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= Rhodium =
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Introduction
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Rhodium is a chemical element; it has symbol Rh and atomic number 45.
It is a very rare, silvery-white, hard, corrosion-resistant transition
metal. It is a noble metal and a member of the platinum group. It has
only one naturally occurring isotope, which is 103Rh. Naturally
occurring rhodium is usually found as a free metal or as an alloy 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 a group 9 element (cobalt group).
Rhodium is found in platinum or nickel ores with the other members of
the platinum group metals. It was discovered in 1803 by William Hyde
Wollaston in one such ore, and named for the rose color of one of its
chlorine compounds.
The element's major use (consuming about 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 to resist tarnishing.
Rhodium detectors are used in nuclear reactors to measure the neutron
flux level. Other uses of rhodium include asymmetric hydrogenation
used to form drug precursors and the processes for the production of
acetic acid.
History
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Rhodium (from , meaning 'rose') was discovered in 1803 by William
Hyde Wollaston, soon after he discovered palladium. He used crude
platinum ore presumably obtained from South America. His procedure
dissolved the ore in aqua regia and neutralized the acid with sodium
hydroxide (NaOH). He then precipitated the platinum as ammonium
chloroplatinate by adding ammonium chloride (). 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 . 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.
For decades, 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. They have
exceptionally good stability in the temperature range of 1300 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.
Characteristics
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!Z !! Element !! No. of electrons/shell
27 cobalt 2, 8, 15, 2
45 rhodium 2, 8, 18, 16, 1
77 iridium 2, 8, 18, 32, 15, 2
109 meitnerium 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 exhibits an
atypical ground state valence electron configuration for that group,
having only one electron in its outermost 's' orbital. This anomaly
is also observed in the neighboring elements niobium (41), ruthenium
(44), and palladium (46).
Chemical properties
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colspan=2|Oxidation states of rhodium
+0 {{chem|Rh|4|( CO)|12}
|-
| +1 ||
|-
| +2 ||
|-
| +3 ||
|-
| +4 ||
|-
| +5 ||
|-
| +6 ||
|}
The common oxidation states of rhodium are +3 and +1. Oxidation states
0, +2, and +4 are also well known. A few complexes at still higher
oxidation states are known.
The rhodium oxides include , , , , and . None are of technological
significance.
All the Rh(III) halides are known but the hydrated trichloride is most
frequently encountered. It is also available in an anhydrous form,
which is somewhat refractory. Other rhodium(III) chlorides include
sodium hexachlororhodate, , and pentaamminechlororhodium dichloride, .
They are used in the recycling and purification of this very expensive
metal. Heating a methanolic solution of hydrated rhodium trichloride
with sodium acetate give the blue-green rhodium(II) acetate, , which
features a Rh-Rh bond. This complex and related rhodium(II)
trifluoroacetate have attracted attention as catalysts for
cyclopropanation reactions. Hydrated rhodium trichloride is reduced by
carbon monoxide, ethylene, and trifluorophosphine to give rhodium(I)
complexes (L = CO, ). When treated with triphenylphosphine, hydrated
rhodium trichloride converts to the maroon-colored , which is known as
Wilkinson's catalyst. Reduction of rhodium carbonyl chloride gives
hexarhodium hexadecacarbonyl, , and tetrarhodium dodecacarbonyl, , the
two most common Rh(0) complexes.
As for other metals, rhodium forms high oxidation state binary
fluorides. These include rhodium pentafluoride, a tetrameric complex
with the true formula ) and rhodium hexafluoride.
Isotopes
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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). Rhodium 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 rhodium).
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.
Occurrence
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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. The concentration of
rhodium in nickel meteorites is typically 1 part per billion. Rhodium
has been measured in some potatoes with concentrations between 0.8 and
30 ppt.
Mining and price
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Rhodium ores are a mixture with other metals such as palladium,
silver, platinum, and gold. Few rhodium minerals are known. The
separation of rhodium from the other metals poses significant
challenges. Principal sources are located in South Africa, river sands
of the Ural Mountains in Russia, and in North America, especially the
copper-nickel sulfide mining area of the Sudbury, Ontario, region.
Although the rhodium abundance 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.
Used nuclear fuels
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Rhodium is a fission product of uranium-235: each kilogram of fission
product contains a significant amount of the lighter platinum group
metals. 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 (101Rh with a
half-life of 3.3 years, and 102mRh with a half-life of 2.9 years), or
about 10 years. These factors make the source unattractive and no
large-scale extraction has been attempted.
Applications
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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 industry.
In 2008, net demand (with the recycling accounted for) of rhodium for
automotive converters made up 84% of the world usage, with the number
fluctuating around 80% in 2015−2021.
Carbonylation
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Rhodium catalysts are used in some industrial processes, notably those
involving carbon monoxide. In the Monsanto process, rhodium iodides
catalyze the carbonylation of methanol to produce acetic acid. This
technology has been significantly displaced by the iridium-based
Cativa process, which effects the same conversion but more
efficiently. Rhodium-based complexes are the dominant catalysts for
hydroformylation, which converts alkenes to aldehydes according to the
following equation:
:
Rh-based hydroformylation underpins the industrial production of
products as diverse as detergents, fragrances, and some drugs.
Originally hydroformylation relied on much cheaper cobalt
carbonyl-based catalysts, but that technology has largely been
eclipsed by rhodium-based catalysts despite the cost differential.
Rhodium is also known to catalyze many reactions involving hydrogen
gas and hydrosilanes. These include hydrogenations and
hydrosilylations of alkenes. Rhodium metal, but not rhodium complexes,
catalyzes the hydrogenation of benzene to cyclohexane.
Ornamental uses
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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). Solid (pure)
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' gave Paul McCartney a rhodium-plated disc for being history's
all-time best-selling songwriter and recording artist.
Other uses
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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
resistance.
* 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
produces.
* Rhodium neutron detectors are used in 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 economically.
In automobile manufacturing, rhodium is also used in the construction
of headlight reflectors.
File:Rhodium 78g sample.jpg|A 78 g sample of rhodium
Image:Aufgeschnittener Metall Katalysator für ein Auto.jpg|Cut-away of
a metal-core catalytic converter
Image:White-gold--rhodium-plated.jpg|Rhodium-plated white gold wedding
ring
Image:Rhodium foil and wire.jpg|Rhodium foil and wire
Precautions
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Being a noble metal, pure rhodium is inert and harmless in elemental
form. However, chemical complexes of rhodium can be reactive. For
rhodium chloride, the median lethal dose (LD50) for rats is 198 mg ()
per kilogram of body weight. Like the other noble metals, rhodium has
not been found to serve any biological function.
People can be exposed to rhodium in the workplace by inhalation. The
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 0.001 mg/m3.
See also
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* 2000s commodities boom
* 2020s commodities boom
* Bullion
* Bullion coin
* Rhodium compounds
External links
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* [
http://www.periodicvideos.com/videos/045.htm Rhodium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
https://www.americanelements.com/rh.html Rhodium Technical and
Safety Data]
* [
https://www.cdc.gov/niosh/npg/npgd0544.html CDC - NIOSH Pocket
Guide to Chemical Hazards]
License
=========
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License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Rhodium
