Oxidation of sulphur (IV) by dodecatungstocobaltate(III) in aqueous acid solution

Abstract

The reaction between dodecatungstocobaltate(III) and sulphite has been studied in the range pH 1.50–3.50 at 25 °C and l= 0.5 mol dm^–3(NaClO₄). The reaction stoicheiometry has been found to be 1 : 1 in the presence of excess of sulphite and the corresponding reaction product obtained is dithionate. However, when the complex: sulphur (IV) mole ratio is > 1.5:1, sulphate is also obtained together with dithionate and their amounts are dependent on the mole ratio of the reactants. The stoicheiometry [S^IV]_consumed:[complex]_consumed varies in the range 0.7–1.0:1 on a par with the sulphate: dithionate ratio. The kinetics of the reaction path which gives a 1:1 stoicheiometry follows the rate law (i). A mechanism has been proposed, considering k₀ and k₁^′–d[complex]/dt=(k₀+k₁′[H⁺]^–1+k₂[HSO₃^–])[complex][HSO₃^–](i) paths to correspond to the reactions between the cobalt(III) complex with HSO₃^– and SO₃^2– respectively. The k₂ path is assigned to the reaction of the complex with S₂O₅^2– or that of sulphur(IV) with an intermediate formed by the reaction between the complex and one sulphur(IV) species. The observed specific alkali-metal-ion catalysis is attributed to the formation of bridging by the alkali-metal ion between the two negatively charged reactants, facilitating the electron-transfer process.