Iridium: the essentials
Ruthenium, rhodium, palladium, osmium, iridium, and platinum together make up a group of elements referred to as the platinum group metals (PGM).
The name iridium is appropriate, for its salts are highly coloured. Iridium is white, similar to platinum, but with a slight yellowish cast. It is very hard and brittle, making it very hard to machine, form, or work. It is the most corrosion resistant metal known, and was used in making the standard metre bar of Paris, which is a 90% platinum-10% iridium alloy. This metre bar has since been replaced as a fundamental unit of length (see Krypton).
Iridium is not attacked by any of the acids nor by aqua regia, but is attacked by molten salts, such as NaCl and NaCN.
Iridium: historical information
Osmium was discovered in 1803 by Smithson Tennant in the dark coloured residue left when crude platinum is dissolved by aqua regia (a mixture of hydrochloric and nitric acid). This dark residue contains both osmium and iridium (named after the Latin word iris meaning rainbow because of the colourful nature of iridium compounds).
Iridium: physical properties
Iridium: orbital properties
Isolation: it would not normally be necessary to make a sample of iridium in the laboratory as the metal is available, at a price, commercially. The industrial extraction of iridium is complex as the metal occurs in ores mixed with other metals such as rhodium, palladium, silver, platinum, and gold. Sometimes extraction of the precious metals such as iridium, rhodium, platinum and palladium is the main focus of a partiular industrial operation while in other cases it is a byproduct. The extraction is complex because of the other metals present and only worthwhile since iridium is useful as a specialist metal and is the basis of some catalysts in industry.
Preliminary treatment of the ore or base metal byproduct is required to remove silver, gold, palladium, and platinum. The residue is melted with sodium bisulphate (NaHSO4) and the resulting mixture extracted with water to give a solution containing rhodium sulphate, Rh2(SO4)3. The insoluble residue contains the iridium. The residue is melted with Na2O2 and extracted into water to remove ruthenium and osmium salts. The residue contains iridium oxide, IrO2. Dissolution of the oxide in regia (a mixture of hydrochloric acid, HCl, and nitric acid, HNO3) gives a solution containing pure (NH4)3IrCl6. Evaporation to dryness and burning under hydrogen gas gives pure iridium.