J. Phys. Chem. 1993,97, 10712-10714
10712
The Branching Ratio of SOs- Radicals Self-Reaction in Aqueous Solution Aleksandr N. Yermakov,' Boris M. Zhitomirsky, Grigoriy A. Poskrebyshev, and Dmitriy M. Sozurakov Institute of Energy Problems of Chemical Physics, Russian Academy of Sciences, 1 17829, Moscow, Russia Received: May 20, 1993'
The radiation-induced chain bisulfite oxidation is of short chain length and is independent of the dose rate (1020-5 X 1021 eV L-I s-l) and the initial concentration (5 X 10-4-2 X M) of bisulfite. The mechanism of oxidation, including the propagation reaction SO; SO< SOL + SOL + 02 (kla) and the termination reaction SO; SO; S20S2- 0 2 ( k l b ) , has been proposed. The branching ratio k l a / k l b = 7 f 1 has been derived at room temperature.
-
+
+
+
-
At the same time the reaction
Introduction Peroxysulfate radical is a key intermediate in the chain autoxidation of aqueous S021-5and in the free-radical decomposition of peroxymonosulfate.6 For the self-reaction of SOSradicals two parallel paths are under discussion:
SO; + SO;- SO; SO; + SO;
-
+ SO; + 0,
s20g2-+ 0,
(Rla) (Rib)
As has been found7in oxygen-18 tracer experiments both oxygen atoms in the 0 2 are from the terminal peroxide oxygen in the peroxysulfate radicals. The formation of sulfate radicals in the self-reaction of SOs- radicals, (Rla), has directly been observed in flash photolysis experimenk8 The unchanged perdisulfate concentration in the dual Ag+-S2Oe2-catalytic decomposition of peroxymonosulfate led to the conclusion7 that S2OS2-was regenerated by (Rlb). Quantitative rate data concerning ( R l ) are quite limited. The overall rate constant kl, + k l b of selfreaction was recently determined8-kla + k l b = lo8 M-' s-l. However, the branching ratio, kla/klb, is still uncertain. From model calculations5of SO2 oxidation in atmospheric droplets the estimated value kla/klb 4 although the calculations were performed with rate constants that were redetermined after the initial e~periment.~.~ The experimental estimation of the branching ratio in a strong acid solution has been undertaken by Thompson:' k l p / k l b = 7-10. However, this value turned out to be quite sensitive to the rate constant of silver (1)-catalyzed perdisulfate decomposition. In this paper we determine the branching ratio, kla/klb, from a kinetic study of radiation-induced bisulfiteoxidation. TheSOsradicals were generated5.8 in N20-02 (4: 1) saturated bisulfite solutions (initial [HSO~-]O = [H+]o l e 3 M):
-
eaq- + N 2 0 -,N 2
OH
+ HS0,SO3- + 0,
-
+ OH- + OH SO3- + H,O
-
-,
(R2) (R3)
SO,-
(R4) Reactions R2-R4 are very fast, and using well-known primary radiation yields of active species (g(eaq)= 2.6, g ( 0 H ) = 2.8) the radiation yield of SO5- radicals (molecules/lOO eV) can be determined: G ( S 0 ; ) = (g(eaq-) + g(0H)) = 5.4 Abstract published in Aduunce ACS Absrructs, August 15, 1993.
(1)
SO;
+ HSO;
-
+ SO; + H+
S O : -
(R5)
is also fast9 (k5= 7.5 X 108 M-l s-l) and therefore at our dose rates and [HS03-]o (see below) the rate of bisulfite oxidation is determined only by reactions of SO; radicals (Rl) and (R6):
SO;
+ HSO;
-
HSO;
-,HS0,-
+ SO;
Ma)
+ SOL
(R6b) Thecompetition of reactions R1 and R6 depends on the initiation rate and the bisulfite concentration. Only an upper limit of the R6 rateconstant is available, k6 < 3 X lo5M-I s-l. At sufficiently high dose rates and low [HSO3-]0 one may neglect chain propagation through (R6), and the chain length of bisulfite oxidation, G(S(IV))/G(SO5-), is to be independent of the initiation rate and the initial bisulfite concentration. Neglecting S(1V) oxidation produced under irradiation, hydrogen peroxide and peroxymonosulfate the radiation yield, G(S(IV)), may be represented: IG(-S(IV))/N
Z
2k1a[SO
