Crystalline Platinum

Moissan observed that on decomposing platinum tetrafiuoride at a high temperature the metal was left as a distinctly crystalline residue. This was indirectly confirmed by Joly, who passed an electric current through pure platinum ribbon dusted over with finely divided topaz. After being maintained at red heat for half an hour microscopic crystals of platinum could be observed clinging to partially decomposed topaz. These gradually increased in size, some reaching 0.1 mm. in the course of two hours. The crystals possessed a high metallic lustre, and belonged to the cubic system, the prevailing form being the octahedron or some modification of it. They were unattacked by hot hydrochloric, sulphuric, or nitric acid, and appeared to be unaffected by cold hydrofluoric acid, although slightly attacked by the last named when hot. Boiling aqua regia completely dissolved the crystals. It is not improbable that fluorine, liberated from the topaz at the high temperature, attacked the platinum, yielding the fluoride which in turn dissociated, leaving the residue of crystalline metal.

Crystalline platinum may also be obtained by ignition of potassium chlor-platinate, and by heating spongy platinum with cupric chloride. Small crystals have been observed in electric furnaces near to platinum electrodes, evidently due to the sublimation of the metal below its melting-point.