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. (C) 2001 Elsevier Science B.V. All rights reserved.