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 tungst en and molybdenum in polyoxometalates (POM) and their lanthanide(III) compl exes (Ln/POM), using disodium-1,2-dihydroxybenzene-3,5-disulfate (Tiron), u sed as colorimetric reagent. Mo(VI) and W(VI) in solutions of polyoxotungst ates and polyoxomolybdates can be determined from calibration graphs obtain ed by recording absorbancies in the wavelength range 352-500 nm for molybde num and 352-400 nm for tungsten, where Beer's law is obeyed. This method ca n be also applied for the simultaneous determination of these elements in p olyoxotungstomolybdates without previous chemical separation. The procedure is based on the recording of absorption spectra using values of absorbanci es at two points on the spectrum. The content of Mo and W can be determined using an appropriate mathematical equation and molar absorptivity coeffici ents for each of two determined Mo and W ions at two points with the absorp tion spectrum. Calibration graphs for 2-10 (also to 40) mu g/ml of Mo(VI) a nd 4-20 (also to 80) mu g/ml of W(VI) were established by measuring the abs orbance 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 measurement s of all samples studied (POMs and their Ln(III) sandwiched and encrypted c omplexes) is <4%. (C) 2000 Elsevier Science S.A. All rights reserved.