AskDefine | Define rhodium

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rhodium n : a white hard metallic element that is one of the platinum group and is found in platinum ores; used in alloys with platinum [syn: Rh, atomic number 45]

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see Rhodium



  1. a metallic chemical element (symbol Rh) with an atomic number of 45.

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Extensive Definition

Rh redirects here. For other uses, see rh (disambiguation)
Rhodium () is a chemical element with the symbol Rh and atomic number 45. A rare silvery-white hard transition metal and a member of the platinum group, rhodium is found in platinum ores and is used in alloys with platinum and as a catalyst. It is usually the most expensive precious metal, though it is sometimes surpassed by rhenium, depending on the market.

Notable characteristics

Rhodium is a hard silvery white and durable metal that has a high reflectance. Rhodium metal does not normally form an oxide, even when heated. Oxygen is absorbed from the atmosphere at the melting point of rhodium, but on solidification the oxygen is released. Rhodium has both a higher melting point and lower density than platinum. It is not attacked by acids: it is completely insoluble in nitric acid and dissolves slightly in aqua regia. A complete dissolution of rhodium in powder form is only obtained in sulfuric acid.


The primary use of this element is as an alloying agent for hardening platinum and palladium. These alloys are used in furnace windings, bushings for glass fiber production, thermocouple elements, electrodes for aircraft spark plugs, and laboratory crucibles. Other uses include:
  • It is used as an electrical contact material due to its low electrical resistance, low and stable contact resistance, and its high corrosion resistance.
  • Plated rhodium, made by electroplating or evaporation, is extremely hard and is used for optical instruments.
  • This metal 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 also may be used in coating sterling silver in order to strengthen the metal from tarnish, as a result from the copper compound found in sterling silver.
  • It is also a highly useful catalyst in a number of industrial processes (notably it is used in the catalytic system of automobile catalytic converters and for catalytic carbonylation of methanol to produce acetic acid by the Monsanto process). It is used to catalyse addition of hydrosilanes to a double bond, a process important in manufacture of certain silicone rubbers.
  • The complex of a rhodium ion with BINAP gives a widely used chiral catalyst for chiral synthesis, as in the synthesis of menthol.
  • It is also used as a filter in mammography systems because of the characteristic x-rays it produces.
  • It is also used in high quality pen surfaces due to its high-resistance characteristics. These pens include Graf von Faber-Castell which is rather less famous than Montblanc, but produces very limited pens.


Rhodium (Greek rhodon meaning "rose") was discovered in 1803 by William Hyde Wollaston, soon after his discovery of palladium. He made this discovery in England using crude platinum ore that he presumably obtained from South America.
His procedure involved dissolving the ore in aqua regia, neutralizing the acid with sodium hydroxide (NaOH). He then precipitated the platinum by adding ammonium chloride, NH4Cl, as ammonium chloroplatinate. The element palladium was removed as palladium cyanide after treating the solution with mercuric cyanide. The material that remained was a red rhodium(III) chloride (hence the name): rhodium metal was isolated via reduction with hydrogen gas.


Normal mining

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 obtained free as a white inert metal which is very difficult to fuse. Principal sources of this element are located in South Africa, in river sands of the Ural Mountains, in North and South America and also in the copper-nickel sulfide mining area of the Sudbury, Ontario region. Although the quantity at Sudbury is very small, the large amount of nickel ore processed makes rhodium recovery cost effective. The main exporter of rhodium is South Africa (>80%) followed by Russia. The annual world production of this element is only about 25 tons and there are very few rhodium minerals. As of October 2007, rhodium cost approximately eight times more than gold, 450 times more than silver, and 27,250 times more than copper by weight. Rhodium's typical historical price is about $1,000/troy oz, but in recent years it has increased to about $9500/troy oz.

As a fission product

It is also possible to extract rhodium from used nuclear fuel, which contains rhodium (1 kg of the fission products of 235U contain 13.3 grams of 103Rh). So as a typical used fuel is 3% fission products by weight it will contain about 400 grams of rhodium per ton of used fuel. The longest lived radioisotope of rhodium is 102mRh which has a half life of 2.9 years, while the ground state (102Rh)has a half life of 207 days.
Each kilo of fission rhodium will contain 6.62 ng of 102Rh and 3.68 ng of 102mRh. As 102Rh decays by beta decay to either 102Ru (80%) (some positron emission will occur) or 102Pd (20%) (some gamma ray photons with about 500 keV are generated) and the excited state decays by beta decay (electron capture) to 102Ru (some gamma ray photons with about 1 MeV are generated). If the fission occurs in an instant then 13.3 grams of rhodium will contain 67.1 MBq (1.81 mCi) of 102Rh and 10.8 MBq (291 μCi) of 102mRh. As it is normal to allow used nuclear fuel to stand for about five years before reprocessing, much of this activity will decay away leaving 4.7 MBq of 102Rh and 5.0 MBq of 102mRh. If the rhodium metal was then left for 20 years after fission, then the 13.3 grams of rhodium metal would contain 1.3 kBq of 102Rh and 500 kBq of 102mRh. At first glance the rhodium might be adding to the resource value of reprocessed fission waste, but the cost of the separation of the rhodium from the other metals needs to be considered.


