Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
Rhodium is considered to be a precious metal. A precious metal is one that is rare and valued. Other precious metals are gold, silver, and platinum. Rhodium is also classified as a member of the platinum group of metals. The platinum group includes five other metals that often occur together in nature: ruthenium, palladium, osmium, iridium, and platinum.
Rhodium falls in the center of the periodic table. The periodic table is a chart that shows how chemical elements are related to one another. Elements in groups 3 through 14 are called the transition elements.
Rhodium was discovered by English chemist and physicist William Hyde Wollaston (1766-1828) in about 1804. He discovered the metal in an ore that apparently came from South America. The rhodium compound he first discovered was a beautiful rose color.
Group 9 (VIIIB)
Rhodium is used primarily to make alloys with other metals. These alloys are used for specialized industrial purposes and in jewelry.
Discovery and naming
In the early 1800s, Wollaston was studying an ore of platinum. Although scientists don't know for sure, they believe the platinum ore came from South America. Wollaston analyzed the ore and found that he could produce a beautiful rose-colored compound from it. He showed that the pink compound contained a new element. Wollaston suggested the name rhodium for the new element because of this rose color. The Greek word for rose is rhodon.
Rhodium is a silver-white metal. It has a melting point of 1,966°C (3,571°F) and a boiling point of about 4,500°C (8,100°F). Its density is 12.41 grams per cubic centimeter. Two of the metal's special properties are its high electrical and heat conductivity. That means that heat and electricity pass through rhodium very easily.
Rhodium is a relatively inactive metal. It is not attacked by strong acids. When heated in air, it combines slowly with oxygen. It also reacts with chlorine or bromine when very hot. It does not react with fluorine, an element that reacts with nearly every other element.
Occurrence in nature
Rhodium is one of the rarest elements on Earth. Its abundance is estimated to be 0.0001 parts per million. That would place it close to the bottom of the list of elements in terms of abundance. Compounds of rhodium are usually found in combination with platinum and other members of the platinum group. Its most common ores are rhodite, sperrylite, and iridosmine.
The first rhodium compound was a beautiful rose color.
Only one naturally occurring isotope of rhodium is known, rhodium-103. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
Rhodium also has a number of radioactive isotopes. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
None of the isotopes of rhodium have any commercial or other use.
Rhodium is usually obtained as a by-product in the recovery of platinum from its ores. Rhodium is separated by a series of chemical and physical reactions from other platinum metals with which it occurs. The mixture of metals is treated with various acids and other chemicals that dissolve some metals, but not others. Rhenium is one of the first metals to be removed from such a mixture.
The cost of pure rhodium was $25 per gram ($600 per troy ounce) in 1997. It cost approximately ten times that in 1991.
Most of the rhodium metal sold in the United States is used to make alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals. Rhodium is often added to platinum to make an alloy. Rhodium is harder than platinum and has a higher melting point. So the alloy is a better material than pure platinum.
Most rhodium alloys are used for industrial or research purposes, such as laboratory equipment and thermocouples. A thermocouple is a device for measuring very high temperatures. Rhodium alloys are also used to coat mirrors and in search-lights because they reflect light very well.
Compounds of rhodium are used as catalysts. A catalyst is a substance used to speed up or slow down a chemical reaction without undergoing any change itself.
There are no studies of the health effects from rhodium or its common compounds. Elements without information about toxicity are usually treated as if they are poisonous.
Rhodium was discovered in 1804 by the English chemist William Wollaston. Its name derives from the Greek word rhodos, meaning rose—the color of solutions containing rhodium salts. The abundance of rhodium in Earth's crust is approximately 0.0004 ppm. Its purification requires its separation from other platinum metals . Treatment of a crude platinum metal concentrate with aqua regia leaves an insoluble portion that is then fused with bisulfate; this dissolves only the rhodium component, which is eventually converted to a solution of chloro complexes, whose treatment with H2 precipitates the metal.
The extremely unreactive, silvery-white element is used in jewelry plating and is part of anodic and thermocouple materials. However, compounds containing Rh in oxidation states from (VI) to (−I) can be produced, the most common being those of Rh(III). Developments in Rh chemistry since the late 1960s have focused on various catalytic processes that use Rh(I) complexes, particularly those containing phosphines such as PR 3 (R is an aryl or alkyl group) and/or CO, because such ligands stabilize this oxidation state.
The catalytic processes, which operate via mechanisms that cycle between Rh(I) and Rh(III) intermediates , include: (1) hydrogenation (the activation of H2 for the reduction of unsaturated organic compounds), (2) hydroformylation (the activation of H2 and CO for their addition to olefins to generate aldehydes or alcohols); and (3) carbonylation (the activation of CO for its addition to organics). Some of the processes have been developed commercially.
- An Rh(I) complex containing a chiral (optically active ) phosphine ligand (i.e., one with three different substituents on the pyramidal P-atom) can catalyze a reaction such as R(R′)C=CHR′ + H2 → R(R′)C*HCH2R (where R, R ′, and R′ are different substituents); the C* atom of the product has four different substituents and is therefore chiral, and thus the use of a small amount of a chiral Rh catalyst generates large amounts of chiral product. Such a catalytic asymmetric hydrogenation has been used for the production of L-dopa, a drug used to treat Parkinson's disease.
- An Rh(I) complex containing CO and PR 3 ligands can catalyze the formation of butan-1-al from propene (CH2=CHCH3 + H2 + CO → CH3CH2CH2CHO), a commercially operated process.
- An Rh(I) complex containing CO is used industrially to synthesize acetic acid from methanol (CH3OH + CO → CH3COOH).
see also Platinum.
Brian R. James
Griffith, W. P. (1967). The Chemistry of the Rarer Platinum Metals (Os, Ru, Ir, and Rh). New York: Interscience Publishers.
Parshall, George W., and Ittel, Steven D. (1992). Homogeneous Catalysis: The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes, 2nd edition. New York: Wiley.
rho·di·um / ˈrōdēəm/ • n. the chemical element of atomic number 45, a hard silvery-white metal of the transition series, typically occurring in association with platinum. (Symbol: Rh)