Iridium

DescriptionIridium metal was detected in the black residue of aqua regia extract of platinum and identified as an element by British chemist Smithson Tennant in 1803. Around the same time, existence of this new metal was proposed by Vauquelin and deFourcroy in France in the course of their extraction of platinum by aqua regia. Tennant named this element Iridium after the Greek word, Iris, meaning rainbow. Iridium occurs in small amounts in native platinum or platinum metal alloys. Iridium and osmium together constitute “osmiridium,” which is resistant to chemical attack and is a byproduct of platinum extraction.

UsesThe most important use of iridium is as an alloying metal for platinum and palladium. Such alloys are used for jewelry, decorative purposes, electrical contacts, thermocouples, crucibles, electrodes, hypodermic needles, and medical accessories. Iridium enhances resistance of platinum to chemical attack and corrosion. It also enhances hardness and tensile strength. The radioisotope Ir-192 is used in examination of ferrous welds and in other radiographic applications.

OccurrenceIridium is the 83rd most abundant element and is found mixed with platinum, osmium,and nickel ores. The minerals containing iridium are found in Russia, South Africa, Canada,and Alaska.Iridium metal is separated from its other metal ores when the combined minerals aredissolved with a strong acid know as aqua regia, which is a mixture of 25% nitric acid and75% hydrochloric acid. Aqua regia is the only acid that will dissolve platinum and gold.Once the platinum and other metals are dissolved, the iridium, which is insoluble in thisstrong acid, becomes the residue. The refined iridium ends up in the form of either powderor crystals.An interesting story as to how most of the iridium appeared on Earth was explainedrecently by scientists who discovered a thin layer of iridium in the sediments that were laiddown in the Earth’s crust at the end of the Cretaceous period. This was a period about 65 millionyears ago when meteors and asteroids crashed into the Earth. These extraterrestrial bodiescontained a high percentage of iridium. Dust from the impact spread around the Earth andblocked the sun for months, resulting in the extinction of many plants and animals, includingthe dinosaurs. This extensive dust cloud also deposited a thin coat of the element iridium thatwas contained in the fiery bolides.

HistoryDiscovered in 1803 by Tennant in the residue left when crude platinum is dissolved by aqua regia. The name iridium is appropriate, for its salts are highly colored. Iridium, a metal of the platinum family, is white, similar to platinum, but with a slight yellowish cast. It is very hard and brittle, making it very hard to machine, form, or work. It is the most corrosion-resistant metal known, and was used in making the standard meterbar of Paris, which is a 90% platinum–10% iridium alloy. This meter bar was replaced in 1960 as a fundamental unit of length (see under Krypton). Iridium is not attacked by any of the acids nor by aqua regia, but is attacked by molten salts, such as NaCl and NaCN. Iridium occurs uncombined in nature with platinum and other metals of this family in alluvial deposits. It is recovered as a by-product from the nickel mining industry. The largest reserves and production of the platinum group of metals, which includes iridium, is in South Africa, followed by Russia and Canada. The U.S. has only one active mine, located at Nye, MT. The presence of iridium has recently been used in examining the Cretaceous-Tertiary (K-T) boundary. Meteorites contain small amounts of iridium. Because iridium is found widely distributed at the K-T boundary, it has been suggested that a large meteorite or asteroid collided with the Earth, killing the dinosaurs, and creating a large dust cloud and crater. Searches for such a crater point to one in the Yucatan, known as Chicxulub. Iridium has found use in making crucibles and apparatus for use at high temperatures. It is also used for electrical contacts. Its principal use is as a hardening agent for platinum. With osmium, it forms an alloy that is used for tipping pens and compass bearings. The specific gravity of iridium is only very slightly lower than that of osmium, which has been generally credited as being the heaviest known element. Calculations of the densities of iridium and osmium from the space lattices give values of 22.65 and 22.61 g/cm3, respectively. These values may be more reliable than actual physical measurements. At present, therefore, we know that either iridium or osmium is the densest known element, but the data do not yet allow selection between the two. Natural iridium contains two stable isotopes. Forty-five other isotopes, all radioactive, are now recognized. Iridium (99.9%) costs about $100/g.

UsesIridium’s most common use is as an alloy metal that, when added to platinum, makes itharder and more durable. It is also mixed with other metals to make electrical contacts, thermocouples(two dissimilar metals joined to form a special type of thermometer), and instrumentsthat will withstand high temperatures without breaking down. It is also used to makespecial laboratory vessels because iridium will not react with most chemical substances. Analloy of iridium and platinum is used as the standard kilogram weight because it is noncorrosiveand will not oxidize and, thus, change its weight over long periods of time.The radioisotope, iridium-192, is used to treat cancer and to take X-ray pictures of metalcastings to detect flaws. Iridium is also used as a catalyst for several chemical reactions.

UsesIn manufacturing crucibles; in hardening platinum; in making nibs for fountain-pen points.

UsesThe primary use of iridium is as a hardening agent in platinum alloys. Other uses are for the making of crucibles and devices that require high temperatures. 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 about 100% due to triplet harvesting. Used in high-dose radiation therapy for the treatment of prostate and other forms of cancer. Iridium is used in tips of ballpoint pens. Iridium is used as a catalyst for carbonylation of methanol to create 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.

DefinitionMetallic element of atomic number 77, one of the platinum metals, group VIII of the periodic table, aw 192.22, valences = 1,2,3,4,6, two stable isotopes.

DefinitionA white transition metal that is highly resistant to corrosion. It is used in electrical contacts, in spark plugs, and in jewelry. Symbol: Ir; m.p. 2410°C; b.p. 4130°C; r.d. 22.56 (17°C); p.n. 77; r.a.m. 192.217.

Definitioniridium: Symbol Ir. A silvery metallictransition element; a.n. 77; r.a.m. 192.20;r.d. 22.42; m.p. 2410°C; b.p. 4130°C.It occurs with platinum and ismainly used in alloys with platinumand osmium. The element forms arange of iridium(III) and iridium(IV)complexes. It was discovered in 1804by Smithson Tennant (1761–1815).