Prospective mechanistic on electron-transfer nature for reduction of permanganate ion by ascorbic acid in aqueous acidic solutions

Abstract

A conventional spectrophotometer has been applied for investigating the kinetics of reduction of permanganate ion by ascorbic acid (vitamin C) in aqueous media. The oxidation process showed complexity kinetics. First- order dependences in each of the reactants and second-order overall kinetics has been observed. The decrease in the oxidation rates with increasing the [H+] indicated that the oxidation was of acid-inhibition nature. Kinetics and spectrophotometric evidences for formation of intermediate complexes through the oxidation process have been revealed. Formation of free-radical intervention was confirmed by the test of acrylonitrile reagent. Formation of either Mn3+ and/Mn4+ as the more reactive species through the progression of reaction were confirmed by the added Mn2+ and F− species. The naked-eyes observation indicated that the purple color of permanganate ion was firstly turned into rose color, which is rapidly disappeared to give a colorless solution. A black brownish of colloidal aggregate sol was formed by aging the colorless solution particularly in the presence of large excess of ascorbic acid over that of permanganate ion concentration, The analysis of the derivatives indicated the formation of dehydroscobic acid and manganese (0) nanoparticles as final oxidation products. An inner-sphere mechanism of two electron changes in a single step was suggested and discussed in terms of the evaluated activation parameters and kinetic observations.