Iridium Trichloride, IrCl₃

Iridium Trichloride, IrCl₃, is most conveniently prepared by heating the reduced metal - obtained by precipitation from an alkali chloriridate solution with magnesium - with sodium chloride in a current of chlorine at 600° to 620° C. The reaction is complete in a few minutes. The limits of stability of the salt under these conditions lie from below 100° C. up to 763° C. Above this latter temperature it dissociates in an atmosphere of chlorine into iridium dichloride and chlorine, and at still higher temperatures (773° C.) into the mono-chloride.

Iridium trichloride is also obtained by heating ammonium chloriridate or iridium tetrachloride in a current of chlorine at 440° C. and cooling the product in an atmosphere of carbon dioxide.

As obtained by the foregoing methods, iridium trichloride is a crystalline substance, olive-green in colour, of density 5.30. It is insoluble in water, alkalies, and acids; even concentrated sulphuric acid and aqua regia having no action upon it. It is appreciably volatile at 470° C.

An almost anhydrous salt is obtained by heating the yellow powder, produced by the action of sulphuric acid upon alkali chloriridites, in hydrogen chloride at temperatures up to 500° C. The colour remains essentially the same, and the composition of the product approaches very closely to that of iridium trichloride, with small quantities of hydrochloric acid and water, the formula being IrCl₃.mHCl.nH₂O, where m and n, however, are but small fractions of unity.

The product is different from ordinary iridium trichloride, however, in that it is deliquescent and soluble in water. On concentrating the solution and drying the product at 100° C. a sesquihydrate is obtained, 2IrCl₃.3H₂O. This dissolves in water, yielding a greenish yellow solution. Silver nitrate gives, with this solution, a precipitate, which upon drying at 110° C. has the composition IrCl₃.AgOH.

The salt described in earlier literature as the tetrahydrate, IrCl₃.4H₂O, appears to be an indefinite mixture of the trichloride with water and hydrogen chloride, as represented by the formula IrCl₃.mHCl.nH₂O, where m is less than unity.