Potassium Hexachlor-rhodite, K₃RhCl₆

Potassium Hexachlor-rhodite, K₃RhCl₆.Aq., was stated by Claus to be formed on dissolving the hydrated sesquioxide in hydrochloric acid and adding a concentrated solution of potassium chloride. The yellow., acid solution becomes gradually red in colour, and in the course of a few weeks dark red, efflorescent crystals separate out. These were found to contain six molecules of water, of which three were readily lost on exposure to air.

Leidie was not able to confirm the above, but succeeded in isolating what he believed to be the trihydrate, K₃RhCl₆.3H₂O, as the result of decomposing potassium rhodium nitrite with warm hydrochloric acid, but which Duff our regards as the monohydrate, K₃RhCl₆.H₂O, isomorphous with the iridium analogue, K₃IrCl₆.H₂O.

Potassium chlor-rhodite is decomposed by water into potassium chloride and the double salt RhCl₃.2KCl, which is anhydrous and crystallises readily from aqueous solution. Owing to this ready decomposition, potassium chlor-rhodite cannot be prepared by simply mixing potassium and rhodium chlorides in solution.

When heated in dry hydrogen chloride gas, the water of crystallisation is liberated without the formation of rhodium sesquioxide, and the salt is transformed, slowly at 360° C., but rapidly at 440° C., into insoluble rhodium trichloride and potassium chloride. The latter may be dissolved away with hot water.