Barium Platinocyanide, BaPt(CN)₄

Barium Platinocyanide, BaPt(CN)₄.4H₂O, is one of the most useful platinocyanides, inasmuch as it may be conveniently used in preparing other salts of hydrogen platinocyanide. Quadrat was the first to prepare it by boiling the copper derivative with barium hydroxide solution. Schafarik obtained it by decomposing barium carbonate with free platinocyanic acid. Martius boiled platinous cyanide with barium cyanide solution and obtained the salt, but recommended the following method as superior: Pulverised platinous chloride, PtCl₂, is added little by little to a boiling solution of barium cyanide in such a manner that no fresh chloride is added until the previous amount has completely dissolved and the solution become colourless. On crystallisation the salt is obtained in a very pure condition.

By electrolysis of a solution of barium cyanide between platinum electrodes, using an alternating current, a solution of barium platino-cyanide has been obtained which, upon concentration, crystallised out in yellow, slightly dichroic crystals, exhibiting a slight fluorescence under radium.

The salt may also be obtained by passing gaseous hydrogen cyanide into a pasty mixture of two parts of platinum dichloride and three of freshly precipitated barium carbonate with ten of water, maintained at nearly the boiling-point. The passage of gas is continued until effervescence ceases, and the solution is then boiled and filtered whilst hot. On cooling, the barium platinocyanide crystallises out in golden yellow, transparent crystals, exhibiting a purple sheen. The crystals frequently exhibit red and green colours on the edges, this effect being particularly noticeable when they are immersed in water. The crystals belong to the monoclinic system, the crystallographic elements being:

a: b: с = 0.86928: 1: 0.47928. β = 103° 54'

At 100° C. the salt loses two molecules of water, and becomes anhydrous at 140°-150° C., a yellowish blue powder remaining, which readily absorbs moisture on exposure to the air. The salt readily dissolves in hot water, but is insoluble in absolute alcohol.

Barium platinocyanide tetrahydrate can exist in two isomeric forms, which, however, are crystallographically identical. They differ in colour, density, and in fluorescence, as indicated in the following table:

	α-salt	β-salt
Colour	Golden yellow	Apple-green
Density at 17° C	2.076	2.085
Fluorescence	Feeble	Intense
Stability	Less stable at the ordinary temperature	More stable at the ordinary temperature
Crystalline character	Same as β-salt	Same as α-salt

When a solution of barium platinocyanide is allowed to crystallise after adding a trace of barium cyanide or barium hydroxide to it, crystals of th,e green β-salt are obtained. If, on the other hand, a trace of hydrogen cyanide, hydrogen chloride, tartaric or acetic acid, or cyanoplatinous acid is present instead, crystals of the yellow α-salt are deposited.

The β-salt appears to be the more stable at ordinary temperatures.

The dihydrate and the anhydrous salt have also been prepared. The former is brick-red in colour and non-fluorescent. The latter is white and but feebly fluorescent.

Barium platinocyanide, after prolonged exposure to radiation from radium compounds, becomes brown or red and loses its fluorescent properties. The only way to completely restore the red crystals to their fluorescent state is by solution and recrystallisation, although gentle heating will partially restore them. This is well illustrated by the following table:

		Fluorescent Value
1	Fresh crystals	100
2	Fresh crystals after exposure to β-rays from radium for eight hours	8
3	Fresh crystals after exposure to β-rays from radium for forty-eight hours	8
4	Red crystals (No. 3 above) after heating to 110° C. in open tube	27
5	Red crystals after heating to 120° C. in sealed tube	33