Rhredirects here. For other uses, see rh (disambiguation)"Rhodium (pronEng|ˈroʊdiəm) is a chemical elementwith the symbol Rh and atomic number45. A rare, silvery-white hard transition metaland 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[ [http://www.taxfreegold.co.uk/preciousmetalpricesindx.html Precious Metal Prices Index ] ] .
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. Oxygenis absorbed from the atmosphereat the melting pointof rhodium, but on solidification the oxygen is released. [ cite book | last = Emsley | first = John | title = Nature's Building Blocks | edition = (Hardcover, First Edition) | publisher = Oxford University Press| date = 2001 | pages = page 363 | id = ISBN 0198503407 ] Rhodium has both a higher melting point and lower densitythan platinum. It is not attacked by acids: it is completely insoluble in nitric acidand 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 and improving the corrosion resistance [Cite book | author=Cramer, Stephen; S., Jr Covino, Bernard | authorlink= | coauthors= | title=ASM handbook | date=1990 | publisher=ASM International | location=Materials Park, OH | isbn=0-87170-707-1 | pages=393 | url=http://books.google.com/books?id=QV0sWU2qF5oC&printsec=frontcover&source=gbs_summary_r&cad=0] of
platinumand palladium. These alloys are used in furnace windings, bushings for glass fiber production, thermocoupleelements, electrodes for aircraft spark plugs, and laboratory crucibles. [Cite book | author= Lide, David R | coauthors= | title= CRC handbook of chemistry and physics : a ready-reference book of chemical and physical data | date=2004 | publisher=CRC Press | location=Boca Raton | isbn=0-8493-0485-7 | pages=4-26 | url=http://books.google.com/books?id=WDll8hA006AC] 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 corrosionresistance.
*Plated rhodium, made by
electroplatingor evaporation, is extremely hard and is used for optical instruments.
*This metal finds use in
jewelryand for decorations. It is electroplated on white goldand 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 silverin order to strengthen the metal from tarnish, as a result from the copper compound found in sterling silver.
*It is also a highly useful
catalystin 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 acidby 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
BINAPgives a widely used chiral catalyst for chiral synthesis, as in the synthesis of menthol.
*It is also used as a filter in
mammographysystems 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, Caran D'ache which are rather less famous than Montblanc, but produces very limited pens.
Rhodium (Greek "rhodon" meaning "rose") was discovered in 1803 by
William Hyde Wollaston, [ [http://www.webelements.com/webelements/elements/text/Rh/hist.html WebElements - The History of Rhodium] ] soon after his discovery of palladium. [cite journal
Platinum Metals Review
url = http://www.platinummetalsreview.com/dynamic/article/view/47-4-175-183
title = Rhodium and Palladium - Events Surrounding Its Discovery
author = W. P. Griffith
volume = 47
issue = 4
year = 2003
pages = 175–183] [cite journal
title = On a New Metal, Found in Crude Platina
first = W. H.
last = Wollaston
authorlink = William Hyde Wollaston
Philosophical Transactions of the Royal Society of London
volume = 94
year = 1804
pages = 419–430.
doi = 10.1098/rstl.1804.0019 ] He made this discovery in
Englandusing crude platinumore that he presumably obtained from South America. [Cite book | author= Lide, David R | coauthors= | title= CRC handbook of chemistry and physics : a ready-reference book of chemical and physical data | date=2004 | publisher=CRC Press | location=Boca Raton | isbn=0-8493-0485-7 | pages=4-26]
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 cyanideafter 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 hydrogengas.
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, and in North America, including the copper- nickel sulfidemining 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, [ [http://periodic.lanl.gov/elements/45.html http://periodic.lanl.gov/elements/45.html] ] but in recent years it has increased to about $4500/troy oz. [ [http://www.kitco.com/charts/rhodium.html KITCO Rhodium Price Charts] ] In 2008 the price briefly rose above $10,000 per ounce.
As a fission product
It is also possible to extract rhodium from
used nuclear fuel, which contains rhodium (1 kg of the fission productsof 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 decayto either 102Ru (80%) (some positron emissionwill occur) or 102Pd (20%) (some gamma rayphotons with about 500 keV are generated) and the excited state decays by beta decay (electron capture) to 102Ru (some gamma rayphotons 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-lifeof 3.3 years, 102Rh with a half-lifeof 207 days, 102mRh with a half-lifeof 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.
decay modebefore the only stable isotope, 103Rh, is electron captureand the primary mode after is beta emission. The primary decay productbefore 103Rh is rutheniumand 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
toxicand possibly carcinogenicFact|date=September 2007. Lethal intake (LD50) for rats is 12.6 mg/kg of rhodium chloride (RhCl3)Fact|date=September 2007. 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.
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 McCartneya 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 [ [http://wonderclub.com/gifts/outrage_deluxe.htm Outrage! Deluxe - The Most Expensive Board Game in the World] ] .
* [http://www.webelements.com/webelements/elements/text/Rh/index.html WebElements.com – Rhodium]
* [http://www.kitco.com/market/ Current Rhodium price]
* [http://www.americanelements.com/rh.html Rhodium Technical and Safety Data]
* [http://periodic.lanl.gov/elements/45.html Los Alamos National Laboratory – Rhodium]
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