A Mechanistic Approach to the Kinetics of Oxidation of Uranium(IV) by Hexachloroplatinate(IV) in Aqueous Perchlorate Solutions. Evidence of the Formation of a Binuclear Intermediate Complex

Abstract

The kinetics of hexachloroplatinate(IV) oxidation of uranium(IV) ion in aqueous perchloric acid solutions at a constant ionic strength of 1.0 mol dm(-3) has been investigated using the stopped-flow and conventional spectrophotometric techniques. The oxidation reaction was found to proceed through two distinct stages. The initial stage was found to be relatively fast corresponding to the formation of [(H(2)O)(n)U(IV)·Cl(6)Pt(IV)](2+) binuclear intermediate complex (with the rate constant k(1) = 1.75 × 10(4) dm(3) mol(-1)s(-1), k(-1) = 6.8 s(-1), and the formation constant K = 2.6 × 10(3) dm(3) mol(-1) at [H(+)] = 1.0 mol dm(-3) and 25 °C for binuclear formation). This stage was followed by a much slower stage corresponding to the transfer of two electrons from U(IV) to Pt(IV) in the rate-determining step (with the rate constant k = 5.32 × 10(-5) s(-1) at [H(+)] = 1.0 mol dm(-3) and 25 °C). The reaction stoichiometry was found to depend on the molar ratio of the reactants concentration. The experimental results indicated the decrease of the observed first-order rate constants with increasing the [H(+)] for the decomposition of the binuclear intermediate complex through the slow-second stage, whereas no change was observed with respect to the rate of formation of the binuclear complex at the initial rapid part. A tentative reaction mechanism consistent with the kinetic results is discussed.