A new spectrophotometric method for the determination and simultaneous determination of tungsten and molybdenum in polyoxometalates and their Ln(III) complexes

A new spectrophotometric method is proposed for the determination of tungsten and molybdenum in polyoxometalates (POM) and their lanthanide(III) complexes (Ln/POM), using disodium-1,2-dihydroxybenzene-3,5-disulfate (Tiron), used as colorimetric reagent. Mo(VI) and W(VI) in solutions of polyoxotungstates and polyoxomolybdates can be determined from calibration graphs obtained by recording absorbancies in the wavelength range 352-500 nm for molybdenum and 352-400 nm for tungsten, where Beer's law is obeyed. This method can be also applied for the simultaneous determination of these elements in polyoxotungstomolybdates without previous chemical separation. The procedure is based on the recording of absorption spectra using values of absorbancies at two points on the spectrum. The content of Mo and W can be determined using an appropriate mathematical equation and molar absorptivity coefficients for each of two determined Mo and W ions at two points with the absorption spectrum. Calibration graphs for 2-10 (also to 40) mu g/ml of Mo(VI) and 4-20 (also to 80) mu g/ml of W(VI) were established by measuring the absorbance at 400 nm for molybdenum and at 352 nm for tungsten, respectively. Limiting concentrations of several interfering ions [P(V), V(V), Si(IV), As(III), Eu(III), Gd(III)] were evaluated. The standard error for measurements of all samples studied (POMs and their Ln(III) sandwiched and encrypted complexes) is <4%. (C) 2000 Elsevier Science S.A. All rights reserved.