The oxidation of iodide by the tetrahydroxoargentate(III) ion in aqueous alkaline media

Abstract

Iodide is rapidly oxidized by [Ag(OH)₄]^– in strongly alkaline media to give hypoiodite and Ag^I according to the stoicheiometric equation (i). Stoicheiometry measurements are consistent with [Ag(OH)₄]^–+ I^–→ Ag^I+ 2OH^–+ IO^–+ H₂O (i) complementary two-electron redox both at excess of Ag^III and excess of I^–. The kinetics of the reaction are first order in the silver(III) and iodide concentrations. The second-order rate constant is 76 ± 1 dm³ mol^–1 s^–1(25 °C, I= 1.2 mol dm^–3) and the enthalpy and entropy of activation are 32.5 ± 0.6 kJ mol^–1 and –100 ± 2 J K^–1, respectively. While precipitation of AgI interferes with spectral measurements at lower (excess) iodide concentrations, the product formation has no effect on the kinetics of the system. The mechanism proposed involves a simultaneous two-electron transfer within an intermediate of composition [Ag(OH)₄I]^2–. The values of the activation parameters are similar to those of other silver(III) reactions.