Iridium (symbol Ir.; atomic weight 193.1), one of the metals of the platinum group, discovered in 1802 by Smithson Tennant during the examination of the residue left when platinum ores are dissolved in aqua regia; the element occurs in platinum ores in the form of alloys of platinum and iridium, and of osmium and iridium. Many methods have been devised for the separation of these metals (see Platinum), one of the best being that of H. St. C Deville and H. J. Debray (Comptes rendus, 1874, 78, p. 1502). In this process the osmiridium is fused with zinc and the excess of zinc evaporated; the residue is then ignited with barium nitrate, extracted with water and boiled with nitric acid. The iridium is then precipitated from the solution (as oxide) by the addition of baryta, dissolved in aqua regia, and precipitated as iridium ammonium chloride by the addition of ammonium chloride. The double chloride is fused with nitre, the melt extracted with water and the residue fused with lead, the excess of lead being finally removed by solution in nitric acid and aqua regia. It is a brittle metal of specific gravity 22.4 (Deville and Debray), and is only fusible with great difficulty. It may be obtained in the spongy form by igniting iridium ammonium chloride, and this variety of the metal readily oxidizes when heated in air.
Two oxides of iridium are known, namely the sesquioxide, Ir2O3, and the dioxide, IrO2, corresponding to which there are two series of salts, the sesqui-salts and the iridic salts; a third series of salts is also known (the iridious salts) derived from an oxide IrO. Iridium sesquioxide, Ir2O3, is obtained when potassium iridium chloride is heated with sodium or potassium carbonates, in a stream of carbon dioxide. It is a bluish-black powder which at high temperatures decomposes into the metal, dioxide and oxygen. The hydroxide, Ir(OH)3, may be obtained by the addition of caustic potash to iridium sodium chloride, the mixture being then heated with alcohol. Iridium dioxide, IrO2, may be obtained as small needles by heating the metal to bright redness in a current of oxygen (G. Geisenheimer, Comptes rendus, 1890, 110, p. 855). The corresponding hydroxide, Ir(OH)4, is formed when potassium iridate is boiled with ammonium chloride, or when the tetrachloride is boiled with caustic potash or sodium carbonate. It is an indigo-blue powder, soluble in hydrochloric acid, but insoluble in dilute nitric and sulphuric acids. On the oxides see L. Wöhler and W. Witzmann, Zeit. anorg. Chem. (1908), 57, p. 323. Iridium sesquichloride, IrCl3, is obtained when one of the corresponding double chlorides is heated with concentrated sulphuric acid, the mixture being then thrown into water. It is thus obtained as an olive green precipitate which is insoluble in acids and alkalis. Potassium iridium sesquichloride, K3IrCl6·3H2O, is obtained by passing sulphur dioxide into a suspension of potassium chloriridate in water until all dissolves, and then adding potassium carbonate to the solution (C. Claus, Jour. prak. Chem., 1847, 42, p. 351). It forms green prisms which are readily soluble in water. Similar sodium and ammonium compounds are known. Iridium tetrachloride, IrCl4, is obtained by dissolving the finely divided metal in aqua regia; by dissolving the hydroxide in hydrochloric acid; and by digesting the hydrated sesquichloride with nitric acid. On evaporating the solution (not above 40° C.) a dark mass is obtained, which contains a little sesquichloride. It forms double chlorides with the alkaline chlorides. For a bromide see A. Gautbier and M. Riess, Ber., 1909, 42, p. 3905. Iridium sulphide, IrS, is obtained when the metal is ignited in sulphur vapour. The sesquisulphide, Ir2S3, is obtained as a brown precipitate when sulphuretted hydrogen is passed into a solution of one of the sesqui-salts. It is slightly soluble in potassium sulphide. The disulphide, IrS2, is formed when powdered iridium is heated with sulphur and an alkaline carbonate. It is a dark brown powder. Iridium forms many ammine derivatives, which are analogous to the corresponding platinum compounds (see M. Skoblikoff, Jahresb., 1852, p. 428; W. Palmer, Ber., 1889, 22, p. 15; 1890, 23, p. 3810; 1891, 24, p. 2090; Zeit. anorg. Chem., 1896, 13, p. 211).
Iridium is always determined quantitatively by conversion into the metallic state. The atomic weight of the element has been determined in various ways, C. Seubert (Ber., 1878, 11, p. 1770), by the analysis of potassium chloriridate obtaining the value 192.74, and A. Joly (Comptes rendus, 1890, 110, p. 1131) from analyses of potassium and ammonium chloriridites, the value 191.78 (O = 15.88).
