Reduction of Ti-IV species in aqueous sulfuric and hydrochloric acids .1. Titanium speciation

The uv-visible absorption of aqueous Ti-IV and Ti-III was measured in the aqueous H2SO4 and HCl supporting electrolytes used in subsequent batch recycle electrolyses for the electrosynthesis of Ti-III. Thermodynamic predictions of the speciation were calculated from published stability constants for the weak sulfate and; chloride complexes. However, when Ti-IV and Ti-III sulfate solutions were mixed, the resulting species absorbed more strongly at visible wavelengths than the Ti-III species alone, possibly indicating the formation of a Ti-IV-Ti-III-sulfate charge transfer complex; no such behaviour was evident in. chloride media. The visible absorption, behaviour was modelled as a function of Ti-III, Ti-IV, proton, and sulfate concentrations to obtain an estimate of the equilibrium constant for the formation of Ti-IV-Ti-III-sulfate complex and its molar absorptivity. As Ti-III species are oxidised by oxygen/air, resulting in possible losses in their generation and utilisation efficiencies, the reaction kinetics were determined spectrophotometrically; again the formation of a Ti-IV-Ti-III-sulfate complex was implied. The voltammetric behaviour of Ti-IV and Ti-III species at a hanging mercury drop electrode in sulfate and chloride media was highly irreversible and indicated the formation of an adsorbed intermediate during Ti-IV reduction, which precluded relating the voltammetric behaviour to bulk solution speciation.