Ammonium Metatungstate, (NH₄)₂W₄O₁₃

Ammonium Metatungstate yields two hydrates: (1) (NH₄)₂W₄O₁₃.8H₂O or (NH₄)₆H₄[H₂(W₂O₇)₆].21H₂O, and (2) (NH₄)₂W₄O₁₃.6H₂O.

The former is obtained (1) by prolonged heating of crystalline ammonium paratungstate at 250° C., when ammonia is expelled, the residue then being dissolved in water and allowed to crystallise; (2) by boiling a solution of ammonium paratungstate for several days and then allowing to crystallise; (3) by the addition of nitric acid to a solution of an acid tungstate of ammonium, filtering and allowing to evaporate (Laurent); this product is contaminated with ammonium nitrate which can be removed only with difficulty.

Ammonium metatungstate crystallises in large, lustrous, tetragonal octahedra, with axial ratio a:c = 1:1.012, isomorphous with the potassium salt and with ammonium borotungstate, 5(NH₄)₂O.B₂O₃.24WO₃.52H₂O. The crystals, which are colourless and transparent, on heating melt in the water of crystallisation and at 100° C. lose 7 molecular proportions of water; the remaining molecule is not expelled below 200° C. The salt begins to lose ammonia at 120° C., and at 250° C. it is converted into a glass-like "colloidal" tungstate of composition (NH₄)₂W₆O₁₉ 4 or 6H₂O.

It dissolves in water: 1 part of salt in 0.84 part of water at 15° C., and 1 part of salt in 0.35 part of water at 45° C. The solution is nearly neutral and is highly refractive. The salt is insoluble in alcohol or ether.

The hexahydrate, (NH₄)₂W₄O₁₃.6H₂O, is formed by treating a hot solution of the salt with alcohol and allowing the mixture to cool, when it separates in monoclinic prisms. Its crystallographic elements are

a:b:c = 0.8121:1:0.7963; β = 95° 9'.

At 100° C. it loses 5 molecular proportions of water.

An acid salt, 3(NH₄)₂O.16WO₃.17H₂O, separates as colourless crystals on evaporating a solution of the metatungstate containing hydrochloric acid. The crystals rapidly effloresce in the air, and at 100° C. lose about 13H₂O. The salt yields an acid solution with water, which on evaporation deposits the metatungstate again.

A double compound of composition (NH₄)₂W₄O₁₃.NH₄NO₃.2H₂O separates on evaporating a solution containing ammonium metatungstate and excess of ammonium nitrate. It yields hexagonal prisms which are decomposed by water.