Vanadium polarographic reduction

Two polarographic methods have been developed for the determination of cohalt(II) at concentrations ranging from approximately 1 to 80 mM in an aqueous sample. For the first method [15], which is suitable for samples containing large amounts of nickel]11), the cobalt(II) is oxidized to Co(NH3)6 in an ammoniacal medium with the aid of sodium perborate, after which the cobalt(III) species is determined. A second procedure [16] entails the use of lead dioxide in an acetic acid-acetate buffer containing oxalate to convert cobalt(II) to the 0(0204)3 ion, which can be subjected to polarographic reduction. This latter approach is well suited to the determination of cobalt in the presence of copper(II), iron(III), nickel(II), tin(IV), and zinc(II), whereas the chief interferences are cerium, chromium, manganese, and vanadium. 

Kinetic and mechanistic studies of yet more complicated species will be noted more briefly. Polarographic reduction of tungstomolybdosilicates (SMo + W=12 Si= 1) starts with a dissociative step. A rate law and initial rates have been reported for the formation of phosphovanadomolybdate (Mo= 10 V=2 P= 1) from dinuclear molybdenum and dinuclear vanadium oxoanions. Further kinetic information on these analytically important phosphomolybdovanadate species is available, in relation to their reactions with hydrazine. Kinetics of exchange are reported between [UfPWnOss).,] - or [UCPaWi OeOa] - and [WO4] -, and of W between [SiWi Nb204ol - and [HsWeOai] -. ... 

The vanadium(ii) reduction of nitrate ion has been investigated using the time dependence of polarographic diffusion currents. At pH < 2... 

Very little information has been given in the literature concerning the controlled-potential oxidation-reduction behavior of vanadium. It seems likely that much of the polarographic data reported (242) can be directly applied to coulometric analysis. As an unexpected complication, however, Meites and Moros (243) were able to show that the reduction of hydrogen ions is induced by the reduction of vanadium (IV) in hydrochloric acid media. 

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