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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 25% nitric acid (HNO₃) and 75% hydrochloric acid (HCl).  The element was named after the Latin word for rainbow (iris; iridium means "of rainbows") because many of its salts are strongly colored.  Some linguists have claimed the word-root is derived from "irid", which means "seven" in the Lezghi Language presently spoken in Azerbaijan and Daghestan.

An alloy of 90% platinum and 10% iridium was used in 1889 to construct the standard meter bar and kilogram mass, kept by the international Bureau of Weights and Measures near Paris.  This bar was replaced as the definition of the meter in 1960 when the meter was redefined in terms of the orange-red spectral line of Krypton-86, but the kilogram prototype is still the international standard of mass.

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 NaCl and NaCN.

The measured density of this element is only slightly lower than that of osmium, which is often listed as the most dense 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.

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.

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 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 in high-dose-radiation therapy for the treatment of prostate and other forms of cancer
• 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.

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

KT Boundary

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 Yucatan 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 Reunion, or example, is still releasing iridium today.

Isotopes

There are two natural isotopes of iridium, and 34 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.