Kinetics and mechanism of oxidation of antimony(III) by Keggin-type 12-tungstocobaltate(III) in aqueous hydrochloric acid medium

The reaction between Sb(III) and \(CoW12O40)-W-III\(5-) proceeds with two, one-electron steps; formation of unstable Sb(IV) is the slow first step followed by its reaction with another oxidant in a fast step. The reaction rate is unaffected by the \H+\ as there are no protonation equlibria involved with both the reactants, whereas the accelerating effect of chloride ion is due to the formation of an active chlorocomplex of the reductant, SbCl63-. Increase in the ionic strength and decrease in the relative permittivity of the medium increases the rate of the reaction, which is attributed to the formation of an outer-sphere complex between the reactants. The activation parameters were also determined and these values support the proposed mechanism. (C) 2002 Wiley Periodicals, Inc.