Potassium Rhodicyanide, K₃Rh(CN)₆

Potassium Rhodicyanide, K₃Rh(CN)₆, may be obtained by heating ammonium chlor-rhodite with a slight excess of potassium cyanide to the fusion-point for ten to fifteen minutes, extraction of the melt with boiling water, and subsequent evaporation.

It may also be conveniently prepared by diluting with water a saturated solution of recently precipitated rhodium sesquioxide in 25 to 30 per cent, potash, until a precipitate begins to form, and pouring the whole into excess of a 20 to 25 per cent, solution of hydrocyanic acid. After standing some time out of contact with air, the liquid is filtered and evaporated in vacuo. The crystals separating out are isomorphous with those of the corresponding ferri-, cobalti- and iridi- cyanides. They possess similar optical properties and crystallographic angles.

When boiled with concentrated acetic acid, rhodium cyanide is precipitated, hydrogen cyanide escaping. Since the corresponding iridicyanide is not affected by similar treatment, this affords a useful method of separating rhodium from iridium.

Like the analogous ferri- and cobalti-cyanides it is decomposed by potassium hydroxide, and also by concentrated hydrochloric acid, in the latter case with evolution of hydrocyanic acid. With certain metallic salts it yields characteristic precipitates. Thus, with ferrous salts a white precipitate is obtained; with ferric, a bright yellow.