Quantitative Determination of Sodium Metatungstate Speciation by
183W N.M.R. Spectroscopy
Bradley J. Smith and Vincent A. Patrick
Australian Journal of Chemistry
53(12) 965 - 970
AbstractThe 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, 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) 10–6.
Keywords: Metatungstates, paratungstates, polyoxometallate.
© CSIRO 2000