Iridium Dioxide, IrO₂

Iridium Dioxide, IrO₂, is obtained when potassium iridate, IrO₃.2K₂O.H₂O, is heated with excess of potassium bromide and chloride. The melt is extracted with water, the insoluble residue treated with aqua regia to dissolve any unchanged iridate, and finally dried at 100° C. The product thus obtained is pure, crystalline iridium dioxide.

The same oxide results, but in an amorphous condition, when finely divided iridium is heated in a current of air or oxygen to bright redness. For complete oxidation the reaction must be continued for many hours, until a constant weight has been reached, the metal becoming black. The optimum temperature for this reaction is 1070° C.

Finally the anhydrous dioxide is obtained by heating the hydrated oxide to 760° C.

When heated, iridium dioxide decomposes into oxygen and the metal. The dioxide and metal are mutually soluble to a certain extent, so that after a certain amount of decomposition has occurred the solid phase consists of two saturated solutions, and yields a constant dissociation pressure.

Iridium dioxide is readily reduced to the metal by heating in hydrogen.

Hydrated Iridium Dioxide, IrO₂.2H₂O. results:

1. when excess of caustic alkali is added to a solution of chloriridate or of iridium tetrachloride. Any sesquioxide simultaneously produced passes into solution, but is converted into the dioxide by boiling in air, by addition of hydrogen peroxide, or by passing a current of oxgen through the solution.
2. On addition of alkali in the presence of air to a double sulphate of iridium.
3. On boiling for several hours crystals of potassium iridate in a solution of ammonium chloride. The crystals are gradually converted into a flocculent precipitate of hydrated dioxide, ammonia being simultaneously evolved. A similar precipitate is obtained on boiling the blue solution formed during the preparation of potassium iridate (vide infra) with excess of ammonium chloride.
   
   The precipitated oxide can be obtained in a very pure state in this maimer, by drying in carbon dioxide at 400° C., then boiling with alkali once more, and finally with sulphuric acid.
4. On heating to redness for two or three hours a mixture of finely divided iridium and sodium hydroxide and nitrate. The mass is extracted with water, leaving the hydrated oxide as residue. Probably a sodium iridate, analogous to the potassium salt, is formed, which, being unstable, is decomposed during the extraction with water.

The colour of iridium dioxide depends to a large extent upon the hydration. Dried over concentrated sulphuric acid it contains two molecules of water and is black in colour. When heated to 760° C. it becomes anhydrous.

Colloidal Iridium Dioxide is obtained by mixing solutions of potassium hydroxide and chloriridate in the cold, the solution gradually becoming violet in colour, and finally the violet dioxide separates out.

When boiled, the violet solution becomes blue, attributable, perhaps, to aggregation of colloidal particles.

Iridium dioxide also exists in colloidal form in solutions obtained by dissolving the amorphous oxide in hydrochloric acid.

The monohydrate, IrO₂.H₂O, does not appear to exist, but the potassium salts, 6IrO₂.K₂O and 12IrO₂.K₂O, are claimed to have been isolated.