Rhodium is a chemical element that is a rare, silvery-white, hard, and chemically inert transition metal and a member of the platinum group. It has the chemical symbol Rh and atomic number 45. It is composed of only one naturally-occurring isotope, 103Rh. Naturally occurring rhodium is usually found as the free metal, alloyed with similar metals, and rarely as a chemical compound in minerals such as bowieite and rhodplumsite. It is one of the rarest precious metals.
Rhodium is a so-called noble metal, resistant to corrosion, found in platinum- or nickel ores together with the other members of the platinum group metals. It was discovered in 1803 by William Hyde Wollaston in one such ore, and named for the rose color of one of its chlorine compounds, produced after it reacted with the powerful acid mixture aqua regia.
The element's major use (more that 80% of world rhodium production) is as one of the catalysts in the three-way catalytic converters in automobiles. Because rhodium metal is inert against corrosion and most aggressive chemicals, and because of its rarity, rhodium is usually alloyed with platinum or palladium and applied in high-temperature and corrosion-resistive coatings. White gold is often plated with a thin rhodium layer to improve its optical impression while sterling silver is often rhodium plated for tarnish resistance.
Rhodium detectors are used in nuclear reactors to measure the neutron flux level.
Rhodium is a hard, silvery, durable metal that has a high reflectance. Rhodium metal does not normally form an oxide, even when heated. Oxygen is absorbed from the atmosphere only at the melting point of rhodium, but is released on solidification. Rhodium has both a higher melting point and lower density than platinum. It is not attacked by most acids: it is completely insoluble in nitric acid and dissolves slightly in aqua regia.
Rhodium is one of the rarest elements in the Earth’s crust, of which it comprises an estimated 0.0002 parts per million (2 × 10−10). Its rarity affects its price, and thus its usage in commercial applications.
The industrial extraction of rhodium is complex as the metal occurs in ores mixed with other metals such as palladium, silver, platinum, and gold. It is found in platinum ores and extracted as a white inert metal which is very difficult to fuse. Principal sources are located in South Africa, in river sands of the Ural Mountains, and in North America, including the copper-nickel sulfide mining area of the Sudbury, Ontario region.
Although the quantity at Sudbury is very small, the large amount of processed nickel ore makes rhodium recovery cost-effective. The main exporter of rhodium is South Africa (approximately 80% in 2010) followed by Russia. The annual world production of this element is less than 30 tonnes and there are very few rhodium-bearing minerals. The price of rhodium is historically highly variable. In 2007, rhodium cost approximately eight times more than gold, 450 times more than silver, and 27,250 times more than copper by weight. In 2008, the price briefly rose above $10,000 per ounce. The economic slowdown of the 3rd quarter of 2008 pushed rhodium prices sharply back below $1,000 per ounce, but they rebounded to $2,750 by early 2010 (over twice the gold price).
The primary use of this element is in automobiles as a catalytic converter, which changes harmful emissions from the engine into less polluting gases. Of 27,200 kg rhodium consumed worldwide in 2010, some 22,500 kg (82.7%) went into and 7,300 kg recovered from this application. About 1,770 kg of rhodium was used in the glass industry, mostly for production of fiberglass and flat-panel glass, and 2,110 kg in the chemical industry.
Rhodium finds use in jewelry and for decorations. It is electroplated on white gold and platinum to give it a reflective white surface. This is known as rhodium flashing in the jewelry business. It may also be used in coating sterling silver to protect against tarnish, which is silver sulfide (Ag2S) produced from the atmospheric hydrogen sulfide (H2S). Solid (pure) rhodium jewelry is very rare, because the metal has both high melting point and poor malleability (making such jewelry very hard to fabricate) rather than due to its high price. Additionally, its high cost assures that most of its jewelry usage is in the form of tiny amounts of powder (commonly called rhodium sponge) dissolved into electroplating solutions.
Rhodium has also been used for honors or to symbolize wealth, when more commonly used metals such as silver, gold, or platinum are deemed insufficient. In 1979, the Guinness Book of World Records gave Paul McCartney a rhodium-plated disc for being history's all-time best-selling songwriter and recording artist.
http://en.wikipedia.org/wiki/Rhodium
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