Rhodium Alums, M₂SO₄.Rh₂(SO₄)₃.24H₂O

Caesium Rhodium Alum, Cs₂SO₄.Rh₂(SO₄)₃.24H₂O, is the most readily prepared of all the rhodium alums on account of its sparing solubility in cold water. The salt crystallises in small yellow octahedra which melt at 110° to 111° C. to a yellowish red liquid. When warmed in a desiccator it loses water, remaining yellow at 100° C., becoming yellowish red at 150° to 180° C., and brown up to 250° C., when it is almost entirely anhydrous.

This alum is of interest inasmuch as its formation renders it easy to separate rhodium from iridium. The sulphates of the metals, dissolved in acidulated water, are treated with caesium sulphate and evaporated. The rhodium alum crystallises out in a pure state, entirely free from iridium.

Rubidium Rhodium Alum, Rb₂SO₄.Rh₂(SO₄)₃.24H₂O, yields stable yellow crystals which melt at 108° to 109° C. to a bright red liquid. The crystals belong to the regular system and exhibit a conchoidal fracture.

Potassium Rhodium Alum, K₂SO₄.Rh₂(SO₄)₃.24H₂O, can only be obtained in a crystalline state by allowing the solution to stand for a prolonged period at a temperature not exceeding 5° C. At higher temperatures a syrupy, uncrystallisable liquid results.

The crystals are light brown in colour, stable in air, but exceedingly soluble in water.

Ammonium Rhodium Alum, (NH₄)₂SO₄.Rh₂(SO₄)₃.24H₂O, yields orange-yellow crystals, readily soluble in water, although less so than the potassium salt. It readily yields large crystals. These are stable in air, melting at 102° to 108° C. to a brownish red liquid. When heated to redness, ammonia and sulphuric acid are expelled, leaving a residue of rhodium, contaminated with sulphate. The last named is not completely removed, even on reheating with ammonium carbonate.

Thallium Rhodium Alum, Tl₂SO₄.Rh₂(SO₄)₃.24H₂O, is somewhat difficult to prepare, partly on account of the small solubility of thallous sulphate. The alum is very soluble in water, and not altogether permanent in air, becoming converted into a whitish powder on prolonged exposure.