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 HSO 3 - and SO 3 2- 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 SO 3 - radicals. Quantum yields for the formation of SO 3 - in argon-saturated solution based on hydrazoic acid and/or ferric oxalate actinometry were 0.19 ± 0.03 for HSO 3 - and 0.39 ± 0.03 for SO 3 2-, 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 k 5 ≈ 2 × 10 5 dm 3 mol -1 s -1 ). N 2 O 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 [S 2 O 6 2- ]/[SO 4 2- ] = 0.43 ± 0.04 for HSO 3 - and 0.61 ± 0.03 for SO 3 2- . In oxygen-saturated solutions, the photolysis of HSO 3 - led to a short chain reaction with sulfate and peroxodisulfate as products. The latter product was assigned to arise from the recombination of SO 5 - radicals. Steady state analysis of the product evolution with time gave rate coefficients for two of the reactions involved: k 16 (SO 5 - + HSO 3 - ) = (1.2 ± 0.4) × 10 4 dm 3 mol -1 s -1 for the main propagation reaction and k 19a (HO 2 + SO 5 - ) = (1.8 ± 1.0) × 10 9 dm 3 mol -1 s -1 for the principal termination reaction. These values agree well with recent data from radiolysis experiments.