Mass and charge transfer kinetics and coulometric current efficiencies. Part VII. Conditional potentials, and single-scan voltammetry of pure vanadium(V)-vanadium(IV) systems in various media at platinum electrodes pre-treated by five methods

Abstract

The limited previous work on vanadium is reviewed. Five methods of electrode pre-treatment have been selected and are described. The variation of the conditional potential of the vanadium(V)-vanadium(IV) system with hydrogen-ion concentration is reported. The experimental work is complicated by isopolymerisation reactions, of which at least one is kinetically slow. The voltammetric reduction of vanadium(V) in saturated potassium sulphate-acetate buffer at pH 4·0 is examined: the benefit of shifting solvent reaction potentials to more negative potentials is nullified by the direct reduction of un-ionised acetic acid.

A similar examination is made in a medium of 5 × 10^–5M sulphuric acid and in one of 2·0 M sulphuric acid. Possible adsorption effects are canvassed. The complex behaviour at intermediate hydrogen-ion concentrations is discussed, with illustrations drawn from a sulphuric acid medium of pH 2·0. The anodic oxidation of vanadium(IV) is briefly examined. It is concluded that the electrochemical behaviour of the vanadium system is strongly dependent on hydrogen-ion concentration and on the electrode pre-treatment. The electrode can be chemically oxidised in vanadium(V) solutions. The mechanism is a one-step one-electron process. No evidence could be found for reduction below the +4 oxidation state at platinum. Vanadium(IV) cannot be oxidised without severe loss of current efficiency, nor reduced to vanadium(III) at platinum electrodes.