Mechanism of the oxidation of thiosulfate with hydrogen peroxide catalyzed by aqua-ethylenediaminetetraacetatoruthenium(III)

Abstract

Catalytic ability of [RuIII(edta)(H2O)]− (edta4−=ethylenediaminetetraacetate) complex toward oxidation of thiosulfate (S2O32−) in presence of H2O2 has been explored in the present work. The kinetics of the catalytic oxidation of thiosulfate (S2O32−) has been studied spectrophotometrically as a function of [RuIII(edta)], [H2O2], [S2O32−] and pH. Spectral analyses and kinetic data indicate a catalytic pathway involving activation of both substrate (S2O32−) and oxidant (H2O2). Substrate activation pathway involves the formation of a red [RuIII(edta)(S2O3)]3− species through the reaction of the [RuIII(edta)(H2O)]− catalyst complex and the substrate (S2O32−). Hydrogen peroxide reacts directly with thiosulfate coordinated to the RuIII(edta) complex to yield sulfite as immediate oxidation product. Peroxide activation pathway is governed by the formation of [RuV(edta)(O)]− catalytic intermediate which oxidize thiosulfate, however, at slower rate (kox2=0.012 M−1 s−1 at 25°C) as compared to the rate of oxidation of the coordinated thiosulfate (kox1=0.93 M−1 s−1 at 25°C). Sulfite and sulfate were found to be the oxidation products of the above described catalytic oxidation process. A detailed mechanism in agreement with the spectral and kinetic data is presented.