Selective determination of ultra trace concentrations of molybdenum by catalytic adsorptive stripping voltammetry

A highly selective and sensitive procedure is presented for the determination of ultra-trace concentrations of molybdenum by catalytic adsorptive stripping voltammetry. The method is based on adsorptive accumulation of the molybdenum (Mo)-methyl thymol blue (MTB) complex onto a hanging mercury drop electrode, followed by reduction of the adsorbed species by voltammetric scan using differential pulse modulation. The reduction current is enhanced catalytically by chlorate. Optimal analytical conditions were found to be a MTB concentration of 0.8 mu M, pH 2.5, a chlorate concentration of 0.5 M, and an adsorption potential at 100 mV versus Ag/AgCl. The peak current is:proportional to the concentration of molybdenum over the range 0.01-150 ng ml(-1) with a 3 sigma detection limit of 2 pg ml(-1) With an accumulation time of 60 s. The effects of possible interfering ions were studied, and it was found that the proposed procedure is free from interferences of common interfering elements such as tungsten, and iron. The procedure is applied to the determination of molybdenum in a mineral water sample and some analytical grade salts with satisfactory results. (C)1999 Elsevier Science B.V. All rights reserved.