Simultaneous second-derivative spectrophotometric determination of cobalt and vanadium using 2-hydroxy-3-methoxy benzaldehyde thiosemicarbazone

2-Hydroxy-3-methoxy benzaldehyde thiosemicarbazone reacts with cobalt(II) and vanadium(V) at pH 6.0 forming, respectively, brown-and green-colored complexes in aqueous dimethyl formamide. The second-derivative spectrum of Co(II) complex shows a zero amplitude at 434.5 nm and a considerably large amplitude at 409.5 nm, while the V(V) complex shows a sufficient amplitude at 434.5 nm and a zero amplitude at 409.5 nm. Furthermore, the derivative amplitudes obey Beer's law at 409.5 and 434.5 nm for Co(II) and V(V) in the range 0.059-3.535 and 0.051-4.074 mu g/mL, respectively. This enables the simultaneous determination of Co(II) and V(V) without separation. A large number of foreign ions do not interfere in the present method. The method is applied to the simultaneous determination of Co(II) and V(V) in synthetic mixtures and alloy steel samples.