Quantitative Determination of Aqueous Dodecatungstophosphoric Acid Speciation by NMR Spectroscopy

Abstract

The speciation and equilibria during base decomposition of α-[PW₁₂O₄₀]^3– have been determined using ³¹P and ¹⁸³W NMR spectroscopy over the pH range 0.7–13.5. NMR spectroscopy was used to directly observe the polytungstate species in aqueous solution at high concentration (0.25 mol L^–1) and follows the progressive decomposition of [PW₁₂O₄₀]^3– to WO₄^2– and PO₄^3–. It was found unexpectedly in the pH range 1.2–2.3 that [PW₁₂O₄₀]^3– did not decompose directly to [PW₁₁O₃₉]^7– but first formed an equilibrium with [P₂W₁₈O₆₂]^6–, [P₂W₂₁O₇₁]^6–, and some new, stable, intermediate species before converting into α-[PW₁₁O₃₉]^7–.