Iridium Ir

Discovery

In 1803 Smithson Tennant in London found iridium, together with osmium, in the black residue left after dissolving crude platinum in aqua regia. Iridium is so hard to melt and work that solid ingots of the pure metal were not produced until the mid-nineteenth century.

In 1889 the International Bureau of Weights and Measures made the international prototype metre and kilogram from an alloy of 90% platinum and 10% iridium, since iridium makes platinum much harder and more stable. That kilogram prototype remained the world's standard of mass until the kilogram was redefined in terms of the Planck constant in 2019.

Origin of the name

The name comes from Iris, the Greek goddess of the rainbow, because Tennant noticed how many different colours iridium's salts display.

Main uses

• Spark plugs: iridium electrodes resist heat and spark erosion, giving long-life automotive spark plugs.
• Crystal-growth crucibles: its high melting point and oxidation resistance make it ideal for crucibles used to grow single crystals for lasers and electronics.
• Electrolysis: iridium oxide is the standard anode catalyst in proton-exchange-membrane water electrolysers, a key technology for green hydrogen, and is used in electrode coatings for chlorine production.
• OLEDs: phosphorescent iridium complexes emit light in many smartphone and TV displays.
• Radiation: iridium-192 is used in industrial radiography of welds and in brachytherapy, where a source is placed close to a tumour.

Isotopes

Natural iridium consists of two stable isotopes, iridium-191 (about 37%) and iridium-193 (about 63%). The most widely used radioactive isotope is iridium-192, with a half-life of about 74 days.

In everyday life

In 1980 a team led by Luis and Walter Alvarez found that a thin clay layer marking the boundary between the Cretaceous and Paleogene periods contained far more iridium than ordinary crustal rock. Since iridium is rare in the crust but relatively common in meteorites, they proposed that a giant asteroid impact caused the mass extinction that ended the dinosaurs. The later identification of the Chicxulub crater in Mexico made the idea widely accepted. Iridium metal has low toxicity, but industrial iridium-192 sources are intensely radioactive; anyone finding a lost source should keep away and alert the authorities.