Abstract
The formation of lacunar heteropolyanions (HPA): [AsW9O33]9–, [As2W19O67(H2O)]14–, and [As2W20O68(H2O)]10– in aqueous solutions was investigated by Raman spectroscopy at [Na2HAsO3]0 = 0.1, [Na2WO4]0 = 0.9 mol L–1 and pH 9.4–1.6. The [AsW9O33]9– HPA is characterized by the most intense band ns (W=O) at 948 cm–1 retaining its position in the pH range from 8.9 to 7.5. Under these conditions, the equilibrium constant of [AsW9O33]9– formation from H2AsO3– and WO42– ions was estimated (logK = 87.0±1.0). The asymmetrical band at 952 cm–1 corresponding to Hx[As2W19O67(H2O)](14–x)– shifts to 960 cm–1 as the pH decreases from 6.5 to 5.5, which is due to the change in HPA protonation. The [As2W20O68(H2O)]10– HPA is formed at pH 3.1—1.6; it is characterized by a band at 972 cm–1.
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