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

12-Tungstosilicic Acid, H8[Si(W2O7)6].xH2O, was first obtained by Marignac by decomposing the mercurous salt by means of hydrochloric acid, and allowing the resulting solution to slowly crystallise. It is more conveniently obtained by the method of Drechsel from concentrated aqueous solutions of its salts, by shaking out with ether after the addition of hydrochloric acid. At ordinary temperatures, large quadratic crystals of the 28-hydrate, H8[Si(W2O7)6].28H2O, are obtained. When these are heated above 33° C., or when the acid is allowed to crystallise from the warm solution, the 22-hydrate, H8[Si(W2O7)6].22H2O, which yields rhombohedral crystals, is formed. The actual transition point is 28.5° C. The 22-hydrate melts at 53° C. A 15-hydrate, H8[Si(W2O7)6].15H2O, separates in fine prisms when concentrated solutions of the acid are treated with fuming hydrochloric acid. The acid is readily soluble in water and in alcohol; it is extremely stable towards hydrolysis, and its solution may be used as a reagent for alkaloids with which it gives distinctive precipitates which are definite and stable compounds. The hydrogen-ion concentration in a 0.05 N solution of the acid is 2.9×10-3.

The basicity of 12-tungstosilicic acid, as indicated by the formula, appears to be 8, and normal alkali salts, for example, the potassium salt K8[Si(W2O7)6].14H2O, were described by Marignac. Subsequent investigators, however, could not verify Marignac's work, and showed that the products obtained by his method of preparation, namely, by saturation of solutions of the paratungstates with silicic acid gel, were salts of 10-tungstosilicic acid and not 12-tungstosilicates. The normal mercurous salt Hg8[Si(W2O7)6].5H2O, however, separates in pale yellow laminae when mercurous nitrate is added to a solution of the free acid.

The 12-tungstosilicates usually obtained correspond to the formula R4H4[Si(W2O7)6].xH2O. They may be prepared by the addition of mineral acid to a boiling solution containing alkali tungstate and silicate; by the addition of alkali silicofluoride to a solution of the corresponding tungstate, until the mixture no longer yields a precipitate of tungstic acid when treated with hydrochloric acid; by treating a solution of the free acid with a metallic carbonate; or by double decomposition. The salts crystallise readily, forming well-defined crystals, and each salt appears to form several well-defined hydrates; for example, the sodium salt Na4H4[Si(W2O7)6].18H2O is dimorphous, and hydrates containing 12H2O and 11H2O respectively are also known. The copper and chromium salts each yield three hydrates. The lithium, calcium, and barium salts yield hydrates which are isomorphous with the rhombohedral form of the free acid, thus:

H8[Si(W2O7)6].22H2O,
Li4H4[Si(W2O7)6].22H2O,
Ca2H4[Si(W2O7)6].22H2O,
Ba2H4[Si(W2O7)6].22H2O.

Isomorphism also exists between certain 12-tungstosilicates and certain metatungstates and borotungstates; for example, the following are isomorphous:

K6H4[H2(W2O7)6].16H2O,
K5H4[B(W2O7)6].16H2O,
K4H4[Si(W2O7)6].16H2O;
and also
Ba3H4[H2(W2O7)6].25H2O,
Ba2.5H4[B(W2O7)6].25H2O,
Ba2H4[Si(W2O7)6].25H2O.

The 12-tungstosilicates form stable double salts with nitrates; for example, 3Na4H4[Si(W2O7)6].4NaNO3.13H2O and Ca2H4[Si(W2O7)6].Ca(NO3)2.13H2O.

The acid salts, Na2H6[Si(W2O7)6].14H2O and K3H5[Si(W2O7)6].12H2O, have been obtained by the action of mineral acids on the ordinary 12-tungstosilicates.

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