Potentiostatic coulometric determination of vanadium, vanadium-manganese and vanadium-iron mixtures and the influence of chromium on the process

Abstract

Earlier voltammetric work permitted the mass and charge transfer kinetic parameters of the vanadium system to be calculated for diverse media and platinum electrode pre-treatments, and command potentials to be selected for potentiostatic determination of vanadium alone and in certain combinations with other steel-forming elements. A simple coulometric cell and an adaptation of a commercial potentiostat are described. Current integration by strip-chart recorder is too inaccurate and so RC integration is discussed. Philbrick SP456 amplifiers refused capacitive feedback, but very satisfactory results were given by Solartron AA 1023 amplifiers. The design of a very high quality integrating capacitor from S.T.C. polystyrene elements is described; leakage and drift tests were very satisfactory. Pre-treatments of electrolytes and electrodes are discussed. Vanadium(V) is determined at –0·128 V in acetate buffer and at +0·247 V in 2·0 M sulphuric acid, in the latter with a relative standard deviation of 0·27 per cent. and a 95 per cent. confidence level result of (1·008 to 1·011)× 10^–1M compared with 1·012 × 10^–1M for a standard solution. Chromium(VI) suppresses all reduction at pH 4·0, and is reduced simultaneously with vanadium in sulphuric acid. Manganese(VII) is reduced to manganese(III) in the first step at +0·7 V at pH 3·5 and manganese(III) and vanadium(V) are simultaneously reduced in the second step at –0·12 V. The separation of iron(III) is possible at +0·9 V but impracticable; simultaneous reduction at +0·25 V in 2·0 M sulphuric acid followed by re-oxidation of the iron(II) at +1·0 V is recommended.