On the identification of molybdophosphate complexes in aqueous solution

Abstract

Aqueous solutions containing sodium phosphate and sodium molybdate ([P]= 0.01–0.07 mol l^–1[Mo] : [P]= 1.25–15) were acidified with concentrated HCl and the formation of molybdophosphate anions was followed by ³¹P n.m.r. spectroscopy with a view to establishing this technique as an easily applicable method for the identification of molybdophosphate species in solution. For comparative purposes and to aid the interpretation of the n.m.r, spectra, Raman spectroscopy and differential pulse polarography were also applied. Of the eight n.m.r. resonances observed, two had already been reported: they stem from [PMo₁₂O₄₀]^3– and [P₂Mo₁₈O₆₂]^6– Lines corresponding to [P₂Mo₅O₂₃]^6–, [PMo₉O₃₁(OH)₃]^6–, and [PMo₁₁O₃₉]^7– were also identified. A controversy concerning the relative importance of the last two species has been resolved. Three lines could not be unequivocally assigned: one may correspond to [PMo₁₀O₃₄]^3–, one to [PMo₆O₂₅]^9–, while for the third there are no further data on which to base a suggestion. It is shown that various reports concerning molybdophosphate solutions are inconsistent with the pertinent ³¹P n.m.r. spectroscopic data.