Rhodium(III) sulfate is an inorganic compound with the chemical formula Rh<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>. The anhydrous form is a red crystalline solid. Several hydrates and related compounds have been characterized.
The anhydrous compound (Rh<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>) forms red, plate-shaped trigonal crystals. The dihydrate (Rh<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>÷2H<sub>2</sub>O) forms orange orthorhombic crystals.
Compounds with the following formulas have been also studied: , , , and . The latter three were characterized by single-crystal X-ray diffraction and found to form monoclinic crystals. All phases are sparingly soluble in ethanol and well soluble in water.
Anhydrous rhodium(III) sulfate can be prepared by heating rhodium metal in concentrated sulfuric acid at 400 ðC. It is reported that at a temperature of 475 ðC the dihydrate is formed instead.
The first attempt to produce rhodium(III) sulfate was in 1929 with the reaction of rhodium(III) hydroxide and sulfuric acid. Two hydrates were reported, a yellow crystalline pentadecahydrate and a red amorphous tetrahydrate. This was not confirmed due to a lack of structural proof.