Rhodium is a chemical element with symbol Rh and atomic number 45. It
is a rare, silvery-white, hard, corrosion-resistant and chemically
inert transition metal. It is a noble metal and a member of the
platinum group. It has only one naturally occurring isotope, 103Rh.
Naturally occurring rhodium is usually found as the free metal,
alloyed with similar metals, and rarely as a chemical compound in
minerals such as bowieite and rhodplumsite. It is one of the rarest
and most valuable precious metals.
Rhodium is found in platinum or nickel ores together with the other
members of the platinum group metals. It was discovered in 1803 by
William Hyde Wollaston
William Hyde Wollaston in one such ore, and named for the rose color
of one of its chlorine compounds, produced after it reacted with the
powerful acid mixture aqua regia.
The element's major use (approximately 80% of world rhodium
production) is as one of the catalysts in the three-way catalytic
converters in automobiles. Because rhodium metal is inert against
corrosion and most aggressive chemicals, and because of its rarity,
rhodium is usually alloyed with platinum or palladium and applied in
high-temperature and corrosion-resistive coatings.
White gold is often
plated with a thin rhodium layer to improve its appearance while
sterling silver is often rhodium-plated for tarnish resistance.
Rhodium detectors are used in nuclear reactors to measure the neutron
2.1 Chemical properties
3.1 Mining and price
3.2 Used nuclear fuels
4.2 Ornamental uses
4.3 Other uses
6 See also
8 External links
William Hyde Wollaston
Rhodium (Greek rhodon (ῥόδον) meaning "rose") was discovered in
1803 by William Hyde Wollaston, soon after his discovery of
palladium. He used crude platinum ore presumably obtained
from South America. His procedure involved dissolving the ore in
aqua regia and neutralizing the acid with sodium hydroxide (NaOH). He
then precipitated the platinum as ammonium chloroplatinate by adding
ammonium chloride (NH
4Cl). Most other metals like copper, lead, palladium and rhodium were
precipitated with zinc. Diluted nitric acid dissolved all but
palladium and rhodium. Of these, palladium dissolved in aqua regia but
rhodium did not, and the rhodium was precipitated by the addition
of sodium chloride as Na
2O. After being washed with ethanol, the rose-red precipitate was
reacted with zinc, which displaced the rhodium in the ionic compound
and thereby released the rhodium as free metal.
After the discovery, the rare element had only minor applications; for
example, by the turn of the century, rhodium-containing thermocouples
were used to measure temperatures up to 1800 °C. The
first major application was electroplating for decorative uses and as
corrosion-resistant coating. The introduction of the three-way
catalytic converter by
Volvo in 1976 increased the demand for rhodium.
The previous catalytic converters used platinum or palladium, while
the three-way catalytic converter used rhodium to reduce the amount of
NOx in the exhaust.
No. of electrons/shell
2, 8, 15, 2
2, 8, 18, 16, 1
2, 8, 18, 32, 15, 2
2, 8, 18, 32, 32, 15, 2 (predicted)
Rhodium is a hard, silvery, durable metal that has a high reflectance.
Rhodium metal does not normally form an oxide, even when heated.
Oxygen is absorbed from the atmosphere only at the melting point of
rhodium, but is released on solidification.
Rhodium has both a
higher melting point and lower density than platinum. It is not
attacked by most acids: it is completely insoluble in nitric acid and
dissolves slightly in aqua regia.
Rhodium belongs to group 9 of the periodic table, but the
configuration of electrons in the outermost shells is atypical for the
group. This anomaly is also observed in the neighboring elements,
niobium (41), ruthenium (44), and palladium (46).
The common oxidation state of rhodium is +3, but oxidation states from
+0 to +6 are also observed.
Unlike ruthenium and osmium, rhodium forms no volatile oxygen
compounds. The known stable oxides include Rh
6 and Sr
Halogen compounds are known in nearly the full range of
possible oxidation states. Rhodium(III) chloride, rhodium(IV)
fluoride, rhodium(V) fluoride and rhodium(VI) fluoride are examples.
The lower oxidation states are stable only in the presence of
The best-known rhodium-halogen compound is the Wilkinson's catalyst
chlorotris(triphenylphosphine)rhodium(I). This catalyst is used in the
hydroformylation or hydrogenation of alkenes.
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-lives
shorter than an hour, except 100Rh (20.8 hours) and 105Rh (35.36
hours). It has numerous meta states, the most stable being 102mRh
(0.141 MeV) with a half-life of about 2.9 years and 101mRh
(0.157 MeV) with a half-life of 4.34 days (see isotopes of
In isotopes weighing less than 103 (the stable isotope), the primary
decay mode is electron capture and the primary decay product is
ruthenium In isotopes greater than 103, the primary decay mode is beta
emission and the primary product is palladium.
Rhodium is one of the rarest elements in the Earth's crust, comprising
an estimated 0.0002 parts per million (2 × 10−10). Its rarity
affects its price and its use in commercial applications.
Mining and price
Rh price evolution.
The industrial extraction of rhodium is complex because the ores are
mixed with other metals such as palladium, silver, platinum, and gold
and there are very few rhodium-bearing minerals. It is found in
platinum ores and extracted as a white inert metal that is difficult
to fuse. Principal sources are located in South Africa; in river sands
of the Ural Mountains; and in North America, including the
copper-nickel sulfide mining area of the Sudbury, Ontario, region.
