Spectrophotometric determination of molybdenum in the presence of tungsten using gallein and benzyldodecyldimethylammonium bromide

The chemical similarity between molybdenum and tungsten makes the direct spectrophotometric determination of these metals impossible. Usually the determination is preceded by a separation step. In order to find out a selective and quantitative isolation method, coprecipitation with thioacetamide and Cu(II) as a carrier; MnO2; cupferron, tannin and crystal violet; quinolin-8-ol, tannin and thioacetamide, were examined. Molybdenum(VI) could be determined in the presence of 100-fold mass excess of tungsten after precipitation with thioacetamide and Cu(II). The remaining methods could only be applied if mass excess of W is equal to or lower with respect to Mo. For the resolution of this problem, the derivative spectrophotometry was used. The studies of different order spectra of gallein complexes of molybdenum, tungsten and their mixtures have shown that the fifth-derivative spectra allows one to eliminate the interfering effects of W on the determination of Mo. At 650 nm the spectral features of tungsten is zeroing while the value of the fifth-derivative spectrum of mixture of Mo and W corresponds only to the concentration of molybdenum in the examined solution. Beer’s law is obeyed in the range 0.32–0.80 μg/mL of Mo. The developed derivative spectrophotometric method and the most selective pre-separation method, based on the precipitation of molybdenum(VI) sulphide, were applied to the determination of Mo in Armco iron and steel. The accuracy of the elaborated methods was confirmed by comparison of the determined content of Mo with certified values as well as with the result obtained by the reference ICP-OES technique.