Iridium Sesquioxide, Ir₂O₃

Iridium Sesquioxide, Ir₂O₃, is obtained by igniting iridium sulphite, or by heating an alkali chloriridate with twice its weight of carbonate. The product is washed with hot water, and the colloidal residue treated with a solution of ammonium chloride to effect its coagulation, and finally with nitric acid and dried, the acid serving to remove the last traces of alkali.

In an impure form iridium sesquioxide is obtained by mixing hot solutions of potassium hydroxide and sodium chloriridite in an atmosphere of carbon dioxide. The solution is evaporated to dryness, and the residue heated to redness in a current of carbon dioxide. Further purification is effected by boiling first with sodium hydroxide solution and then with sulphuric acid.

When heated above 400° C. iridium sesquioxide decomposes, yielding metallic iridium and its dioxide, a little oxygen being likewise liberated. It dissolves in hydrochloric acid, yielding a colloidal solution.

Hydrated Iridium Sesquioxide, Ir₂O₃.5H₂O, is obtained as an olive- green precipitate on addition of the requisite quantity of potassium hydroxide solution to one of chloriridite. If excess of potash is taken no precipitate appears, since the sesquioxide dissolves as rapidly as formed, yielding an olive-green solution, from which, however, it is precipitated on addition of acid.

The precipitated oxide readily oxidises in air. It dissolves in acids to a yellowish green solution. The alkaline solutions readily oxidise on boiling in air or on addition of hydrogen peroxide, the dioxide being precipitated (vide infra). Boiling them with alcohol causes reduction to iridium black.