Spectrophotometric determination of vanadium(IV, V) in chloride-rich samples by its catalytic effect on the oxidative coupling of N,N-diphenylhydrazine with chromotropic acid

A kinetic method has been developed for the spectrophotometric determination of subnanogram per milliliter levels of vanadium in the presence of large amounts of sodium chloride. The method is based on the catalytic effect of vanadium(V) on the oxidative coupling of N,N-diphenylhydrazine with chromotropic acid in the presence of bromate. In this reaction, chloride enhanced the catalysis of vanadium(V). Vanadium(IV) could be also determined because of its oxidation by bromate. By measuring the change in absorbance of the dye produced at a fixed time, vanadium(IV) and vanadium(V) in the range of 0.01-0.2 ng mL(-1) could be determined. The relative standard deviation was 2.0% for ten determinations of 0.1 ng mL(-1) vanadium(V). Interferences from diverse ions could be suppressed by the use of ethylenediamine-N,N,N',N'-tetrakis(methylenephosphonic acid) as a masking agent. The method was successfully applied to the determination of vanadium(IV) and vanadium(V) in chloride-rich samples.