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May
11, 2009 - Eitan Cohen, owner of Cohen Mint, the first
ever to produce investment rhodium bullion coins.
Rhodium is a chemical
element that is a rare, silvery-white, hard, and
chemically inert transition metal and a member of the
platinum group. |
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It has the chemical symbol Rh
and atomic number 45. It is composed of only one isotope,
103Rh. Naturally occurring rhodium is found as the free
metal, alloyed with similar metals, and never as a chemical
compound. It is one of the rarest precious metals and the
most costly (recently took over gold's and platinum's top
spots of cost per ounce).
Rhodium is a so-called noble metal, resistant to corrosion,
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 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 (about 81% of world rhodium
production) is as one of the catalysts in the three-way
catalytic converters of 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 optical impression
while sterling silver is often rhodium plated for tarnish
resistance.
Rhodium detectors are used in nuclear reactors to measure
the neutron flux level.
Rhodium 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 may also be used in coating
sterling silver to protect against tarnish, which is silver
sulfide (Ag2S) produced from the atmospheric hydrogen
sulfide (H2S). Solid (pure) rhodium jewelry is very rare,
because the metal has both high melting point and poor
malleability (making such jewelry very hard to fabricate)
rather than due to its high price.[37] Additionally, its
high cost assures that most of its jewelry usage is in the
form of tiny amounts of powder (commonly called rhodium
sponge) dissolved into electroplating solutions.
Rhodium has also been used for honors 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. |