Zinc Cyanide, Zn(CN)₂

Zinc Cyanide, Zn(ON)₂, is prepared by passing hydrogen cyanide into a solution of zinc acetate, or by precipitating solutions of zinc salts with solutions of alkali cyanides.

It has been obtained in a reasonably pure condition by precipitation with carbonate-free alkali cyanide, washing the precipitate, dissolving in excess of the cyanide solution, filtering, and treating with a slight excess of sulphuric acid over the quantity required to decompose the alkali cyanide and liberate Zn(CN)₂.

As ordinarily prepared it is an amorphous white powder, though it can be obtained in orthorhombic prisms by arranging water, zinc acetate solution, and a solution of hydrogen cyanide in layers, ordered according to their density, in a tall jar.

It is insoluble in water, though it is gradually hydrolysed, insoluble in hydrocyanic acid, and perceptibly soluble in a solution of zinc acetate. It decomposes on heating and ammonia is evolved when it stands in the moist state. Carbon dioxide slowly decomposes it in the presence of water, and dilute acids dissolve it - readily when amorphous and slowly when crystalline.

Its heat of formation from solid zinc and gaseous cyanogen is 29.3 Cal., and from its elements –8.8 Cal.

Prismatic crystals of Zn(CN)₂.2NH₄.H₂O, and transparent crystals of Zn(CN)₂.2NH₄, have been obtained by the action of ammonia upon zinc cyanide in the presence of water and alcohol respectively. Both substances are very soluble in alcohol or water, and are unstable.

Precipitated zinc cyanide dissolves readily in solutions of alkali cyanides, and zinc sulphide is precipitated from these solutions by alkali sulphides. When zinc cyanide dissolves in sodium cyanide, the salt Na₂Zn(CN)₄ is present in the solution. The same salt occurs in solutions obtained by acting with sodium cyanide solution on zinc oxide, or with aqueous caustic soda on excess of zinc cyanide. From solutions of sodium zinc cyanide that are reasonably free from less soluble salts and free alkali, the trihydrate, Na₂Zn(CN)₄.3H₂O, can be crystallised in efflorescent orthorhombic crystals. This salt dissolves in less than half its own weight of water and is stable in air up to 105° C.

Solutions of potassium zinc cyanide can be analogously prepared, and contain K₂Zn(CN)₄ in solution. The anhydrous salt can be crystallised from them in regular octahedra. They can be dried at 110° C. and withstand a temperature short of fusion. 100 c.c. of water at 20° C. dissolve 11 grm. of the salt, though there is a tendency to supersaturation. The solubility increases with the temperature. Carbon dioxide has almost no effect on the aqueous solution. The salt is nearly insoluble in alcohol and completely so in the other ordinary organic solvents.