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

摘要

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_2O)_nU ~(IV)·Cl_6Pt~(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.