Hydrated Rhodium Sesquioxide, Rh₂O₃.5H₂O

Hydrated Rhodium Sesquioxide, Rh₂O₃.5H₂O, is precipitated from solutions of rhodium salts, for example, rhodium trichloride, on addition of not too great a quantity of potassium or sodium hydroxide. On allowing to stand the red liquid deposits a rose-coloured precipitate consisting of the sesquioxide and some unaltered rhodium salt. The precipitate, however, gradually loses its red colour, becoming yellow, and remains such after thorough washing.

Obtained in the above manner rhodium sesquioxide is a yellow powder, and on being heated loses water and oxygen, yielding a black powder, which Claus regarded as the anhydrous sesquioxide, but for which Leidie's analyses suggest the empirical formula Rh₃O₄.

In the freshly precipitated condition rhodium sesquioxide dissolves in concentrated potassium hydroxide solution, from which it is precipitated again on dilution with water. If, however, chlorine is passed through the concentrated alkaline solution, a dark green precipitate of uncertain composition is formed, for which Claus suggested the formula Rh₂O₃.RhO₂ or Rh₃O₅.

The sesquioxide readily dissolves in acids, giving yellow, noncrystalline salts of herbal taste.