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 [(H2O)nUIV 3 Cl6PtIV]2+ binuclear intermediate complex (with the rate constant k1
= 1.75  104 dm3 mol
1s
1, k
1 = 6.8 s
1, and the formation constant K = 2.6  103 dm3 mol
1 at [H+] = 1.0mol 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 UIV to PtIV 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.