Ammonium Tungstates

The normal salt cannot be obtained by dissolving tungstic acid in aqueous ammonia, since on concentration ammonia is lost and an acid salt remains. That the normal tungstate is present in solution would appear evident from the fact that calcium chloride precipitates the normal calcium tungstate. It may be obtained as a white mass by the addition of hydrated tungstic acid to liquid ammonia. The normal tungstate is very soluble in water and readily loses ammonia.

The acid tungstate, 2(NH₄)₂O.3WO₃.3H₂O, sometimes crystallises from a very concentrated neutral solution of tungstic acid in ammonia, which, however, more often yields the paratungstate. The crystals, which are warty and indistinct, give off ammonia in contact with the air, leaving the paratungstate.

Ammonium paratungstate, 5(NH₄)₂O.12WO₃.11H₂O, is the product most frequently obtained by the interaction of tungstic acid and ammonia; for example, when tungstic acid is dissolved in aqueous ammonia, or when ammonia is added to a solution of ammonium meta- tungstate, on concentration of the resulting solution the paratungstate crystallises out. It is dimorphous and yields both acicular needles and laminated plates. The former are the more stable and are pseudo-rhombic (a:b:c = 0.7995:1:0.4582). The plates are triclinic pinacoidal, a:b:c = 0.9785:1:1.2154; α = 65° 47', β = 117° 33', γ = 119° 2'. Both forms are stable at ordinary temperatures, but when heated, begin to lose ammonia at 60° C.; at 100° C. lose 7 molecules of water; and at 250° C. yield the glass-like colloidal tungstate (see below). If the crystals are heated in contact with dry ammonia, they lose water and absorb ammonia. When boiled in aqueous solution for several days, the metatungstate is formed.

This salt was regarded by Berzelius and Anthon as the ditungstate; Lotz and Scheibler formulated it as 3(NH₄)₂O.7WO₃.6H₂O, but the formula given above is due to Marignac. Rosenheim suggests (NH₄)₁₀H₄[H₄(WO₄)₆(W₂O₇)₃].7H₂O.

The hydrate, 5(NH₄)₂O.12WO₃.5H₂O or 3(NH₄)₂O.7WO₃.3H₂O, is obtained by evaporation of a solution of ammonium paratungstate at temperatures near the boiling-point. It yields small, glistening, monoclinic prisms,

a:b:c = 1.0442:1:0.7871; β = 109° 50'.

On heating at 100° C. it loses 2 molecules of water. The electrical conductivity of solutions of this salt has been investigated. At 25° C. the equivalent conductivity of a N/128 solution increased from 95.1 to 109.0 reciprocal ohms in six days. The rate of increase was greater when the temperature was raised and was complete in about three hours at 80° C. The following table shows the equivalent conductivities at 25° C. of solutions containing 1/10 of a molecular weight in v litres, (a) when freshly made, (b) after heating at 80° C. for three hours and then allowing to cool:

v =	32	64	128	256	512	1024
(a)Λ =	62.0	71.7	83.7	93.3	103.9	120.2
(b)Λ =	102	112	121	131	139	148

A hydrate, 5(NH₄)₂O.12WO₃.6H₂O, has been obtained by the addition of acetic acid to a solution of ammonium tungstate.

The acid salt, 2(NH₄)₂O.5WO₃.5H₂O, is produced by cooling a hot saturated solution of ammonium paratungstate, when it separates in small octahedral plates with crystallographic elements:

a:b:c = 1.1204:1:1.7190; α = 105° 46', β = 95° 17', γ = 90° 1'.

It dissolves in 26 to 29 parts of water. On heating at 100° C. it loses about seven-tenths of its water of crystallisation.

The octatungstate, 3(NH₄)₂O.8WO₃.8H₂O, is sometimes obtained when a solution of the preceding salt is allowed to crystallise; but from a hot solution, the trihydrate, 2(NH₄)₂O.5WO₃.3H₂O, has been obtained.

A colloidal tungstate, of composition (NH₄)₂O.6WO₃.4 or 6H₂O, results as a vitreous mass when ammonium para- or meta-tungstate is heated to 250° C. It is miscible with water in all proportions and absorbs ammonia readily, thus reverting to the meta-salt.

The following double ammonium sodium tungstates have been prepared: 3(NH₄)₂O.3Na₂O.16WO₃.22H₂O; 16(NH₄)₂O.4Na₂O.50WO₃.50H₂O; 6(NH₄)₂O.5Na₂O.24WO₃.27H₂O.

The compound, WO₃.3NH₃, probably ammonium imidotungstate, NH:WO(O.NH₄)₂, is obtained by heating the oxychloride, WO₂Cl₂, with liquid ammonia under pressure.

A tungstate of hydroxylamine, 4NH₂OH.3WO₃.3H₂O, is produced when hydroxylamine hydrochloride is added to a solution of sodium tungstate; it appears as a white precipitate which turns pale yellow on drying.

Hydroxylamine ammonium tungstate, NH₄O.WO₄.NH₄, is prepared by treating finely powdered sodium paratungstate with an aqueous solution of hydroxylamine hydrochloride, and then dissolving the product in 15 per cent, ammonia. It crystallises in black tablets on evaporating the solution over potassium hydroxide. It is a strong reducer, and immediately reduces ammoniacal silver nitrate and Fehling's solution.

The following ammonio-tungstates have been described:

Ag₂WO₄.4NH₃;
CuWO₄.2NH₃.H₂O;
CuWO₄.4NH₃;
CuO.4WO₃.6NH₃.8H₂O;
ZnWO₄.4NH₃.3H₂O.