PHOTOOXIDATION OF SO32- IN AQUEOUS-SOLUTION

Aqueous solutions of sodium sulfite, with or without additives, were exposed to 254-nm radiation, and the yields of products were determined. In the absence of oxygen the major products were dithionate and sulfate with a molar ratio of 0.5 ± 0.08. From their total yield the quantum yield for the primary products, SO3- and eaq-, was determined as 0.85 ± 0.04. In the presence of oxygen, dithionate was not observed and the formation of sulfate was extremely rapid, occurring with a quantum yield of about 500. The addition of benzene as a radical scavenger led to the formation of phenol. The further addition of tert-butyl alcohol, which reacts rapidly with OH but slowly with SO4-, did not cause a reduction in the yield of phenol, indicating that the SO4- radical is the main chain carrier. When benzene was replaced by either ethanol or 2-propanol, the products were acetaldehyde and acetone, respectively, in addition to sulfate. From the product ratios [SO42-]/[carbonyl compound] as a function of the concentration ratio [SO32-]/[alcohol], the rate constant for reaction 16, SO4- + SO3 → SO3- + SO42-, was derived by competition kinetics: k16 = (5.5 ± 1.5) × 108 M-1 s-1. The data also provided the branching ratio for the two channels of reaction 3: (a) SO5- + SO32- → SO3- + SO52- and (b) SO5- + SO32- → SO4- + SO42-. The value obtained was k3a/k3b = 0.41 ± 0.25 or k3b/k3 = 0.72 ± 0.11. The reaction mechanism discussed involves also the reaction of peroxymonosulfate with sulfite. At pH 7-8 a rate constant of 350 ± 90 M-1 s-1 was obtained for this reaction. © 1990 American Chemical Society.