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Precious Metals
Platinum
Atomic Number: 78
Atomic Symbol: Pt
Atomic Weight: 195.09
History
(Sp. platina, silver) Pre-Columbian Indians used platinum but it was re-discovered in South America by Ulloa in 1735 and by Wood in 1741.
Sources
Most of the world's platinum comes from South African mines, but platinum is also found in its native form in alluvial deposits of the mountains of Russia, Canada, United States and Columbia. These deposits of platinum are often accompanied by small quantities of iridium, osmium, palladium, ruthenium, and rhodium - a related group of metals also known as Platinum Group of Metals or PGM. Sperrylite, occurring with the nickel-bearing deposits of Sudbury, Canada, is another source of platinum.
Platinum is very rare and heavily recycled to keep up with market demands.
Properties
Platinum is a malleable and ductile silvery-white metal. It has a coefficient of expansion similar to that of soda-lime-silica glass, making platinum useful for sealed electrodes in glass systems. Platinum does not oxidize in air at any temperature, but is corroded by halogens, cyanides, sulfur, and caustic alkalis. Platinum is insoluble in hydrochloric and nitric acid, but dissolves when they are mixed as aqua regia, forming chloroplatinic acid. Alloyed with even small quantities of cobalt, platinum becomes magnetic. Like palladium, platinum absorbs large volumes of hydrogen, retaining it at ordinary temperatures but giving it up when heated.
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.
Uses
Platinum is mostly used in jewelry, automobile manufacturing, specialty wire (electrodes, thermocouples, etc.), and vessels for laboratory use. Platinum resistance wires are used for constructing high-temperature electric furnaces. Platinum is used to coat missile nose cones and jet engine fuel nozzles, which must perform reliably at high temperatures for long periods of time. 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.
Palladium
Atomic Number: 46
Atomic Symbol: Pd
Atomic Weight: 106.4
History
Palladium was named after the asteroid Pallas, which was discovered in 1802.
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
Palladium is a steel-white metal. It does not tarnish in air, but oxidizes heavily when heated. Palladium 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, Palladium 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 means for its purification.
Uses
Finely divided palladium is a good catalyst and is used for hydrogenation and dehydrogenation reactions. When alloyed, palladium is used in jewelry production. Like gold, palladium can be beaten into leaf as thin as 1/250,000 in. Palladium is used in automobile manufacturing, dentistry, jewelry, watchmaking, and in making surgical instruments and electrical contacts.
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 seawater 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 seawater 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 square feet of sheet or drawn out to 50 miles of fine wire. It is a soft metal and is usually alloyed to give it more strength. Gold is a good conductor of heat and electricity; it is a good reflector of infrared and is inert.
Uses
Gold is used in coinage and was a standard for monetary systems in many countries. Gold is also extensively used for jewelry, decoration, dental work, and for plating. In addition gold is used for coating certain space satellites.
The most common gold compounds are auric chloride and chlorauric acid, the latter used in traditional 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.
Karat (K) is a measure of gold’s purity. 14K and 18K gold alloys are most popular in the United States. Pure gold is mixed with copper and silver to achieve specific color, cost and hardness levels. Add more copper – it will be reddish, more silver – it will be whitish, with yellow and green shades in between. The karat scale is 24 karats, so 18K has 75% gold in it.
Silver
Atomic Number: 47
Atomic Symbol: Ag
Atomic Weight: 107.868
History
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.
Sources
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.
Properties
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.
Uses
Sterling silver (.925 pure silver) is used for jewelry, silverware, etc. where appearance is paramount. Silver is of the utmost importance in traditional photography. It is widespread in dentistry. 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, silver 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 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.
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