Abstract
Polarographic determination of vanadium in strong phosphoric acid (SPA) and SPA-EDTA solutions was studied. In SPA solutions, no waves were obtained for vanadium (III) and vanadium (IV). Vanadium (V) gave a single wave with E1/2=-1.03-1.09 V (vs. SCE) at pH 10.011.5, the wave height being proportional to vanadium (V) concentration.
In SPA-EDTA solutions, vanadium (III, IV and V) all gave reduction waves.
The half-wave potential of vanadium (III) was-1.28-1.29 V (vs. SCE) at pH 5.09.0, corresponding to the diffusion-controlled and reversible reduction of vanadium (III) to vanadium (II). Both vanadium (V) and vanadium (IV) gave double wave at the same pH range, the second wave corresponding to the reversible reduction of vanadium (III) to vanadium (II). The double wave heights for vanadium (IV) decreased drastically at above pH 5.5. The second wave for vanadium (V) decreased with the increase of pH or the SPA concentration. Since the value Id√η was constant in this pH range, it was caused by the change of viscosity of the solution and the second wave was diffusion-controlled.
The optimum condition for the determination is: vanadium in oxidation state vanadium (V) and in concentration below 10-2 M is determined using 200°C-SPA 5 ml/50 ml+0.06 M EDTA at pH 7.0 as supporting electroyte, and 0.004% polyacrylamide as a maximum suppressor.
