Precious Metals Primer (Source: Los Alamos National Laboratory)
Platinum
Atomic Number: 78
Atomic Symbol: Pt
Atomic Weight: 195.09
History
(Sp. platina, silver) Discovered in South America by Ulloa in 1735
and by Wood in 1741. The metal was used by pre-Columbian Indians.
Sources
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.
Properties
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.
Uses
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
Atomic Number: 46
Atomic Symbol: Pd
Atomic Weight: 106.4
History
Palladium was named after the asteroid Pallas, which was discovered at
about the same time. Pallas was the Greek goddess of wisdom.
Sources
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 of Palladium.
Properties
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.
Uses
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 contacts.
Iridium
Atomic Number: 77
Atomic Symbol: Ir
Atomic Weight: 192.22
History
(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.
Properties
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.
Sources
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.
Uses
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 bearings.
Gold
Atomic Number: 79
Atomic Symbol: Au
Atomic Weight: 196.9665
History
(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.
Sources
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.
Properties
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.
Uses
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.