrations of H202 and Xe03. Under oxygen and at pH 1.25, HZO2/XeO3 increases from 1 to 3 with increase in H2Oz concentration while in neutral solution the ratio increases from 0.5 to 1. The results for aerated solutions at p H 1.25 are explained with the following reaction scheme.
H + 02’HOz XeO3
-
+ HOZ
10.5
aDose rateof 5.6 X 1 0 ’ 8 e V y - 1 h r - l
1.30 1.45 1.60
2
+0 2
(2) (3)
HzOz
(41
(1-3)HzO
+ (1-2)02
(5)
In argon-saturated solutions, reaction 1 is absent and reaction 6 H
+ Xe03
-
XeOz
+ OW
(6)
is substituted for reaction 2. With the primary radiolytic yields at pH 1.25 (G(H) = 3.65, G ( 0 H ) = 2.95, G(Hz0z) = O B ) , G = 5.02 for HzOz/Xe03 = 3 and G = 7.75 for HzOz/ Xe03 = 1. Thus, in the reaction scheme proposed, the effect of dose rate on G is related to an effect of dose rate on HzOz concentration and, thereby, on the consumption ratio H202/Xe03. Experiments have been undertaken for test of the proposal. The large values of G and dose-rate effect for aerated solutions at pH 6 suggest a chain reaction for which a reaction scheme is proposed that includes reactions 3 and 4 and the following reactions. 0 2
202-
+ eaq-
-
+02-
+ H2O
XeO3 + (0.5-1)Hz02 10.0 9.6
Xe
XeOz
XeO3 + 0 3 -
0.17 0.38 0.85
-
2 OH
XeO3 TABLE II: Dependence of G(-Xe03) on Xe03 Concentration in y Radiolysis of Aerated Solutions at pH 6 a
XeO2 i- OH + 0
- +
XeOz XeO3 + (1-3)HzOz
(1)
-
-+
+
-
t 7)
02-
XeOz i- 0 3 -
XeO2 + 0
HO2-
2
(8)
+ OZ-
+ OH- +
0 2
XeO2 + (0.5-1)HzO + (0.75-1)Oz
(9)
(10) (11)
12.3 18.7
20.8
C. Heitzand F. Simon, Radiochem. Radioanal. Lett., 5, 341 (1970). E. H. Appelman and J. G. Malm, J . Amer. Chem. SOG.,86, 2141 (1964). J. G. Maim and E. H. Appeiman, Atom. Energy Rev., 7,3 (1969).
The Journal of Physical Chemisfry, Vol. 76, No. 25, 1972
391 0
COMMUNICATIONS TO THE EDITOR
In argon-saturated solutions, reactions 6, 12, and 13 re-
X e 0 3 + eaq- -'XeOz 0-
+ HzO+OH-
+ 0+ OH
(12) (13)
place reactions 7-10. With the primary radiolytic yields at pH 6 (G(eaq-) = 2.65, G(H) = 0.55, G ( 0 H ) = 2.7, G(H202) = 0.7). G = 6.85 for HzOz/Xe03 = 1 and G = 10.5 for H20z/Xe03 = 0.5 in the argon solutions. From the absence of a chain reaction in the argon solutions, it is con-
The Journal of Pi?yslca/ Chemistry, Vol. 76, No. 25, 1972
cluded that OH does not react with %Os. Again, for small concentrations of Xe03 in the argon solutions at pH 6, the effect of dose rate on G is explained in terms of its effect on the ratio H202/Xe03. Laboratoire de Chimie Nucieaire Sfrasbourg-Cronenbourg (67/, France Laboratoire C w e Paris, France Received September 20, / 9 7 2
@. Heitz* M. Haissinsky
