Determination of subnanomolar concentrations of tungsten (VI) by catalytic adsorption polarography

A new differential pulse polarographic method for the determination of W(VI) using a catalytic adsorption wave is described. W(VI) is first chelated by 7-iodo-8-hydroxyquinoline-5-sulfonic acid at pH 0.5. The complex ion formed is strongly adsorbed on the surface of a dropping mercury electrode. At a potential of –0.95 V versus the Ag/AgCl (3M KCl) reference electrode the adsorbed complex is reduced by the polarographic current and oxidized very fast by hydrated hydrogen ions providing the oxidized form of the complex ion for repeated redox cycles. As the redox process taking place in the electric double layer, the diffusion of the complex does not limit the polarographic current. Therefore, high currents occur, and consequently, a very high sensitivity is obtained. The practical detection limit (PDL) is 3.7 ng W/kg solution corresponding to 2 × 10^–11 M. The standard deviation of single values is 1.2 ng/kg at the concentration of 91 ng/kg lying in the middle of the linear part of the calibration curve. Because Mo (VI) gives a very similar catalytic adsorption wave, serious mutual interferences occur in the analysis of mixtures of both species. An effective separation of Mo(VI) was worked out. Using 1% (w/v) solution of trioctylphosphinoxide in kerosene, Mo(VI) can almost completely be extracted from 1.8M HCl with a threefold extraction resulting in a separation factor of 40000.