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Iridium is a chemical element with the symbol Ir and atomic number 77 It is one of the transition metals in the periodic table it is in the group 9 ( in the older part of the census sub-group 8 ) or cobalt group. The very heavy, hard, brittle, silvery - white lustrous precious metal from the platinum group of metals is considered the most corrosion-resistant element. Under 0.11 Kelvin it switches over to the superconducting state.


Iridium (Greek ίριοειδής " rainbow colored" by the variegation of its compounds ) was discovered in 1804 by Smithson Tennant in London together with osmium. When resolving a Rohplatins in aqua regia both platinum metals were in the insoluble black residue. The color variety of iridium salts inspired Tennant to the name Iridium. Also, the " International Prototype Kilogram " and the " standard meter " consisting of an iridium alloy. It is kept in Paris in the Bureau International des Poids et Mesures since 1898.


Iridium is rare than gold or platinum. It is after rhenium together with rhodium and ruthenium the rarest non -radioactive metal. Its share in the continental crust is only 1 ppb. In nature, it occurs elementary in the form of small grains or in the company of the platinum. With osmium, it forms two naturally occurring minerals:

  • Osmiridium composed of 50% iridium, the remainder consisting of osmium, platinum, ruthenium and rhodium, and
  • Iridosmium, made up to 55-80 % from 20-45 % osmium, and iridium.

Important deposits are in South Africa, in the Urals, North and South America, Tasmania, Borneo and Japan.

Free iridium as well as other platinum group elements are found in river sands. In addition, Iridium accumulates during the smelting of nickel ores.


Physical Properties

Because of its hardness and brittleness of iridium is difficult to be processed. At a red heat it incompletely oxidized to black IrO2, which decays back above 1140 ° C. Even as iridium osmium in the heat and especially at higher oxygen content than IrO3 oxide is volatile. In cold places, however, it differs in contrast to osmium metal or IrO2 again.

In the isotopic composition of naturally occurring iridium is the second densest element after osmium. The stable iridium isotope 193Ir has, however, with 22.65 g/cm3, the highest density of all non-radioactive isotopes. Whether iridium or osmium is the densest element, that is a matter of definition. In the Anglo-Saxon literature mainly applies osmium as the densest element.

Chemical Properties

In mineral acids, even in aqua regia, it is stable. In chloride melts in the presence of chlorine but it is dissolved.


There are two natural isotopes of iridium, 34 and 21 Kernisomere radioisotopes, of which the Kernisomer 192m2Ir with a half -life of 241 years is the most stable. It decays by internal conversion to the 192Ir with 73.831 days half-life of the isotope with the longest half-life ist.192Ir decays as beta emitters for platinum isotope 192mPt, most of the other to osmium. The remaining isotopes have half-lives and Kernisomere between 300 microseconds in 165Ir and 190Ir at 11.78 days.

192Ir is suitable due to its gamma radiation with an energy of about 550 keV (kilo electron volts ) for the irradiation testing of components. For workpieces with a wall thickness of about 20 mm mostly resorted to this isotope is ( normatively regulated, see eg DIN EN 1435 ).

For the NDT of iridium emitter is usually sealed in the form of a 2-3 mm large tablet in a source holder, and this is housed in a lockable type B work bowl that is lined to shield the gamma radiation with depleted uranium.

Working containers for iridium emitters have the following dimensions: 20 cm long, 10 cm wide and 15 cm high. The weight is due to the uranium jacket depending on the activity about 13 to 20 kg. Thus they are more suitable for mobile use as an industrial X-ray systems.

→ List of iridium isotopes


Iridium is often a component of alloys, which it imparts hardness and / or brittleness. Platinum - iridium alloys are employed in a precision measurements in medicine and engineering.

Another use of it:

  • As part of the alloy of the original kilogram and the third Ur -Meter,
  • For encapsulating plutonium in nuclear batteries,
  • In the form of containers and crucibles for high -temperature applications,
  • As an electrical contact,
  • In jewelry as platinum -iridium alloy ( PtIr 800 and PtIr 900), for highly stressed parts ( rings, tie pins, fasteners, machine heads and springs)
  • At the spark plug electrodes,
  • In pens, the front at the top for a fountain pen Os / Ir alloy is usually used,
  • As part of ballpoint pen refills ( writing ball )
  • In alloy with platinum as the spray tip in the flame atomic absorption spectrometry,
  • In the measurement of surface tension by the ring method you Noüy, where it is used in alloys with platinum because of its optimum wettability,
  • In sputtering targets for the generation of electron- lead trends coatings of electrical insulators in high-resolution scanning electron microscopy,
  • In the form of the UV protective layer on quality sunglasses
  • In dental alloys,
  • Increasing chemical reactions as a catalyst ( used in the synthesis of industrially important acetic acid ),
  • Because of its high density and its high melting point as a target in nuclear physics, for example, for antiproton production at CERN.

KT- Impact

Iridium comes in relatively high concentration in the sediment layer in front that separates the age of the Cretaceous from the Tertiary, and serves as an important proof of a large meteorite impact that wiped out the dinosaurs, among other things. See also iridium anomaly and KT impact event ( Cretaceous-Tertiary impact ).


Metallic iridium is non-toxic because of its resistance. As a powder or dust is highly flammable, non-flammable in a compact form. Iridium compounds should be classified as toxic.


Many iridium salts are colored: with chlorine it forms olive iridium (III ) chloride or dark blue-black, not quite defined iridium (IV ) chloride. Fluorine reacts to yellow, slightly volatile iridium (VI ) fluoride or yellow-green iridium (V ) fluoride.