Rhodium

Rh redirects here. For other uses, see rh (disambiguation)

Rhodium (pronounced /ˈroʊdiəm/) 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^[2], though it is sometimes surpassed by rhenium, depending on the market.

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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.^[3] 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.

Applications

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.

History

Rhodium (Greek rhodon meaning "rose") was discovered in 1803 by William Hyde Wollaston,^[4] 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.

Occurrence

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 it 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. It is generally difficult to determine if a rock sample contains platinum group elements. 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,^[5] but in recent years it has increased to about $6,150/troy oz.^[6]

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.

Isotopes

Main article: isotopes of rhodium

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.

Precautions

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 highly toxic and carcinogenic^{[citation needed]}. Lethal intake (LD50) for rats is 12.6 mg/kg of rhodium chloride (RhCl₃)^{[citation needed]}. Rhodium compounds can stain human skin very strongly. The element plays no biological role in humans. However, if used plainly, without 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.

See also

• Rhodium compounds

References

1. ^ Rhodium: rhodium(I) fluoride compound data. OpenMOPAC.net. Retrieved on 2007-12-10.
2. ^ http://www.taxfreegold.co.uk/preciousmetalpricesindx.html
3. ^ Emsley, John (2001). Nature's Building Blocks, (Hardcover, First Edition), Oxford University Press, page 363. ISBN 0198503407. 
4. ^ WebElements - The History of Rhodium
5. ^ http://periodic.lanl.gov/elements/45.html
6. ^ KITCO Rhodium Price Charts