Tungstocyanic Acid, H₄[W(CN)₈]

Tungstocyanic Acid, H₄[W(CN)₈].6H₂O, may be prepared by treating the silver salt with a slight excess of cold dilute hydrochloric acid, and saturating the clear filtrate with hydrogen chloride below 0° C. The acid separates in yellow needles, and may be dried first in carbon dioxide and then in a desiccator over potassium hydroxide. It may also be obtained by the action of concentrated acid in the cold on the potassium salt. The product, which is soluble in water and in alcohol, is a strong acid and readily decomposes carbonates. The crystals deliquesce in moist air; if dried over sulphuric acid, they lose about three-fourths of their water content. When heated above 60° C. they decompose.

The salts of this acid are analogous to the corresponding molybdo-cyanides. Those of the alkali and alkaline earth metals are soluble in water, those of the heavier metals are insoluble, and all are insoluble in ordinary organic solvents. Many of the cyanides - for example, those of cadmium, copper, nickel, silver, and zinc - dissolve in concentrated ammonia solutions and yield, on recrystallisation, metal ammine derivatives. The tungstocyanides, which are generally yellow in colour, yield yellow neutral solutions which are generally stable and are unaffected by dilute acids or alkalies. Hot dilute nitric acid causes slight oxidation, whilst hot concentrated nitric or sulphuric acid causes complete decomposition. In dilute sulphuric acid solution the compounds of this series are oxidised by potassium permanganate in such a way that for each tungsten atom present one half atom of oxygen is used up. This fact appeared to contradict the presence of tetravalent tungsten, since in the usual course of oxidation, i.e. in passing from W^IV to W^VI, one atom of tungsten requires one atom of oxygen. However, investigation confirmed the tetravalency of tungsten, the product of oxidation being not tungstic acid, but a complex cyanide, containing pentavalent tungsten, of the type R^•₃[W(CN)₈]. The transformation of the anion may therefore be expressed

[W^IV(CN)₈]'''' → [W^V(CN)₈]'''

The valency of the tungsten in both these series of compounds is further confirmed by determining the amount of silver deposited on heating them for ten to twelve hours at 140° to 160° C. with ammoniacal silver nitrate in a sealed tube. The reaction takes place according to the equation

W^VI-x + nAg^• = W^VI + xAg + (n-x)Ag^•