Redox potentials of vanadium-containing couples. Part 3. The formal redox potential of the V3+–V2+ couple

Abstract

A method for the chemical and electrochemical preparation of mixtures of vanadium-(III) and -(II) in 1 mol dm^–3(H⁺, Na⁺) Cl^– aqueous solutions has been elaborated. The concentration of each ion can be determined under nitrogen in a completely closed system comprised essentially of a spectrophotometric cuvette and a junction-free cell containing mercury and glass electrodes. By changing the vanadium ion concentrations in ‘titrations’ of different kinds and by measuring the electromotive force of the galvanic cell the formal redox potential of the V^III–V^II couple has been deduced. Its value at 298 K against the normal hydrogen half cell and in 1 mol dm^–3(H⁺, Na⁺) Cl^– is –242 ± 2 mV over a large range of V³⁺/V²⁺ ratio and of total vanadium concentration. Using the specific interaction theory (s.i.t.) approach the standard redox potential is estimated to be –196 mV. The methodology reported seems to be appropriate for studying the solution chemistry of the rather oxidizable V²⁺ ion.