Platinum Arsenides

Platinum and arsenic combine readily on heating; Gehlen observed deflagration when the spongy metal was heated with excess of arsenic. The system Pt-As, up to about 28 per cent, of arsenic, has been studied. There is a eutectic containing 13 per cent, of arsenic solidifying at 597° C., and from the times of eutectic solidification indications of the probable existence of the arsenide Pt₂As₃ were obtained. The formation of an arsenide of composition Pt₃As₂ was described by Tivoli who, by the action of arsine on aqueous platinic chloride, obtained black crystalline scales which he considered to be a hydroxy arsenide, PtAs(OH). This substance could be dried at 130° C. without decomposition, but above that temperature it yielded the arsenide in accordance with the equation:

6PtAs(OH) = As₂O₃ + 2Pt₃As₂ + 3H₂O

Wohler, however, has shown that the product of the interaction of arsine and an aqueous solution of platinic chloride or potassium chlor-platinite is a mixture of variable composition, and he was unable to obtain the hydroxyarsenide.

Platinum Di-arsenide, PtAs₂, occurs in Nature as the rare mineral sperrylite, and may be prepared in the laboratory by heating a mixture of the elements in a sealed tube at 270° C. The reaction is explosive, but can be moderated by using a large excess of arsenic, which is afterwards removed in a stream of carbon dioxide at 500° C. The residual grey powder resembles natural sperrylite, and is attacked with difficulty by concentrated nitric acid or aqua regia. The di-arsenide has also been obtained in a pure form by heating platinic chloride with an excess of arsenic in a current of hydrogen. The crystal structure has been investigated by the powder method; a = 5.957 ± 0.003 A.