Magnesium Tungstates

Normal magnesium tungstate, MgWO₄, is formed by fusion of sodium tungstate with sodium and magnesium chlorides, and remains after washing out the more soluble salts as lustrous orthorhombic crystals isomorphous with the calcium salt. The trihydrate, MgWO₄.3H₂O, was obtained by Lefort by mixing solutions of sodium tungstate and magnesium acetate in presence of alcohol; the precipitate was readily soluble in water. The heptahydrate, MgWO₄.7H₂O, is obtained by boiling tungstic acid with magnesium carbonate in suspension in water; a clear solution results, which, on cooling, deposits glistening prisms which readily dissolve in hot water.

Magnesium ditungstate, MgO.2WO₃.8H₂O, was prepared by Lefort by heating a suspension of magnesium hydroxide with excess of tungstic acid, and allowing the solution formed to crystallise; rhomboidal prisms separate, which are readily soluble in water.

Magnesium paratungstate, 3MgO.7WO₃.24H₂O, is obtained as white crystals by mixing warm solutions of alkali paratungstate and magnesium sulphate, the latter in excess. The salt is slightly soluble in cold water, more so in hot, and is easily decomposed by acids. If molecular proportions of sodium paratungstate and magnesium sulphate are mixed in solution at 70° C., the double salt, 3Na₂O.3MgO.14WO₈.33H₂O, is produced. Similar double salts containing ammonium, 2(NH₄)₂O.3MgO.12WO₃.24H₂O and (NH₄)₂O.2MgO.7WO₃.10H₂O, have been prepared. It is probable that these two salts are identical.

Magnesium tritungstate, MgO.3WO₃.4H₂O, results as a white precipitate on the addition of alcohol to a mixture of sodium tritungstate and magnesium acetate solutions; it is soluble in water.