Rates and mechanism of carbonyl sulfide oxidation by peroxides in concentrated sulfuric acid

We measured the rates of carbonyl sulfide (OCS) oxidation by hydrogen perox ide H2O2 (HP) and peroxymonosulfuric acid HOOSO2OH (PSA) in 13.5-18.0 M (76 -96 wt %) sulfuric acid (SA) between 290 and 306 K. The reaction is first o rder in both [OCS] and [peroxide], having nearly identical second-order rat e constants k(1) for HP or PSA as oxidants. k(1) increases exponentially wi th Hammett acidity H-o, in the range investigated: log(k(1)/M-1 s(-1)) = - (9.57 +/- 0.41) - (0.80+/- 0.05) H-o , (H-o < 0),at 306 K. OCS is, however, inert toward HP in concentrated perchloric acid at equivalent no values. W e infer that OCS oxidation by HP in SA proceeds by an acid-catalyzed proces s involving the intermediacy of PSA rather than H3O2+. k(1) depends on temp erature according to log(k(1)/M-1 s(-1)) = (6.64+/- 1.58) - (2606 +/- 472)/ T, in 18 M SA. The observed kinetic behavior and parameters are typical of sulfide oxidations by PSA in very acidic media. Present data, in conjunctio n with data on ambient HP and sulfate aerosol levels in the lower stratosph ere, lead to reaction Lifetimes that are many orders of magnitude longer th an the currently estimated OCS atmospheric residence time of about 4 years.