Rhodium Sulphydrate, Rh₂S₃.3H₂S

When hydrogen sulphide is passed in excess into a solution of a rhodium salt, the sulphydrate, Rh₂S₃.3H₂S, is formed as a black precipitate, slowly in the cold, rapidly on boiling. In the cold complete precipitation, particularly in the presence of excess of acid, may take several months. But given sufficient time for precipitation, whether in the cold or on boiling, the reaction is quantitative and the liquid becomes colourless. If only the theoretical quantity of hydrogen sulphide is added to precipitate the rhodium in solution, the same sulphydrate appears first to form, and to decompose, very slowly in the cold, more rapidly on prolonged boiling, yielding the normal sesquisulphide, the hydrogen sulphide set free during decomposition effecting the precipitation of the remaining rhodium.

The same series of reactions appears to take place when the rhodium salt is in excess.