Photodecomposition and Photooxidation of Hydrogen Sulfite in Aqueous Solution

A zinc arc lamp and a mercury lamp, respectively, were used to study the photodecomposition of HSO3- and SO32- in aqueous solutions saturated with either argon or nitrous oxide. The main products in both cases were sulfate and dithionate, which are attributed to arise from the self-reaction of SO3- radicals. Quantum yields for the formation of SO3- in argon-saturated solution based on hydrazoic acid and/or ferric oxalate actinometry were 0.19 ± 0.03 for HSO3- and 0.39 ± 0.03 for SO32-, 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 hydrated electrons with dithionate (rate coefficient k5 ≈ 2 × 105 dm3 mol-1 s-1). N2O as a scavenger for these radicals removed 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. The latter product was assigned to arise from the recombination of SO5- radicals. Steady state analysis of the product evolution with time gave rate coefficients for two of the reactions involved: k16(SO5- + HSO3-) = (1.2 ± 0.4) × 104 dm3 mol-1 s-1 for the main propagation reaction and k19a(HO2 + SO5-) = (1.8 ± 1.0) × 109 dm3 mol-1 s-1 for the principal termination reaction. These values agree well with recent data from radiolysis experiments.