Iridium

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Iridium

Iridium is a chemical element in the periodic table 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 element known and for its association with the demise of the dinosaurs, in the form of a meteorite strike. It is used in high temperature apparatus, electrical contacts, and as a hardening agent for platinum.

Notable characteristics

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

The measured density of this element is only slightly lower than that of osmium, which is often listed as the heaviest 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 22650 kg/m³ for iridium versus 22610 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:

For making crucibles and devices that require high temperatures.
Electrical contacts (notable example: Pt/Ir sparkplugs).
Osmium/iridium alloys are used for tipping fountain pen nibs and 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.
Iridium is used as a catalyst for carbonylation of methanol to produce acetic acid

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.

History

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

An alloy of 90% platinum and 10% iridium was used in 1889 to construct the standard metre bar and kilogram 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.

The KT event of 65 million years ago, marking the temporal border between the Cretaceous and Tertiary eras 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 near what is now Yucatán Peninsula. Their theory is widely accepted to explain the demise of the dinosaurs. 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.

Iridium is rare on Earth, but relatively common in meteorites.

Isotopes

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.

Precautions

Iridium metal is mostly non-toxic due to its relative unreactivity, but iridium compounds should be considered highly toxic.

References

Los Alamos National Laboratory - Iridium

External links

Chemical elements | Precious metals | Transition metals

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