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

Abstract

The kinetics of oxidation of thiosulfate to tetrathionate by trans-dihydroxotetraoxoosmate(VII1) in aqueous alkaline media have been studied. The oxidation follows a rate expressionwhere K a is the formation constant of trans-dihydroxotetraoxoosmate (VIII), and K2 and ka, respectively, represent the formation constants of the intermediate complex involving Os(VII1) and S20;and its decomposition constant. The K h , K P , and k3 values have been computed to be (19.5 f 3) dm3/mol, (6.12 f 0.5) and (3.32 f 0.3) X lo-' dm3/mol s a t 303 K, and I = 0.32 mol/dm3, respectively. The rate law is consistent with a mechanism envisaging the equilibrium formation of an intermediate complex involving Os(VII1) and S*O$-, followed by a rate-determining decomposition of the complex with concomitant electron transfer. 1002PANDA, NEOGI, AND RAMASWAMY complexes [2-61 formed prior to electron transfer or without a change in the inner coordination sphere of the metal [7-111. Keeping this in view, the electron transfer in the OS( VIII)-S20;-system has been studied by stopped-flow spectrophotometry. Materials and MethodsAll the chemicals used, except Os04 (which was a sample of Johnson and Matthey), were of AnalaR grade. Doubly distilled water was exclusively used for the preparation of solutions. Stock Os(VII1) solution, prepared by dissolving Os04 in a weakly aqueous alkaline solution, was estimated by iodometry [12] and stored in a cool dark place. The solutions for kinetic runs were prepared from this stock solution by dilution. The ionic strength

Electron transfer to trans‐dihydroxotetraoxoosmate(VIII). II. Kinetics of the oxidation of dimethylsulfoxide in aqueous alkaline media

“…(2)- (4) in terms of interactions of dmso with the two Os(VII1) species existing in alkaline solutions [8,11,12] : …”

“…The results in the case of thiosulfate oxidation [8] revealed that the electron transfer was substitution controlled. The present paper describes the results of our attempts to understand the kinetics of the Os(VII1)-dimethylsulfoxide reaction in aqueous alkaline medium.…”

“…Dimethylsulfoxide (referred to hereafter as dmso) was a Baker Analysed sample and was used after redistillation under reduced pressure. The preparation of trans-dihydroxotetraoxoosmate(VIII), hitherto abbreviated as Os(VIII), from osmium tetroxide and alkali and the method of its estimation were described elsewhere [8]. All other chemicals used were of analytical grade quality.…”

“…We have recently carried out mechanistic investigations on oxidations of thiosulfate [8] and thiocyanate [9] by the oxohydroxoosmate(VII1) complex in alkaline media. The results in the case of thiosulfate oxidation [8] revealed that the electron transfer was substitution controlled.…”

“…We have recently carried out mechanistic investigations on oxidations of thiosulfate [8] and thiocyanate [9] by the oxohydroxoosmate(VII1) complex in alkaline media. The results in the case of thiosulfate oxidation [8] revealed that the electron transfer was substitution controlled. The present paper describes the results of our attempts to understand the kinetics of the Os(VII1)-dimethylsulfoxide reaction in aqueous alkaline medium.…”