Rhodium is a rare, silvery-white, hard and chemically inert transition metal chemical element which is a member of the platinum group. Its chemical symbol is Rh and atomic number is 45. Alloyed with similar metals, and never as a chemical compound, naturally occurring rhodium is found as the free metal, and it is one of the rarest precious metals, and the most costly.
Discovered in 1803 by William Hyde Wollaston rhodium is the so-called noble metal. It is resistant to corrosion, as found in platinum, nickel ores together with the other members of the platinum group metals. It turns rose color because of its chlorine compounds, which is produced after it reacted with the powerful acid mixture aqua regia.
About 80% of world rhodium production element's major use is as one of the catalysts in the three-way catalytic converters of automobiles. Because it is resistant against corrosion and most aggressive chemicals, and because of its rarity rhodium metal is inert. Rhodium is always alloyed with platinum or palladium and applied in high-temperature and numerous corrosion-resistive coatings. Sterling silver is often rhodium plated for tarnish resistance and white gold is plated with a thin rhodium layer to improve its shiny look.
Soon after his discovery of palladium, Wollaston discovered Rhodium. He used crude platinum that he obtained from South America. He dissolved the ore in aqua regia and neutralized the acid with sodium hydroxide. By adding ammonium chloride, as ammonium chloroplatinate he then precipitated the platinum. To precipitate other metals like copper, lead, palladium and rhodium zinc was used.
Electroplating was the first major application for decorative uses and as corrosion resistant coating. The demand for rhodium increased due to the introduction of the three way catalytic converter by Volvo in 1976. Platinum or palladium was used in the previous catalytic converters and the three way catalytic converter used rhodium to reduce the amount of NOx in the exhaust.
Rhodium is used in jewelry and for decorations. It gets electroplated onto white gold and platinum to give it a reflective, shiny white surface. This is known as rhodium flashing or plating in the jewelry business. Also with sterling silver it may also be used in coating to protect against tarnish. Because the metal has a high melting point and poor malleability making jewelry very hard to fabricate, solid or pure rhodium jewelry is very rare. And because of its high cost, for its jewelry usage it is in the form of tiny amounts of powder usually called a rhodium sponge which is dissolved into electroplating solutions.
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 in 1979. Since then rhodium has been used for honors, or to symbolize wealth, when more commonly used metals such as silver, gold or platinum are said to be insufficient.
Rhodium is used as a mixing agent for hardening and improving the corrosion resistance of platinum and palladium. But it is used in a lot of other things like furnace windings, bushings for glass fiber production, thermocouple elements, electrodes for aircraft spark plugs, and laboratory crucibles, electrical contact material due to its low electrical resistance, low and stable contact resistance, and high corrosion resistance. Rhodium plating, made by electroplating or evaporation, is extremely hard and is used for visual instruments. In the medical field it is used as a filter in mammography systems because of the sharp X-rays it produces.
High-quality fountain pens like Graf von Faber-Castell and Caran D'ache use rhodium to coat some parts due to its high reflectance, as well as its chemical and mechanical resistance. Although being a noble metal, rhodium compounds can strongly stain human skin so some allergy to it may be found.
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