Mass and charge transfer kinetics and coulometric current efficiencies. Part VIII. Single-scan voltammetry of vanadium(V)-vanadium(IV) in the presence of chromium, manganese and iron, and the kinetic parameters of the vanadium system, at platinum electrodes pre-treated by five methods

Abstract

Continuing the earlier examination of the vanadium system alone, under various conditions and with various electrode pre-treatments, the effect of neighbouring steel-forming d-block elements has been investigated. Chromium(VI) at pH 4·0 suppresses the vanadium(V) reduction wave, and the degree of suppression is quantitatively proportional to the chromium(VI) concentration. Activated electrodes are deactivated by dipping them in a chromium(VI) solution, and remain so even when well washed thereafter, so that chromium(VI) as well as chromium(III) is adsorbed strongly on platinum. In 2·0 M sulphuric acid, chromium(VI) and vanadium(V) are reduced at the same rate. Manganese(VII) in acetate buffer gives a fast, well separated wave, but the separation is not as good in 2·0 M sulphuric acid; slowing the vanadium(V) reduction by using an oxidised electrode effects no improvement: the manganese wave is similarly affected. Addition of chromium(VI) to the manganese-vanadium mixture at pH 4 suppresses the manganese wave only slightly, even when the vanadium wave is completely suppressed. In 2·0 M sulphuric acid, the manganese wave is undistorted and chromium and vanadium are simultaneously reduced. Iron(III) in 2·0 M sulphuric acid does not interfere, but the separation of the vanadium and iron waves is not good. Iron(II) can, however, act as a potentiostatic intermediate. The kinetic parameters of the vanadium system are reproducible in acetate buffer, but only when the electrode is fouled in 2·0 M sulphuric acid. Pattern theory and diffusion-corrected Lewartowicz methods give results that agree. The charge-transfer kinetic parameters are shown to be potential dependent in acidic media. The results are compared with those in earlier reports. The generation current efficiency for vanadium(IV) in acetate buffer was computed.