Sodium Rhodium Sulphate, Na₂SO₄.Rh₂(SO₄)₃

Sodium Rhodium Sulphate, Na₂SO₄.Rh₂(SO₄)₃, was first prepared by Bunsen by heating sodium rhodium sulphite with concentrated sulphuric acid. This salt was regarded by Seubert and Kobbe as an anhydrous rhodium alum. Since then, however, true rhodium alums have been obtained.

Rhodium sulphate, like its analogues the sulphates of cobalt and iridium, yields stable salts with sulphates of the alkali metals known as alums. These are well-defined crystalline salts, isomorphous with the better known iron and aluminium alums. They thus form an interesting link between these metals and the central vertical column in Group VIII, of which rhodium is the middle member. These alums are obtained when a solution of rhodium sesquioxide in sulphuric acid is added to an alkali sulphate and allowed to crystallise. It is essential that excess of acid be present, and not more than about two-thirds of the theoretical quantity of alkali sulphate. The temperature must not be allowed to rise too high.