Naturally occurring rhodium is composed of only one isotope, 103Rh. The most stable radioisotopes are 101Rh with a half-life of 3.3 years, 102Rh with a half-life of 207 days, 102mRh with a half-life of 2.9 years, and 99Rh with a half-life of 16.1 days. Twenty other radioisotopes have been characterized with atomic weights ranging from 92.926 u (93Rh) to 116.925 u (117Rh). Most of these have half-lifes that are less than an hour except 100Rh (half-life: 20.8 hours) and 105Rh (half-life: 35.36 hours). There are also numerous meta states with the most stable being 102mRh (0.141 MeV) with a half-life of about 207 days and 101mRh (0.157 MeV) with a half-life of 4.34 days. See isotopes of rhodium.
The primary decay mode before the only stable isotope, 103Rh, is electron capture and the primary mode after is beta emission. The primary decay product before 103Rh is ruthenium and the primary product after is palladium.


Rhodium metal is, as a noble metal, inert. However, when rhodium is chemically bound, it is reactive. Rhodium compounds are not often encountered by most people and should be considered to be toxic and possibly carcinogenic. Lethal intake (LD50) for rats is 12.6 mg/kg of rhodium chloride (RhCl3). Rhodium compounds can strongly stain human skin. The element plays no biological role in humans. If used in elemental form rather than as compounds, the metal is harmless.

Symbolic uses

Rhodium has been used for honours, 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. Guinness has also noted items such as the world's "Most Expensive Pen" or "Most Expensive Board Game" as containing rhodium.


rhodium in Afrikaans: Rodium
rhodium in Arabic: روديوم
rhodium in Bengali: রোহডিয়াম
rhodium in Belarusian: Родый
rhodium in Bosnian: Rodijum
rhodium in Catalan: Rodi
rhodium in Czech: Rhodium
rhodium in Corsican: Rodiu
rhodium in Danish: Rhodium
rhodium in German: Rhodium
rhodium in Estonian: Roodium
rhodium in Modern Greek (1453-): Ρόδιο
rhodium in Spanish: Rodio
rhodium in Esperanto: Rodio
rhodium in Basque: Rodio
rhodium in Persian: رادیوم
rhodium in French: Rhodium
rhodium in Friulian: Rodi
rhodium in Manx: Roydjum
rhodium in Galician: Rodio
rhodium in Korean: 로듐
rhodium in Armenian: Ռոդիում
rhodium in Croatian: Rodij
rhodium in Ido: Rodio
rhodium in Indonesian: Rhodium
rhodium in Icelandic: Ródín
rhodium in Italian: Rodio
rhodium in Hebrew: רודיום
rhodium in Javanese: Rodium
rhodium in Swahili (macrolanguage): Rodi
rhodium in Haitian: Rodyòm
rhodium in Kurdish: Rodyûm
rhodium in Latin: Rhodium
rhodium in Latvian: Rodijs
rhodium in Luxembourgish: Rhodium
rhodium in Lithuanian: Rodis
rhodium in Lojban: zgujinme
rhodium in Hungarian: Ródium
rhodium in Malayalam: റോഡിയം
rhodium in Marathi: र्‍होडियम
rhodium in Dutch: Rhodium
rhodium in Japanese: ロジウム
rhodium in Norwegian: Rhodium
rhodium in Norwegian Nynorsk: Rhodium
rhodium in Occitan (post 1500): Ròdi
rhodium in Uzbek: Rodiy
rhodium in Polish: Rod (pierwiastek)
rhodium in Portuguese: Ródio
rhodium in Romanian: Rodiu
rhodium in Russian: Родий
rhodium in Sicilian: Rodiu
rhodium in Simple English: Rhodium
rhodium in Slovenian: Rodij
rhodium in Serbian: Родијум
rhodium in Serbo-Croatian: Rodijum
rhodium in Finnish: Rodium
rhodium in Swedish: Rodium
rhodium in Tamil: ரோடியம்
rhodium in Thai: โรเดียม
rhodium in Vietnamese: Rhodi
rhodium in Turkish: Rodyum
rhodium in Ukrainian: Родій
rhodium in Chinese: 铑
rhodium in Slovak: Ródium
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