Potassium Platinocyanide, K₂Pt(CN)₄

Potassium Platinocyanide or Gmelin's Salt, K₂Pt(CN)₄.3H₂O, was the first platinocyanide to be discovered, namely, by Ittner, who allowed mixed solutions of potassium cyanide and platinic chloride to crystallise. Ittner, however, does not appear to have understood the nature of the compound he had prepared. Gmelin, who discovered the ferricyanides, observed that by heating to redness a mixture of platinum sponge and potassium ferrocyanide, a compound results in which platinum replaces iron. Analysis of the product crystallised from water showed it to have the formula K₂Pt(CN)₄.3H₂O, when translated into modern symbols. Another method consists in mixing ammonium chlor-platinite with water, adding caustic potash and raising to 100° C. Finally, saturated potassium cyanide solution is added. Ammonia is evolved, and potassium platinocyanide passes into solution:

2KOH + (NH₄)₂PtCl₄ + 4KCN = 2NH₃ + 4KCl + K₂Pt(CN)₄ + 2H₂O.

After prolonged boiling to remove all the ammonia the solution is allowed to crystallise, when the salt separates out in long rhombic needles, density 2.4548 at 16° C. Upon exposure to air the crystals effloresce, and at 120° C. lose three molecules of water, becoming pale golden yellow. The crushed crystals are sulphur-yellow in colour, but the crystalline faces exhibit a blue fluorescence. When dehydrated at ordinary temperatures the colour becomes reddish yellow. On heating, the salt becomes white, then yellow, and finally melts at 400-600° C. It is soluble in cold water, easily so in hot, as also in alcohol and ether. Dilute mineral acids do not affect it, but concentrated sulphuric acid effects the precipitation of platinous cyanide.

Wilm drew attention to the fact that the properties of potassium platinocyanide vary somewhat according to the method of preparation. When white, fluorescent crystals of the salt are boiled with hydrogen peroxide (20 volume solution) the crystals remaining after evaporation are pale yellowish green in colour, but devoid of fluorescence. They constitute, in fact, an isomeride, into which the white, fluorescent salt has been converted, probably through the catalytic agency of some impurity in the peroxide. This catalytic theory is supported by the fact that by boiling the white, fluorescent crystals with pure perhydrol, no change takes place.

Contrary to the statements of early investigators, oxidation of potassium platinocyanide with the halogens, nitric acid or lead peroxide, does not yield potassium platinicyanide. The halogens convert it into a dihalogenide, of general formula K₂Pt(CN)₄.X₂. If, however, the solution of potassium platinocyanide is treated with hydrogen peroxide (20-volume solution) acidulated with dilute sulphuric acid, potassium platinicyanide, KPt(CN)₄, is obtained united with three molecules of platinocyanide, thus: KPt(CN)₄.3K₂Pt(CN)₄.6H₂O.

The crystallographic elements of the hydrated salt are: