REDOX CHEMISTRY OF H2S OXIDATION BY THE BRITISH GAS STRETFORD PROCESS .4. V-S-H2O THERMODYNAMICS AND AQUEOUS VANADIUM (V) REDUCTION IN ALKALINE-SOLUTIONS

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 HV2O73-/V4O92- 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, V18O4212- 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 almost-equal-to 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 HV2O73- 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 V18O4212- ions probably predominate at equilibrium. In the presence of the large excess of HS-ions required to form VS43- ions, the electrochemical behaviour of a gold electrode was dominated by the former species.