Iridium is the chemical element with symbol Ir and atomic number 77. It is a transitional group metal, part of the so called platinum group metals. Iridium is one of the rarest elements in the Earth's crust, with annual production and consumption of only three metric tonnes.
Iridium is the most corrosion resistant metal, even at high temperatures. At standard conditions, no chemical compound can attack it. Finely divided however, it is more reactive. At high temperatures, molten sodium cyanide and potassium cyanide will attack iridium, as well as oxygen and fluorine.
Iridium tends to form mixed-valence compounds, most notably iridum(IV) chloride (whose name is particularly unusual).
Iridium's organometallic chemistry is extensively studied, as iridium forms very stable bonds with carbon.
Iridium is a silvery white, very hard metal. It has excellent mechanical properties, which makes it difficult to work with. It's high melting point of 2446 °C makes it difficult to melt and cast, so powdered metallurgy is usually employed. It is the only metal to maintain good mechanical properties in air at temperatures above 1600 °C, making it useful as a high temperature catalyst. It is the second densest element after osmium, with a value of 22.56 g/cm3. Because of this property, most of the elemental iridium has sunk in the Earth crust.
In nature it usually found together with osmium, usually in meteorites. However due to it's low concentration, it's not a particularly good source for the amateur chemist.
Iridium can be found in certain high-performance car spark plugs, in the tip of the electric contact. Because it's immune to most corrosive substances, it's recommended to dissolve the welding around the iridium. It is not much, but being very dense, just a few spark tips can build up a significant quantity.
Lastly, iridium can be bought as bullions and coins, though in recent years it has risen to 20$/gram.
Iridium's high melting point and chemical resistance allows it to be used in the creation of high-performance crucibles who, unlike the ceramic ones, are very thin, thus reducing the amount of iridium used. Such crucibles are used for single crystal growth.
As it sits at the bottom of the metal reactivity series, it can be reduced very easy from it's compounds.
- Organoiridium compounds
- Single crystal growth in iridium crucibles
Being the least reactive metal, iridium is non-toxic. Finely divided iridium powder can be hazardous to handle, as it is an irritant and may ignite in air.
Bulk iridium does not require any special storage. Powdered iridium should be stored in closed containers.
Due to iridium's rarity and price, it's best to recycle it.