PHOTODECOMPOSITION AND PHOTOOXIDATION OF HYDROGEN SULFITE IN AQUEOUS-SOLUTION

A zinc are lamp and a mercury lamp, respectively, were used to study t he photodecomposition of HSO3- and SO32- in aqueous solutions saturate d with either argon or nitrous oxide. The main products in both cases were sulfate and dithionate, which are attributed to arise from the se lf-reaction of SO3- radicals. Quantum yields for the formation of SO3- in argon-saturated solution based on hydrazoic acid and/or ferric oxa late actinometry were 0.19 +/- 0.03 for HSO3- and 0.39 +/- 0.03 for SO 32-, essentially independent of S(IV) concentration. In both systems, the rate of sulfate formation rose with time at the expense of that of dithionate, This is explained by reactions of hydrogen atoms and hydr ated electrons with dithionate (rate coefficient k(5) approximate to 2 x 10(5) dm(3) mol(-1) s(-1)). N2O as a scavenger for these radicals r emoved the effect and raised the quantum yields to 0.25 +/- 0.03 and 0 .75 +/- 0.04, respectively. The product ratios under these conditions were [S2O62-]/[SO42-] = 0.43 +/- 0.04 for HSO3- and 0.61 +/- 0.03 for SO32-. In oxygen-saturated solutions, the photolysis of HSO3- led to a short chain reaction with sulfate and peroxodisulfate as products. Th e latter product was assigned to arise from the recombination of SO5- radicals. Steady state analysis of the product evolution with time gav e rate coefficients for two of the reactions involved: k(16)(SO5- + HS O3-) = (1.2 +/- 0.4) x 10(4) dm(3) mol(-1) s(-1) for the main propagat ion reaction and k(19a)(HO2 + SO5-) = (1.8 +/- 1.0) x 10(9) dm(3) mol( -1) s(-1) for the principal termination reaction. These values agree w ell with recent data from radiolysis experiments.