Chlor-iridates

When the reaction has subsided, nitric and sulphuric acids are added, whereby barium sulphate is precipitated. The clear solution is decanted and distilled until one-quarter of its volume has been collected as distillate. This latter liquor is rich in osmium and may be worked for that metal (see p. 208). The residue is evaporated to small bulk. Ammonium chloride and a small quantity of nitric acid are added, and the mixture evaporated to dryness on the water-bath. A violet-black crystalline solid is obtained which is washed with water half saturated with ammonium chloride until the filtrate is colourless. The insoluble residue consists of fairly pure ammonium chlor-iridate containing ruthenium, from which the spongy metals are obtained by ignition. 

Exact Atomic Weight.—The first determination of the atomic weight of iridium is due to Berzelius.8 His single analysis of potassium chlor-iridate leads to the very high value Ir = 196-7. 

Determinations of the percentage of iridium in ammonium chlor-iridate were made by Hoyermann 2 in 1911 ... 

At Hanau the method adopted is similar to the foregoing. The crude Russian platinum is treated with a mixture of aqua regia (one part) and water (two parts) in a glass retort under a pressure of 12 inches of water. The resulting solution is evaporated to dryness and heated to 125° C., whereby the palladium is converted to the palladous condition. The product is extracted with water, and acidified with hydrochloric acid. On addition of ammonium chloride a precipitate of ammonium chlor-platinate is obtained, contaminated with some ammonium chlor-iridate. On concentration of the mother-liquor a further small amount of ammonium chlor-iridate separates out. The liquid is now treated with scrap-iron, which precipitates all the other platinum metals, together with any platinum and iridium not hitherto precipitated. The deposit is washed free from undissolved iron by treatment with hydrochloric acid, and in turn dissolved in aqua regia. From this... 

The residue is heated in retorts with aqua regia, whereby most of the platinum passes into solution, together with some. rhodium, osmium, palladium, and a little iridium, whilst the undissolved portion consists of osmiridium, ruthenium, and a little platinum. The solution is distilled, the osmium passing off as the volatile tetroxide, and the mother-liquor then neutralised with sodium carbonate. Addition of mercuric cyanide throws out the palladium as cyanide. Ammonium chloride added to the filtrate effects the precipitation of ammonium chlor-platinate and chlor-iridate. Ignition of these salts yields the spongy metal, which may be worked up as already indicated into the compact form. 

The salts are wanned gently with a solution of potassium cyanide. The ammonium chlor-iridate is reduced to chlor-iridite, which passes into solution, leaving the chlor-platinate as an insoluble residue.2... 

The chlor-platinates are mostly of a yellow colour, the insoluble salts, namely, those of potassium, rubidium, caesium, ammonium, and silver, crystallising in the anhydrous condition in the form of cubic octahedra, isomorphous with the chlor-iridates and chlor-osmates. The more soluble chlor-platinates, namely, those of sodium, magnesium, zinc, etc., crystallise with 6 molecules of water, and are isomorphous with the chlor-stannates.1... 

Reactions of Salts of Iridium,.—Hydrogen sulphide in excess causes the precipitation of dark brown iridium sesquisulphide, Ir2S3. If the iridium is present in solution in the tetravalent condition, as for example in the form of alkali chlor-iridates, M2IrCl8 or 2MQ.IrCld, the dark solution is first decolorised and a yellowish white precipitate of sulphur forms in consequence of the reduction of the iridium salt to the trivalent condition, namely, as IrCl3. When this has been accomplished the brown sesquisulphide is obtained. 

Potassium chloride and ammonium chloride precipitate from concentrated solutions their respective chlor-iridates, K IrClg and (NH1)2IrCl6, as brownish red deposits. 

Addition of reducing agents such as ferrous sulphate or stannous chloride to solutions of the alkali chlor-iridates reduces them to double salts of iridium trichloride, namely, 3MCl.IrCls or M3IrCl6, known generally as chlor-iridites. The solution is simultaneously decolorised, and the salts crystallise out on cooling. 

Estimation.—Iridium is conveniently estimated by igniting ammonium chlor-iridate in hydrogen, and weighing as metal. To this end, solutions containing iridium salts are evaporated with aqua regia, ammonium chloride added, and the chlor-iridate, (NH4)2IrCl6, collected and ignited. 

---