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.
Cartoon by Nick D Kim ([Science and Ink], used by permission).
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 around us Read more »
Iridium has no biological role.
Iridium is found as the free element alloyed with platinum and other platinum metals. Naturally occurring alloys include osmiridium and iridiosmium, both mixtures of iridium and osmium.
|Location||ppb by weight||ppb by atoms||Links|
|Human||(no data) ppb by weight||(no data) atoms relative to C = 1000000|
Physical properties Read more »
Heat properties Read more »
- Melting point: 2739 [2466 °C (4471 °F)] K
- Boiling point: 4701 [4428 °C (8002 °F)] K
- Enthalpy of fusion: 26 kJ mol-1
Crystal structure Read more »
The solid state structure of iridium is: ccp (cubic close-packed).
Iridium: orbital properties Read more »
Iridium atoms have 77 electrons and the shell structure is 18.104.22.168.15.2. The ground state electronic configuration of neutral Iridium is [Xe].4f14.5d7.6s2 and the term symbol of Iridium is 4F9/2.
- Pauling electronegativity: 2.20 (Pauling units)
- First ionisation energy: 880 kJ mol‑1
- Second ionisation energy: 1600 kJ mol‑1
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.
Iridium isotopes Read more »
Iridium has two isotopes, Ir-191 and Ir-193 and both are used in the production of radioactive material. Ir-191 is used for the production of radioactive Ir-192. This Ir-192 is used as a radiation source in gamma cameras that are used for non-destructive testing. Ir-192 sources are also used in so-called brachytherapy procedures whereby radioactive materials are placed in close contact with the tissue being treated. Although Ir-192 can be produced from natural Ir, the use of enriched Ir-191 gives a much higher specific activity and allows the use of smaller sources. The use of Ir-193 has been suggested for the production of the therapeutic radioisotope Pt-195m.
|191Ir||190.960584 (4)||37.3 (2)||3/2||0.1462|
|193Ir||192.962917 (4)||62.7 (2)||3/2||0.1592|