Autocatalytic and Non-Catalytic Oxidation of Manganese(II) on a Platinum Electrode in Acetic Buffer

摘要

The influence of acetate on Mn2+ oxidation by a platinum electrode has been investigated in aqueous solution with cyclic voltammetry. It is found that acetate at concentrations below 0.5 M gives rise to autocatalytic oxidation of Mn2+ with MnO2 serving as the catalyst, while in 0.5-1 M acetate (as well as in the absence of acetate) Mn2+ is oxidized non-catalytically. The autocatalytic oxidation is revealed by a separate voltammetric wave which has a much sharper rise and a lower potential compared to non-catalytic waves, by a sigmoid kinetics during chronoamperometry and by an enhanced Mn2+ uptake with electrodeposited MnO2. The increase in temperature and Mn2+ concentration promotes autocatalysis while acidification and the increase in sweep rate inhibits it that is thought to be due to the acceleration or delay of MnO2 formation from Mn3+. It is suggested that acetate acts in two ways: (1) thermodynamically stabilizes Mn3+ by complexation to enable the autocatalytic reaction Mn2+ + MnO2 + OAc- + H+ <-> Mn3+ - OAc + (MnOOH)-O-III and (2) delays the formation of MnO2 (presumably on the stage of Mn(III) crystallization) which leads to the inhibition of autocatalysis at high acetate concentrations.