Silicotungstic acid is a heteropoly acid. It is a white (or light grey, pale-yellow) solid with a formula H4[W12SiO40] and molar mass 2878.2 g.mol-1. It is comercially available as a x-hydrate and it is used as a solid state catalyzator in industry. In the last decade more articles reported the acid as an excellent catalyst in a synthesis of diesters, acetals and ketals [1,2]. Heteropolyanion derived from the acid adopts Keggin structure typical for such heteropolyanions with a formula [XM12O40]n-. In the structure of the anion the heteratom is tetrahedrally surrounded by four oxygen atoms. The tetrahedron MO4 is enclosed in the cluster made up of twelve octahedrons MO6. The whole structure belongs to Td point group.
Keggin structure of heteropolyanions [photo: Wikipedia]
Sodium tungstate diyhdrate (15 g, 0.045 mol) was dissolved in 30 mL of distilled water and the solution was heated up to 90 °C. 0.77 mL of sodium silicate solution (density 1.5 g.cm-3) was slowly added to the solution under constant stirring. Through a stopcock 10 mL of concentrated hydrochloric acid was added from a burette over 30 minutes to the hot mixture of sodium tungstate and sodium silicate. After the end of the reaction, the mixture was cooled down in an ice-bath and 5 mL of concentrated hydrochloric acid was added. The mixture was transfered to a separatory funnel and 12 mL of diethylether was added. After shaking, three layers were observed in the funnel. Bottom oily-ether layer was separated and the residue was extracted three more times. The extracts were combined together and the solvent was distilled of using RVE. Yield: 53.3 %.
 Brahmkhatri, V. Patel A. Ind. Eng. Chem. Res. ISSN 0888-5885. 2011, Vol. 50, p. 13693-13702.
 Yang, S.-J. et al. J Zheijang Univ SCI. ISSN 1009-3095. 2005, Vol. 6B, No. 5, p. 373-377.