Sodium Pentatungstate/Sodium Hexatungstate

Sodium pentatungstate, Na2O.5WO3, is obtained by fusing together sodium tungstate and tungstic anhydride (1:2), or by heating sodium paratungstate to incipient fusion and extracting the fused mass with water, when it remains in brilliant plates or scales which are only slightly soluble in water.

Sodium hexatungstate, Na2O.6WO3.9H2O, is obtained according to Marignac by prolonged boiling of tungstic acid with sodium paratungstate. Ullik, by decomposing a solution of sodium metatungstate with hydrochloric or nitric acid and allowing the solution to evaporate, obtained large yellowish crystals of what he considered to be the octa-tungstate, Na2O.8WO3.12H2O, but Friedheim could not confirm his results, and Leontowitsch, using the reagents in different proportions, obtained crystals of the hexatungstate, of compositionNa2O.6WO3.15H2O. The anhydrous octatungstate, Na2O.8WO3, was obtained by von Knorre by oxidation of fused metatungstate at a bright red heat, and extraction of the mass with water, when lustrous scales of the octatungstate remain. The relation of these higher acid salts to one another and to metatungstic acid has not yet been determined.