Estimation of Iridium

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.

Separation of Iridium and Platinum. - Quennessen recommends dissolving the alloy in a mixture of one part by volume of nitric acid (density 1.32) with two parts of hydrochloric acid (density 1.18) and evaporating to dryness. The residue is heated to 120° C., dissolved in water and precipitated with magnesium. The deposited metals are ignited at dull redness to render the iridium insoluble, treated first with dilute sulphuric acid to extract the magnesium, and then with diluted aqua regia to extract the platinum, leaving the iridium as an insoluble residue.

Another method consists in melting the alloy of platinum and iridium with ten times its weight of lead in a carbon crucible, and repeatedly digesting the button thus obtained with hot, dilute nitric acid. The insoluble residue is then digested with aqua regia, diluted with water, and the residue of iridium washed, ignited and weighed.

The iridium content of commercial platinum may be rapidly determined colorimetrically as follows: The metal is dissolved in aqua regia, evaporated to dryness and taken up with concentrated hydrochloric acid. The colour of the solution thus obtained is compared with that of a standard solution of platinum and iridium in the same solvent.

Osmiridium, in a finely divided condition, is intimately mixed with four times its weight of sodium peroxide and added to fused sodium hydroxide in a nickel crucible. On treating the product with water and washing the residue with sodium hypochlorite solution, a solution is obtained which holds all the osmium and ruthenium as alkali osmate and ruthenate respectively, and some of the iridium as iridate.

The solution is distilled with chlorine at 70° C., the vaporised tetroxides of osmium and ruthenium being collected in dilute hydrochloric acid, and the metals may be separated and estimated as explained on page 338. The liquid in the distilling-flask must be kept alkaline to prevent iridium also passing over as chloride with the osmium and ruthenium. The residual solution containing the iridium is acidified with hydrochloric acid, the residue from the fusion dissolved with it, and the iridium estimated as indicated above.