Iridium

Main article: Isotopes of iridium
iso	NA	half-life	DM	DE (MeV)	DP
¹⁸⁹Ir	syn	13.2 d	ε	0.532	¹⁸⁹Os
¹⁹⁰Ir	syn	11.8 d	ε	2.000	¹⁹⁰Os
¹⁹¹Ir	37.3%	¹⁹¹Ir is stable with 114 neutrons
¹⁹²Ir	syn	73.83 d	β	1.460	¹⁹²Pt
¹⁹²Ir	syn	73.83 d	ε	1.046	¹⁹²Os
^192mIr	syn	241 y	IT	0.155	¹⁹²Ir
¹⁹³Ir	62.7%	¹⁹³Ir is stable with 116 neutrons
¹⁹⁴Ir	syn	19.3 h	β<	2.247	¹⁹⁴Pt
¹⁹⁵Ir	syn	2.5 h	β<	1.120	¹⁹⁵Pt

References

Iridium (pronounced /ɪˈrɪdiəm/) is a chemical element that has the symbol Ir and atomic number 77. A dense, very hard, brittle, silvery-white transition metal of the platinum family, iridium is used in high-strength alloys that can withstand high temperatures and occurs in natural alloys with platinum or osmium. Iridium is notable for being the most corrosion-resistant metal known and for its significance in the determination of the probable cause of the extinction, by an asteroid impact, of the dinosaurs. It is used in high-temperature apparatuses, electrical contacts, and as a hardening agent for platinum.

1 Characteristics
2 Applications
3 History
- 3.1 K-T boundary
4 Occurrence
5 Isotopes
6 Compounds
7 Production
8 Precautions
9 References
10 External links

Characteristics

A platinum group metal, iridium is white, resembling platinum, but with a slight yellowish cast. Due to its extreme hardness and brittleness, iridium is difficult to machine, form, or work. It is the most corrosion-resistant metal known: iridium cannot be attacked by any acids or by aqua regia. It can, however, be attacked by molten salts, such as NaCl and NaCN.

The measured density of iridium is only slightly lower than that of osmium, which is often listed as the densest element known. However, calculations of density from the space lattice may produce more reliable data for these elements than actual measurements and give a density of 22,650 kg/m³ for iridium versus 22,610 kg/m³ for osmium. Definitive selection between the two is therefore not possible at this time.

Applications

The principal use of iridium is as a hardening agent in platinum alloys. Other uses:

Crucibles and devices that require high temperatures.
As a radiography source for non-destructive testing
Electrical contacts (notable example: Pt-Ir spark plugs).
Osmium-iridium alloys are used for compass bearings.
Iridium is commonly used in complexes like Ir(mppy)₃ and other complexes in polymer LED technology to increase the efficiency from 25% to almost 100% due to triplet harvesting.
Used as a source of radioactivity (Ir-192) for the treatment of cancer (in particular High Dose Rate prostate brachytherapy and intracavitary cervix brachytherapy)
Iridium is used as a catalyst for carbonylation of methanol to produce acetic acid
Iridium is used in supercolliders in the production of antimatter, specifically antiprotons

At one time iridium, as an alloy with platinum, was used in bushing the vents of heavy ordnance, and in a finely powdered condition (iridium black), for painting porcelain black.

Iridium was used to tip some early-twentieth-century fountain pen nibs. The tip material in modern fountain pens is still conventionally called "iridium," although there is seldom any iridium in it.

History

Iridium was discovered in 1803 by British scientist Smithson Tennant in London, England along with osmium in the dark-coloured residue of dissolving crude platinum in aqua regia (a mixture of hydrochloric and nitric acid). The element was named after the Greek word for rainbow (ίρις, iris; iridium means "of rainbows") because many of its salts are strongly coloured.

An alloy of 90% platinum and 10% iridium was used in 1889 to construct the standard metre bar and kilogramme mass, kept by the International Bureau of Weights and Measures near Paris. The metre bar was replaced as the definition of the fundamental unit of length in 1960 (see krypton), but the kilogram prototype is still the international standard of mass.

K-T boundary

Main article: Cretaceous–Tertiary extinction event

Main article: K–T boundary

The K–T boundary of 65 million years ago, marking the temporal border between the Cretaceous and Tertiary periods of geological time, was identified by a thin stratum of iridium-rich clay. A team led by Luis Alvarez (1980) proposed an extraterrestrial origin for this iridium, attributing it to an asteroid or comet impact. Their theory is now widely accepted to explain the demise of the dinosaurs. A large buried impact crater structure with an estimated age of about 65 million years was later identified near what is now Yucatán Peninsula (the Chicxulub crater). Dewey M. McLean and others argue that the iridium may have been of volcanic origin instead. The Earth's core is rich in iridium, and Piton de la Fournaise on Réunion, for example, is still releasing iridium today.

Occurrence

Iridium is found uncombined in nature with platinum and other platinum group metals in alluvial deposits. Naturally occurring iridium alloys include osmiridium and iridiosmium, both of which are mixtures of iridium and osmium. It is recovered commercially as a by-product from nickel mining and processing.

According to the CRC Handbook, ruthenium and rhodium are the only non-radioactive elements rarer than iridium. Although iridium is one of the uncommon in Earth's crust, it is relatively common in meteorites. It is thought that the concentration of iridium in meteorites matches the concentration of iridium in the Earth as a whole; as Iridium and osmium are the densest elements, it is believed that they descended below the Earth's crust and toward the core at a time when the Earth was young and still molten.

Isotopes

Main article: isotopes of iridium

There are two natural isotopes of iridium, and many radioisotopes, the most stable radioisotope being Ir-192 with a half-life of 73.83 days. Ir-192 beta decays into platinum-192, while most of the other radioisotopes decay into osmium.

Compounds

See also: Category:Iridium compounds

Iridium(III) bromide IrBr₃

Iridium(III) chloride IrCl₃

Iridium(IV) oxide IrO₂

Tetrairidium dodecacarbonyl Ir₄(CO)₁₂

Vaska's complex IrCl(CO)[P(C₆H₅)₃₂

Production

Please help improve this article or section by expanding it. Further information might be found on the talk page or at requests for expansion. (January 2008)

A new method for the quantitative extraction and determination of trace amounts of iridium from hydrochloric acid media has been established based on the formation of an ion-association complex of iridium hexachloro anion IrCl6(2-) with dicyclohexyl-18-crown-6 (DC18C6) oxonium cation in chloroform, then determination by inductively coupled plasma atomic emission spectrometry (ICP-AES). The effect of various factors (solvent, acid concentration, crown ether, reagent concentration, shaking time, composition of the extracted species, foreign ions, etc.) on the extraction and back-extraction of iridium has been investigated. The procedure was used to determine traces of iridium in palladium chloride and rhodium chloride.

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