Hydrated Rhodium Dioxide, Rh(OH)₄

Hydrated Rhodium Dioxide, RhO₂.2H₂O or Rh(OH)₄, has been obtained in a variety of ways.

1. By passing chlorine into a solution of the freshly precipitated sesquioxide in concentrated potassium hydroxide solution. The reaction between the chlorine and potash causes the temperature to rise and the hydrated sesquioxide is precipitated, only to be oxidised by the hypochlorite in solution to the dioxide.
   
   2KOH + Cl₂ = KCl + KClO + H₂O
   KClO + Rh₂O₃ = KCl + 2RhO₂.
   
   The precipitate is green, whilst the solution assumes a deep violet- blue colour, probably in consequence of the production of an alkali rhodate.
2. Rhodium salts may be oxidised direct with alkali hypochlorites in alkaline solution to the dioxide. This has been effected with sodium chlor-rhodite, and with rhodium chloropurpureochloride.
3. A green anodic deposit of dioxide is formed by electrolysing a double oxalate of rhodium and an alkali metal, or a double chloride in the presence of oxalic acid.

The dioxide is insoluble in water, but dissolves in hydrochloric acid, yielding free chlorine and rhodium sesquichloride.

The compounds K₂O.6RhO₂, Na₂O.8RhO₂, and BaO.12RhO₂ are obtained on heating to incipient redness the corresponding double nitrites. These are analogous to the cobaltites, chromites, and man- ganites, and prove that rhodium dioxide can exert an acidic action.