Electron transfer between trans ‐dihydroxotetraoxoosmate(VIII) and thiosulfate. A kinetic study in aqueous alkaline media

The kinetics of oxidation of thiosulfate to tetrathionate by trans ‐dihydroxotetraoxoosmate(VIII) in aqueous alkaline media have been studied. The oxidation follows a rate expression\documentclass{article}\pagestyle{empty}\begin{document}$$ - \frac{{d[{\rm Os(VIII})]}}{{dt}} = k3{\rm K}_{OS} {\rm K}_2 [{\rm S}_2 {\rm O}_3^{2 - }][{\rm Os(VIII})](1 + {\rm K}_{os} {\rm K}_2 [{\rm S}_2 {\rm O}_3^{2 - }])^{ - 1} $$\end{document}where K Os is the formation constant of trans ‐dihydroxotetraoxoosmate (VIII), and K 2 and k 3 , respectively, represent the formation constants of the intermediate complex involving Os(VIII) and S 2 O   3 2−and its decomposition constant. The K Os , K 2 , and k 3 values have been computed to be (19.5 ± 3) dm 3 /mol, (6.12 ± 0.5) and (3.32 ± 0.3) × 10 −1 dm 3 /mol s at 303 K, and I = 0.32 mol/dm 3 , respectively. The rate law is consistent with a mechanism envisaging the equilibrium formation of an intermediate complex involving Os(VIII) and S 2 O   3 2− , followed by a rate‐determining decomposition of the complex with concomitant electron transfer.