Chemical elements
    Physical Properties
    Chemical Properties
      Iridium Monochloride
      Iridium Dichloride
      Iridium Trichloride
      Potassium Chloriridite
      Sodium Chloriridite
      Ammonium Chloriridite
      Aquo Chloriridites
      Iridium Tetrachloride
      Potassium Chloriridate
      Sodium Chloriridate
      Ammonium Chloriridate
      Silver Chloriridate
      Thallium Chloriridate
      Iridium Tribromide
      Iridium Tetrabromide
      Potassium Bromiridate
      Sodium Bromiridate
      Ammonium Bromiridate
      Iridium Oxybromide
      Iridium Tri-iodide
      Potassium Iodiridite
      Iridium Tetra-iodide
      Potassium Iodiridate
      Iridium Monoxide
      Iridium Sesquioxide
      Iridium Dioxide
      Iridium Trioxide
      Iridium Monosulphide
      Iridium Sesquisulphide
      Iridium Disulphide
      Iridium Sesquisulphite
      Potassium Iridium Sulphite
      Iridium Sesquisulphate
      Potassium Iridium Alum
      Ammonium Iridium Alum
      Caesium Iridium Alum
      Rubidium Iridium Alum
      Iridium Disulphate
      Iridium Sesquiselenide
      Hydrogen Iridi-nitrite
      Potassium Iridi-nitrite
      Sodium Iridi-nitrite
      Ammonium Iridi-nitrite
      Hydrogen Iridicyanide
      Potassium Iridicyanide
      Barium Iridicyanide
    PDB 1c1k-4enb

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

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

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.
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