Calcium Tungstates

The normal tungstate, CaWO₄, occurs native as scheelite (see p. 182). It may be obtained by fusing together sodium tungstate and anhydrous calcium chloride, either with or without the addition of common salt; the resulting crystals are similar in form to scheelite, that is, tetragonal bipyramidal, a:c = l:1.5356, and of density 6.06 to 6.08. The amorphous form is readily obtained by mixing solutions of a calcium salt and sodium tungstate. The precipitated calcium tungstate is converted to the crystalline variety by fusion with sodium chloride, or by heating with lime in a current of hydrogen chloride. This salt is used for preparing the phosphorescent screens used in experiments with Rontgen rays.

Calcium ditungstate in the anhydrous state was supposed by Lefort to result from the double decomposition of sodium ditungstate and calcium acetate, whilst concentration of the mother-liquor yielded crystals presumed to be the trihydrate. However, the existence of these compounds is not established 4 since the products obtained vary in composition.

Calcium paratungstate, 3CaO.7WO₃.18H₂O, is prepared in a similar manner to the corresponding barium salt. The salt sometimes contains 19H₂O. It is more soluble than the barium and strontium para-tungstates, and may be heated to redness without melting. The double salt, 3Na₂O.2CaO.12WO₃.34H₂O, is obtained by adding lime-water or a solution of a calcium salt in excess to a boiling solution of sodium paratungstate, and allowing to cool. White crystals separate, which, when heated, fuse, and on cooling set to a black mass.

The salt, 4CaO.10WO₃.25H₂O, is obtained in a similar manner to the corresponding barium salt. It is a white, infusible, granular powder.

Calcium tritungstate, CaO.3WO₃.6H₂O, is obtained by mixing concentrated solutions of sodium tritungstate and calcium acetate, or calcium chloride (Lefort). The precipitate is decomposed by hot water into the di- and meta-tungstates.