Mechanism of osmium(VIII)-catalysed oxidation of selenium(IV) by aqueous alkaline ferricyanide ion

Abstract

The kinetics of the osmium(VIII)-catalysed oxidation of selenium(IV) by alkaline ferricyanide has been studied for 0·1M-perchlorate solutions. The reaction shows first-order dependence to the concentrations of Se^IV, Fe(CN)₆^3–, OH^– and Os^VIII. Increase in ionic strength of the medium has an accelerating effect on the reaction rate, while an addition of ferrocyanide ions retards the rate. The mechanism indicates that the reaction occurs by the interaction between HSeO₄^3– and Fe(CN)₆^3– ions forming an unstable HSeO₄^2– intermediate which is then attacked by Os^VIII to give the oxidised product HSeO₄^– and the reduced form of osmium rapidly reacts with another molecule of ferricyanide to regenerate the Os^VIII. The rate law expression has been obtained.