Determination of the nature of the diperiodatocuprate(III) species in aqueous alkaline medium through a kinetic and mechanistic study on the oxidation of iodide ion

Abstract

The nature of the diperiodatocuprate(III) (DPC) species present in aqueous alkaline medium has been investigated by a kinetic and mechanistic study on the oxidation of iodide by DPC. The reaction kinetics were studied over the 1.0 × 10⁻³–0.1 mol dm⁻³ alkali range. The reaction order with respect to DPC, as well as iodide, was found to be unity when [DPC] ≪ [I⁻]. In the 1.0 × 10⁻³–1.0 × 10⁻² mol dm⁻³ alkali region, the rate decreased with increase in the alkali concentration and a plot of the pseudo-first order rate constant, k versus 1/[OH⁻] was linear. Above 5.0 × 10⁻² mol dm⁻³, a plot of k versus [OH⁻] was also linear with a non-zero intercept. An increase in ionic strength of the reaction mixtures showed no effect on k at low alkali concentrations, whereas at high concentrations an increase in ionic strength leads to an increase in k. A plot of 1/k versus [periodate] was linear with an intercept in both alkali ranges. Iodine was found to accelerate the reaction at the three different alkali concentrations employed. The observed results indicated the following equilibria for DPC.

[Cu(H₂IO₆)₂]^3- ⇋ [Cu(H₂IO₆)]^- + H₂IO₆ ^3-

[Cu(H₂IO₆)] + OH^- ⇋ [Cu(HIO₆)]^- + H₂O

A suitable mechanism has been proposed on the basis of these equilibria to account for the kinetic results.