Although the quantity at Sudbury is very small, the large amount of
processed nickel ore makes rhodium recovery cost-effective.
The main exporter of rhodium is South Africa (approximately 80% in
2010) followed by Russia. The annual world production is 30
tonnes. The price of rhodium is highly variable. In 2007, rhodium cost
approximately eight times more than gold, 450 times more than silver,
and 27,250 times more than copper by weight. In 2008, the price
briefly rose above $10,000 per ounce ($350,000 per kilogram). The
economic slowdown of the 3rd quarter of 2008 pushed rhodium prices
sharply back below $1,000 per ounce ($35,000 per kilogram); the price
rebounded to $2,750 by early 2010 ($97,000 per kilogram) (more than
twice the gold price), but in late 2013, the prices were less than
Political and financial problems[clarification needed] led to very low
oil prices and oversupply, causing most metals to drop in price. The
economies of China, India and other emerging countries slowed in 2014
and 2015. In 2014 alone, 23,722,890 motor vehicles were produced in
China, excluding motorbikes.[clarification needed] This resulted in a
rhodium price of 740.00 US-$ per
Troy ounce (31.1 grams) in late
Used nuclear fuels
Main article: Synthesis of precious metals
Rhodium is a fission product of uranium-235: each kilogram of fission
product contains a significant amount of the lighter platinum group
Used nuclear fuel
Used nuclear fuel is therefore a potential source of rhodium,
but the extraction is complex and expensive, and the presence of
rhodium radioisotopes requires a period of cooling storage for
multiple half-lives of the longest-lived isotope (about 10 years).
These factors make the source unattractive and no large-scale
extraction has been attempted.
The primary use of this element is in automobiles as a catalytic
converter, changing harmful unburned hydrocarbons, carbon monoxide,
and nitrogen oxide exhaust emissions into less noxious gases. Of
30,000 kg of rhodium consumed worldwide in 2012, 81%
(24,300 kg) went into this application, and 8,060 kg was
recovered from old converters. About 964 kg of rhodium was used
in the glass industry, mostly for production of fiberglass and
flat-panel glass, and 2,520 kg was used in the chemical
Rhodium is preferable to the other platinum metals in the reduction of
nitrogen oxides to nitrogen and oxygen:
x → x O
2 + N
Rhodium catalysts are used in a number of industrial processes,
notably in catalytic carbonylation of methanol to produce acetic acid
by the Monsanto process. It is also used to catalyze addition of
hydrosilanes to molecular double bonds, a process important in
manufacture of certain silicone rubbers.
Rhodium catalysts are
also used to reduce benzene to cyclohexane.
The complex of a rhodium ion with
BINAP is a widely used chiral
catalyst for chiral synthesis, as in the synthesis of menthol.
Rhodium finds use in jewelry and for decorations. It is electroplated
on white gold and platinum to give it a reflective white surface at
time of sale, after which the thin layer wears away with use. This is
known as rhodium flashing in the jewelry business. It may also be used
in coating sterling silver to protect against tarnish (silver sulfide,
Ag2S, produced from atmospheric hydrogen sulfide, H2S).
rhodium jewelry is very rare, more because of the difficulty of
fabrication (high melting point and poor malleability) than because of
the high price. The high cost ensures that rhodium is applied only
as an electroplate.
Rhodium has also been used for honors or to signify elite status, when
more commonly used metals such as silver, gold or platinum were deemed
insufficient. In 1979 the
Guinness Book of World Records
Guinness Book of World Records gave Paul
McCartney a rhodium-plated disc for being history's all-time
best-selling songwriter and recording artist.
Rhodium is used as an alloying agent for hardening and improving the
corrosion resistance of 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:
Electrical contacts, where it is valued for small electrical
resistance, small and stable contact resistance, and great corrosion
Rhodium plated by either electroplating or evaporation is extremely
hard and useful for optical instruments.
Filters in mammography systems for the characteristic X-rays it
Rhodium neutron detectors are used in combustion engineering nuclear
reactors to measure neutron flux levels – this method requires a
digital filter to determine the current neutron flux level, generating
three separate signals: immediate, a few seconds delay, and a minute
delay, each with its own signal level; all three are combined in the
rhodium detector signal. The three Palo Verde nuclear reactors each
have 305 rhodium neutron detectors, 61 detectors on each of five
vertical levels, providing an accurate 3D "picture" of reactivity and
allowing fine tuning to consume the nuclear fuel most
A 78 g sample of rhodium
Cross section of a metal-core catalytic converter
Rhodium-plated white gold wedding ring
Rhodium foil and wire
Being a noble metal, pure rhodium is inert. However, chemical
complexes of rhodium can be reactive.
Median lethal dose (LD50) for
rats is 198 mg of rhodium chloride (RhCl
3) per kilogram of body weight. Like the other noble metals, all
of which are too inert to occur as chemical compounds in nature,
rhodium has not been found to serve any biological function. In
elemental form, the metal is harmless.
People can be exposed to rhodium in the workplace by inhalation. The
Occupational Safety and Health Administration
Occupational Safety and Health Administration (OSHA) has specified the
legal limit (Permissible exposure limit) for rhodium exposure in the
workplace at 0.1 mg/m3 over an 8-hour workday, and the National
Institute for Occupational Safety and Health (NIOSH) has set the
recommended exposure limit (REL), at the same level. At levels of
100 mg/m3, rhodium is immediately dangerous to life or
health. For soluble compounds, the PEL and REL are both
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