Tungsticyanic Acid, H₃[W(CN)₈]

Tungsticyanic Acid, H₃[W(CN)₈].6H₂O, is obtained from its silver salt by the action of hydrochloric acid. It forms orange-yellow crystals which over sulphuric acid turn reddish brown, with loss of water. It is unstable, and decomposes in daylight and in moist air. As has already been mentioned (see p. 260), the compounds of this series are not oxidised by potassium permanganate, although the tungsten is only pentavalent, as is shown by the liberation of silver from ammoniacal silver nitrate.

The potassium salt, 2K₃[W(CN)₈].5H₂O, is obtained by oxidising a solution of the tungstocyanide, K₄[W(CN)₈].2H₂O, with potassium permanganate, adding silver nitrate, which precipitates the red silver tungsticyanide, and treating this with a solution of potassium chloride. The silver chloride is removed by filtration, and the solution on evaporation yields light yellow crystals of the potassium salt. The crystals are fairly stable in dry air, but undergo decomposition when exposed to moist air in daylight. The salt readily forms additive products; for example, with potassium chloride it yields red slender prisms of composition K₃[W(CN)₈].KCl.5H₂O.

The following salts have also been obtained by the action of the metallic chlorides on the silver salt:

Na₃[W(CN)₈].4H₂O,
Rb₃[W(CN)₈].2H₂O,
Cs₃[W(CN)₈].2H₂O,
Ba₃[W(CN)₈]₂.10H₂O,
Sr₃[W(CN)₈]₂.11H₂O.

Thiocyanates of tungsten have not been prepared. Complex salts containing pyridine and of composition (C₅H₅NH)₂.WO(SCN)₅.xH₂O, (C₅H₅N)₂.W(OH)₂(SCN)₃, and 2(C₅H₅N.HCNS).WO₂(SCN) have been isolated.

Various complex compounds of tungstic acid with organic acids, such as oxalic, tartaric, citric, malic, and lactic acids, have been described.