Mass and charge transfer kinetics and coulometric current efficiencies. Part IX. An examination of the titanium(IV)-titanium(III) system and the effects of ultratrace impurities in sulphuric acid

Abstract

Titanium(III) has found considerable use as a coulometric intermediate, and is the strongest reductant that can be generated with good efficiency in aqueous media. The medium is restricted to strong sulphuric acid. At such high concentrations, a surfactant impurity becomes adsorbed on the working electrode and very seriously reduces the speed of the charge-transfer process and therefore the current efficiency. A method of purification of the sulphuric acid by electrosorption is shown to give a charge-transfer rate constant in excess of 10^–5 l cm^–2 s^–1, but charcoal column purification is without effect. Both 7 and 10 M sulphuric acid media were examined, but because the mass-transfer rate is decreased by the higher viscosity of the 10 M acid there is little difference in current efficiency. Addition of iron(III) or iron(II) hinders the adsorption of the impurity and the deactivation of the electrode, but offers no special benefit. Electrode kinetic parameters are reported for various treated and untreated media, and current efficiencies for the generation of titanium(III) are computed. The behaviour in perchloric acid is re-interpreted.