A voltammetric study of the kinetics of the hexacyanomanganate(III) disproportionation in acidic medium

Abstract

The fast disproportionation of hexacyanomanganate(III) in acidic medium yields Mn(CN)62− and Mn(H2O)62+. The reaction has been studied by amperometry, with a rotating platinum-disk electrode, at a 1.0 V (vs. SCE) constant potential, in the acidity range 0.01–2.0 M and at temperatures between −5 and 15 °C. A reversible one-electron process takes place with a 0.85 V (vs. SCE) half-wave potential that is not dependent on acidity. The reaction kinetics is pseudo-first order in [Mn(CN)63−] and the rate constant depends on acidity according to the equation: kobs = ka + kb[H3O+] + kc[H3O+]2. At 5 °C and ionic strength 2.0, the determined values of ka, kb, and kc were (2.20 ± 0.08) × 10−3 s−1, (4.9 ± 0.2) × 10−3 s−1 M−1 and (1.40 ± 0.10) × 10−3 s−1 M−2, respectively. A mechanism is proposed where the three terms of kobs are assigned to pathways involving hexacyanomanganate(III) in three different states of protonation. From the effect of temperature the activation parameters, ΔH≠ and ΔS≠, for each pathway are also reported.Key words: hexacyanomanganate(IV), disproportionation, kinetics, amperometry, rotating platinum-disk electrode.