(Sp. platina, silver) Discovered in South America by Ulloa in 1735 and by Wood in
1741. The metal was used by pre-Columbian Indians.
Platinum occurs native, accompanied by small quantities of iridium, osmium, palladium,
ruthenium, and rhodium, all belonging to the same group of metals (PGM). These are
found in the alluvial deposits of the Ural mountains, of Columbia, and of certain
western American states. Sperrylite, occurring with the nickel-bearing deposits
of Sudbury, Ontario, is the source of a considerable amount of metal. Most of the
world's platinum, however, comes from the South African mines.
Platinum is a beautiful silvery-white metal, when pure, and is malleable and ductile.
It has a coefficient of expansion almost equal to that of soda-lime-silica glass,
and is therefore used to make sealed electrodes in glass systems. The metal does
not oxidize in air at any temperature, but is corroded by halogens, cyanides, sulfur,
and caustic alkalis. It is insoluble in hydrochloric and nitric acid, but dissolves
when they are mixed as aqua regia, forming chloroplatinic acid.
The metal is extensively used in jewelry, wire, and vessels for laboratory use,
and in many valuable instruments including thermocouple elements. It is also used
for electrical contacts, corrosion-resistant apparatus, and in dentistry.
Platinum-cobalt alloys have magnetic properties. Platinum resistance wires are used
for constructing high-temperature electric furnaces.
The metal is used for coating missile nose cones, jet engine fuel nozzles, etc.,
which must perform reliably at high temperatures for long periods of time. The metal,
like palladium, absorbs large volumes, of hydrogen, retaining it at ordinary temperatures
but giving it up when heated.
In the finely divided state platinum is an excellent catalyst, having long been
used in the contact process for producing sulfuric acid. It is also used as a catalyst
in cracking petroleum products. Much interest exists in using platinum as a catalyst
in fuel cells and in antipollution devices for automobiles.
Platinum anodes are extensively used in cathodic protection systems for large ships
and ocean-going vessels, pipelines, steel piers, etc. Fine platinum wire will glow
red hot when placed in the vapor of methyl alcohol. It acts here as a catalyst,
converting the alcohol to formaldehyde. The phenomenon has been used commercially
to produce cigarette lighters and hand warmers. Hydrogen and oxygen explode in the
presence of platinum.
Palladium was named after the asteroid Pallas, which was discovered at about the
same time. Pallas was the Greek goddess of wisdom.
Discovered in 1803 by Wollaston, Palladium is found with platinum and other metals
of the platinum group in placer deposits of Russia, South America, North America,
Ethiopia, and Australia. It is also found associated with the nickel-copper deposits
of South Africa and Ontario. Palladium's separation from the platinum metals depends
upon the type of ore in which it is found. Russia is the largest world supplier
The element is a steel-white metal, it does not tarnish in air, and it is the least
dense and lowest melting of the platinum group of metals. When annealed, it is soft
and ductile; cold-working greatly increases its strength and hardness. Palladium
is attacked by nitric and sulfuric acid.
At room temperatures, the metal has the unusual property of absorbing up to 900
times its own volume of hydrogen, possibly forming Pd2H. It is not yet clear if
this is a true compound. Hydrogen readily diffuses through heated palladium, providing
a means of purifying the gas.
Finely divided palladium is a good catalyst and is used for hydrogentation and dehydrogenation
reactions. It is alloyed and used in jewelry trades.
White gold is an alloy of gold decolorized by the addition of palladium. Like gold,
palladium can be beaten into leaf as thin as 1/250,000 in. The metal is used in
dentistry, jewelry, watchmaking, and in making surgical instruments and electrical
(L. iris, rainbow) Tennant discovered iridium in 1803 in the residue left when crude
platinum is dissolved by aqua regia. The name iridium is appropriate because its
salts are highly colored.
Iridium, a metal of the platinum family, is white, similar to platinum, but with
a slight yellowish cast. Because iridium is very hard and brittle, it is hard to
machine, form, or work.
It is the most corrosion-resistant metal known, and was used in making the standard
meter bar of Paris, which is a 90 percent platinum and 10 percent iridium alloy.
This meter bar was replaced in 1960 as a fundamental unit of length (see under Krypton).
Iridium is not attacked by any of the acids nor by aqua regia, but is attacked by
molten salts, such as NaCl and NaCN. The specific gravity of iridium is only very
slightly lower than osmium, which is generally credited as the heaviest known element.
Calculations of the densities of iridium and osmium from the space lattices give
values of 22.65 and 22.61 g/cm^3, respectively. These values may be more reliable
than actual physical measurements. At present, therefore, we know that either iridium
or osmium is the densest known element, but the data do not yet allow selection
between the two.
