Quantitative Determination of Sodium Dodecatungstosilicate Speciation by 183W Nuclear Magnetic Resonance Spectroscopy

Abstract

The speciation and equilibria of sodium dodecatungstosilicate has been determined using ¹⁸³W nuclear magnetic resonance (NMR) spectroscopy over the pH range 3-13.5. The use of NMR allowed the direct observation of polytungstate anions in aqueous solution and at high concentration (0.5 mol L^-1). Using this technique, the speciation of α-[SiW₁₂O₄₀]^4-, α-[SiW₁₁O₃₉]^8-, α-[NaSiW₁₁O₃₉]^7-, α-[H₂W₁₂O₄₀]^6-, [H₈W₁₁O₄₀]^6-, [H₇W₁₁O₄₀]^7-, [W₇O₂₄]^6-, [H₂W₁₂O₄₂]^10-, and WO₄^2- was quantified as a function of pH. This work has allowed stability constants for α-[SiW₁₂O₄₀]^4- (log K 46) and α-[SiW₁₁O₃₉]^8- (log K 86) to be estimated.