Iridium Tetrachloride, IrCl₄

Iridium Tetrachloride, IrCl₄, has been prepared in a variety of ways, notably:

1. By evaporating ammonium chloriridate, (NH₄)₂IrCl₆, with aqua regia or chlorine water, whereby the ammonia is expelled.
2. By dissolving iridium black or the dioxide in hydrochloric acid. The solution is concentrated and any trichloride converted into tetrachloride by addition of aqua regia. The product is dried in vacuo, whereby a brownish black amorphous residue is obtained which is very hygroscopic and soluble in water.
3. Iridium tetrachloride may also be obtained by direct union of the elements, the chlorine being introduced under high pressure. Thus at 60° C., in the presence of liquid chlorine, under a pressure of about 20 atmospheres, iridium was gradually converted into the tetrachloride in five days; whilst a year was required at 15° C. under a pressure of 8 atmospheres.
   
   Iridium tetrachloride has been obtained crystallised in the form of tetrahedra, but in this form it contains water, which is expelled, together with hydrogen chloride, on heating. At higher temperatures metallic iridium alone is left as residue.
   
   Iridium tetrachloride is readily reduced to the trichloride. Its aqueous solution, on dilution, yields hypochlorous acid and the trichloride. On boiling, a precipitate of oxychloride is obtained. Addition of excess of alkali precipitates part of the iridium as dioxide, the remainder staying in solution as sesquioxide, being precipitated only upon neutralisation of the alkali. Addition of alcohol to the alkaline solution precipitates metallic iridium, aldehydes and alkali formates being simultaneously produced. Reducing agents, such as stannous chloride, sulphur dioxide, nitric oxide, hydrogen sulphide, ferrous sulphate, etc., convert the tetrachloride into trichloride.