Potassium Pentachlor-rhodite, K₂RhCl₅

Potassium Pentachlor-rhodite, K₂RhCl₅, is obtained in the anhydrous condition when finely divided rhodium is heated to redness with potassium chloride in a current of chlorine (Berzelius).

The mass is extracted with water and crystallised. Berzelius suggested the formula RhCl₃. 2KCl. 2H₂O, but Leidie was unable to confirm the presence of water. It seems probable, therefore, that Berzelius had obtained a mixture of salts.

Another method of preparing this salt consists in mixing solutions of rhodium trichloride and an excess of potassium chloride; on allowing to crystallise, the anhydrous salt, K₂RhCl₅, separates out in ortho- rhombic crystals, of reddish colour, but slightly soluble in water and insoluble in alcohol.

The dihydrate of this salt, K₂RhCl₅.2H₂O, was stated by Claus1 to be formed on adding potassium chloride to a solution of sodium chlor-rhodite. It crystallises in small brown prisms which do not effloresce, and are rather difficultly soluble in water.

The monohydrate, K₂RhCl₅.H₂O, is obtained on fusing spongy rhodium with twice its weight of potassium chloride in a stream of chlorine. On cooling, the mass is extracted with water, and the solution, evaporated under reduced pressure, deposits deep red crystals of the monohydrated salt. If the mother-liquors are now saturated with hydrogen chloride to precipitate the excess of potassium chloride, and evaporated, dark red, sparingly soluble crystals of potassium chlor-rhodite, K₃RhCl₆.3H₂O, are obtained.