The kinetics and mechanism of the oxidation of inorganic oxysulfur compounds by potassium ferrate.: Part I. Sulfite, thiosulfate and dithionite ions

The kinetics and mechanism of the oxidation of the sulfite (SO32−), thiosulfate (S2O32−) and dithionite (S2O42−) ions by ferrate (FeO42−) ions was studied under pseudo-first-order conditions and, in addition, the reaction with thiosulfate ions was studied under non-pseudo-first-order conditions. Previous work on sulfite and thiosulfate was repeated and extended, specifically over a larger pH range, and using non-pseudo-first-order kinetics. Dithionite was only studied at high pH values because of rapid hydrolysis below a pH of about 10. The kinetics for sulfite and thiosulfate showed a first-order dependence on hydrogen, oxysulfur and ferrate ion concentrations at high pH values, but became independent of the hydrogen ion concentration below a pH of about 8.5. The kinetics for dithionite involved two terms, one being first-order in the hydrogen, dithionite and ferrate ion concentrations, and the other being first-order in only the dithionite and ferrate ions. For all three reductants the proposed mechanism has a rate-determining step involving reaction between the protonated ferrate and the oxysulfur ions. For dithionite there is also a rate-determining step involving reaction between unprotonated ferrate and dithionite. All the oxysulfur ions gave Fe(III) as a product. Sulfite produced sulfate, whereas thiosulfate and dithionite gave sulfite.