Quantitative Determination of Sodium Metatungstate Speciation by 183W N.M.R. Spectroscopy

Abstract

The speciation and equilibria of sodium metatungstate, Na₆[H₂W₁₂O₄₀], has been determined by a combination of ¹⁸³W n.m.r. spectroscopy and gravimetry over the p[H] range 4.5–10.0. The use of n.m.r. spectroscopy allowed the direct observation of polytungstate anions in aqueous solution and high concentration (0.25 mol l^–1). Sodium metatungstate, with the progressive addition of base, was found to decompose to paratungstate A, [W₇O₂₄]^6–. Paratungstate A then slowly converted to paratungstate B, [H₂W₁₂O₄₂]^10–. The kinetics of equilibrium was found to be slow, requiring up to 8 months to stabilize. With the addition of further base paratungstate B decomposed to orthotungstate, WO₄^2–. Logarithmic constants (logK) for these transformations were determined as 195, 20 and 118, respectively. The solubility product for sodium paratungstate B was determined to be 6.89 ( 0.80) 10^–6.