Iso-12-tungstosilicic Acid, H₈[Si(W₂O₇)₆]

Iso-12-tungstosilicic Acid, H₈[Si(W₂O₇)₆].20H₂O. - This compound is distinct in properties from the 12-tungstosilicic acid described above, and forms a series of salts which differ from the 12-tungsto- silicates. The two isomeric forms were recognised by Marignac, who distinguished them by the names silicotungstic acid and tungstosilicic acid. A similar isomerism occurs with the 12-tungstoboric acids. The iso-12-tungstosilicic acid is best prepared by heating at 100° C. a solution containing sodium tungstate and sodium silicate, made slightly acid with sulphuric acid. On adding sulphuric acid and ether, oily drops separate and both isomeric forms of the acid are produced, the iso-acid being in the greater quantity. The acids may be separated by fractional crystallisation of their potassium salts, the 12-tungstosilicate crystallising first, and then the rhombic potassium iso-salt, K₄H₄[Si(W₂O₇)₆].7H₂O. From this the acid may be obtained by Drechsel's method. The acid yields triclinic crystals. If the potassium salt is heated with water in a sealed tube at 150° C. it is gradually transformed into its isomeride, K₄H₄[Si(W₂O₇)₆].16H₂O. The following salts are described by Marignac:

Na₄H₄[Si(W₂O₇)₆].10H₂O,
(NH₄)₄H₄[Si(W₂O₇)₆].6H₂O,

as well as the normal salts

K₈[Si(W₂O₇)₆].20H₂O,
(NH₄)₈[Si(W₂O₇)₆].16H₂O.

The normal barium salt, Ba₄[Si(W₂O₇)₆].27H₂O, is obtained as a fine crystalline powder by adding barium chloride to a boiling solution of ammonium paratungstate saturated with silicic acid gel, and allowing the oil which separates to remain in contact with the mother-liquor.