Redox chemistry of H₂S oxidation by the British Gas Stretford process part IV: V-S-H₂O thermodynamics and aqueous vanadium (v) reduction in alkaline solutions

Abstract

The thermodynamics of V-H₂O and V-S-H₂O systems at 298 K are summarized in the form of potential-pH and activity-pH diagrams calculated from recently published critically assessed standard Gibbs energies of formation. At pH 9, as used in Stretford Processes, V-H₂O potential-pH diagrams predicted that the V(v)/V(iv) couple involves HV₂O₇ ³⁻/V₄O₉ ²⁻ ions. However, in neutral and alkaline solutions there is difficulty in discriminating between kinetic intermediates and thermodynamically stable solution species, some with V(v)/V(iv) mixed oxidation states. Hence, V₁₈O₄₂ ¹²⁻ ions may be the stable V(iv) species, depending on the concentration, though no thermodynamic data are available to enable them to be included in potential-pH calculations. Potential-pH diagrams for V-S-H₂O systems predicted an area of stability for VS₄ in the pH range ≈2–8 and over a restricted potential range; neither VS nor VS₂ were predicted to be stable under any conditions considered. In cyclic voltammetric experiments at Hg, Au and vitreous carbon electrodes, reduction of vanadium (v) species (probably HV₂O₇ ³⁻ ions) was found to be irreversible on a variety of electrode surfaces and, at lower potentials, led predominantly to the formation of solid oxide films (V₃O₅, V₂O₃ and VO) rather than to V(iv) solution species, of which V₁₈O₄₂ ¹²⁻ ions probably predominate at equilibrium. In the presence of the large excess of HS⁻ ions required to form VS₄ ³⁻ ions, the electrochemical behaviour of a gold electrode was dominated by the former species.