Kinetics and mechanism of the osmium(VIII)-catalysed autoxidation of aqueous sulfur dioxide in acidic and alkaline mediaψ

Kinetics of the osmium(VIII)-catalysed oxidation of sulfur(IV) by dioxygen in stirred solutions obeys the rate law (A) in phosphate buffered alkaline medium, and the rate law (B) in acetate buffered acidic medium. R o b s = k[Os V I I I ][S I V ][OH - ] - 1 (A) R o b s = k 3 [Os V I I I ][S I V ] 2 [H + ] - 1 (B) The kinetics order in [O 2 ] is zero. The rate is comparatively faster in alkaline medium. The proposed mechanisms involve the Os V I I I /Os V I cycle perpetuated by reduction of Os V I I I by S I V , and regeneration of Os V I I I by oxidation of osmium(VI) by O 2 as cooxidant. There is spectrophotometric evidence for the formation of an intermediate complex in acid medium.