Quantitative determination of sodium metatungstate speciation by W-183 NMR spectroscopy

The speciation and equilibria of sodium metatungstate, Na-6[H2W12O40], has been determined by a combination of W-183 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, WO42-. 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) x 10(-6).