Oxidation of S(IV) by the dodecatungstocobaltate(III) ion over an extended pH range using acetic acid-acetate buffer: Different kinetics, oxidation product and mechanism

The reinvestigation of the title reaction over the extended pH range, 3.25 ⩽ pH ⩽ 6.3, has shown that the reaction is first-order in [Co(III)W]5−, [SIV] and [M+] where M+ is the alkali metal ion, with SO42- as the exclusive product. The alkali metal ions form ion-pairs with SO32- and [Co(III)W]5− (abbreviation for 12-tungstocobaltate(III)) ions and also act as a bridge between the ion-pairs in the formation of the activated complex, the decomposition of which is rate limiting. A mechanism based on the above fact explains the kinetics results. Because of the nature of the rate dependence on M+ and H+, the application of the extended Marcus theory involving work terms could not be applied, but the application in its simple form does support that the reaction is outer-sphere.