Iridium occurs uncombined in nature with platinum and other metals of this family
in alluvial deposits. It is recovered as a by-product from the nickel mining industry.
Although its principal use is as a hardening agent for platinum, iridium is also
used to make crucibles and devices requiring high temperatures. It is also used
for electrical contacts.
The element forms an alloy with osmium which is used for tipping pens and compass
(Sanskrit Jval; Anglo-Saxon gold; L. aurum, gold) Known and highly valued from earliest
times, gold is found in nature as the free metal and in tellurides; it is very widely
distributed and is almost always associated with quartz or pyrite.
It occurs in veins and alluvial deposits, and is often separated from rocks and
other minerals by mining and panning operations. About two thirds of the world's
gold output comes from South Africa, and about two thirds of the total U.S. production
comes from South Dakota and Nevada. The metal is recovered from its ores by cyaniding,
amalgamating, and smelting processes. Refining is also frequently done by electrolysis.
Gold occurs in sea water to the extent of 0.1 to 2 mg/ton, depending on the location
where the sample is taken. As yet, no method has been found for recovering gold
from sea water profitably.
It is estimated that all the gold in the world, so far refined, could be placed
in a single cube 60 ft. on a side. Gold is metallic, having a yellow color when
in a mass, but when finely divided it may be black, ruby, or purple. It is the most
malleable and ductile metal; 1 oz. of gold can be beaten out to 300 ft^2. It is
a soft metal and is usually alloyed to give it more strength. It is a good conductor
of heat and electricity, and is unaffected by air and most reagents.
It is used in coinage and was a standard for monetary systems in many countries.
It is also extensively used for jewelry, decoration, dental work, and for plating.
It is used for coating certain space satellites, as it is a good reflector of infrared
and is inert.
The most common gold compounds are auric chloride and chlorauric acid, the latter
being used in photography for toning the silver image. Gold has 18 isotopes; 198Au,
with a half-life of 2.7 days, is used for treating cancer and other diseases. Disodium
aurothiomalate is administered intramuscularly as a treatment for arthritis. A mixture
of one part nitric acid with three of hydrochloric acid is called aqua regia (because
it dissolved gold, the King of Metals). Gold is available commercially with a purity
of 99.999+%. For many years the temperature assigned to the freezing point of gold
has been 1063.0C; this has served as a calibration point for the International Temperature
Scales (ITS-27 and ITS-48) and the International Practical Temperature Scale (IPTS-48).
In 1968, a new International Practical Temperature Scale (IPTS-68) was adopted,
which demands that the freezing point of gold be changed to 1064.43C. The specific
gravity of gold has been found to vary considerably depending on temperature, how
the metal is precipitated, and cold-worked.
The Latin word for silver is argentum. Silver has been known since ancient times.
It is mentioned in Genesis. Slag dumps in Asia Minor and on islands in the Aegean
Sea indicate that man learned to separate silver from lead as early as 3000 B.C.
Silver occurs natively and in ores such as argentite (Ag2S) and horn silver (AgCl);
lead, lead-zinc, copper, gold, and copper-nickel ores are principal sources. Mexico,
Canada, Peru, and the U.S. are the principal silver producers in the western hemisphere.
Pure silver has a brilliant white metallic luster. It is a little harder than gold
and is very ductile and malleable, being exceeded only by gold and perhaps palladium.
Pure silver has the highest electrical and thermal conductivity of all metals, and
possesses the lowest contact resistance. It is stable in pure air and water, but
tarnishes when exposed to ozone, hydrogen sulfide, or air containing sulfur. The
alloys of silver are important.
terling silver is used for jewelry, silverware, etc. where appearance is paramount.
This alloy contains 92.5% silver, the remainder being copper or some other metal.
Silver is of the utmost importance in photography, about 30% of the U.S. industrial
consumption going into this application. It is used for dental alloys. Silver is
used in making solder and brazing alloys, electrical contacts, and high capacity
silver-zinc and silver-cadmium batteries. Silver paints are used for making printed
circuits. It is used in mirror production and may be deposited on glass or metals
by chemical deposition, electrode position, or by evaporation. When freshly deposited,
it is the best reflector of visible light known, but is rapidly tarnished and loses
much of its reflectance. It is a poor reflector of ultraviolet. Silver fulminate,
a powerful explosive, is sometimes formed during the silvering process. Silver iodide
is used in seeding clouds to produce rain. Silver chloride has interesting optical
properties as it can be made transparent; it also is a cement for glass. Silver
nitrate, or lunar caustic, the most important silver compound, is used extensively
in photography. Silver for centuries has been used traditionally for coinage by
many countries of the world. In recent times, however, consumption of silver has
greatly exceeded the output.