Platinum and the platinum group metals
Where Found (Encyclopedia of Global Resources)
The platinum metals are extremely rare in the Earth’s crust. All occur together, with platinum and palladium predominating. The mineral sperrylite (platinum arsenide) is a major source in Canada. Significant deposits are also located in South Africa and the former Soviet Union. Smaller deposits have been found in Colombia, Australia, and the United States, chiefly in Alaska and Montana.
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Primary Uses (Encyclopedia of Global Resources)
Technical Definition (Encyclopedia of Global Resources)
Chemists generally refer to the block of six transition metals—ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt)—as the platinum metals. Their atomic numbers are, respectively, 44, 45, 46, 76, 77, and 78. Using the recommended group designations of the International Union of Pure and Applied Chemistry, ruthenium and osmium belong to Group 8, rhodium and iridium to Group 9, and palladium and platinum to Group 10. In the older system of group numbering, the platinum metals were placed in Group VIII.
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Description, Distribution, and Forms (Encyclopedia of Global Resources)
Ruthenium has seven naturally occurring isotopes with an average atomic mass of 101.07. It has another thirteen artificial (radioactive) isotopes. Pure ruthenium is a hard metal and has a gray-white appearance. Rhodium has only one natural isotope, with an atomic mass of 102.906. More than thirty artificial isotopes are known. Rhodium has a silvery white metallic luster. Palladium has six natural isotopes with an average atomic mass of 106.42. It has eighteen artificial isotopes. Palladium is a steel-white metal that does not tarnish in air. Osmium has seven natural isotopes and an average atomic mass of 190.2. It has nearly thirty artificial isotopes. The metal has a slight bluish color because of a thin surface film of the oxide. Iridium has only two natural isotopes, with an average atomic mass of 192.2. It has nearly forty artificial isotopes. The metal has a white appearance with a slight yellowish tinge and is hard and brittle. Platinum has six natural isotopes and an average atomic mass of 195.08. It has thirty artificial isotopes. It is a silvery-white metal with a lustrous appearance. Ruthenium, rhodium, and palladium all have densities of about 12 grams per cubic centimeter (12.45, 12.41, and 12.02, respectively), while osmium, iridium, and platinum are about twice as dense (22.61, 22.65, and 21.45 grams per cubic centimeter, respectively). The melting points increase in the order: palladium, platinum,...
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History (Encyclopedia of Global Resources)
Hundreds of years before Europeans explored the Americas, the Indians of Colombia and Ecuador used platinum-gold alloys to make small artifacts by heating and hammering the alloy. Because of platinum’s high melting point, it was not possible for these people to melt and work pure platinum. In their relentless search for gold in the late seventeenth century, the invading Spanish conquistadores discovered the Indians’ platinum. Being a white-colored metal it was called platina, derived from the Spanish word for silver. Initially it was considered a rather annoying contaminant rather than a precious metal. By the mid-eighteenth century, samples of platinum had reached Europe. In 1803, William Wollaston produced the first pure samples of the metal after dissolving crude platinum in aqua regia (a mixture of hydrochloric and nitric acids).
In the same year, Wollaston’s studies with platinum ores led to his discovery of two new metals, palladium and rhodium, in the samples of crude platinum. After dissolving the ore in aqua regia and neutralizing it with sodium hydroxide, he added ammonium chloride to remove the platinum as ammonium chloroplatinate. By then adding mercuric cyanide, he removed the palladium as palladium cyanide. Metallic palladium was recovered by reduction of the palladium cyanide compound. After the palladium cyanide was extracted, the residue was washed and dried; it yielded a red compound of rhodium,...
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Obtaining Platinum Metals (Encyclopedia of Global Resources)
Because of platinum metals’ low natural abundances and the difficulty in extracting them, commercial production of the platinum metals is often viewed as a by-product of the mining of other metals such as nickel, copper, and silver. For example, if not for the huge tonnage of nickel ore processed annually, the extraction of the rarer platinum metals would not be economically feasible. In general, the platinum metals are obtained by subjecting the ores to a series of complicated and costly chemical reactions. Not surprisingly, the platinum metals are among the most expensive of all elements to produce. Prices can fluctuate enormously depending on economic and environmental conditions. The high cost and rarity of the platinum metals are also responsible for their extensive recycling.
Platinum is obtained from crude ores by a process which eliminates other impurities: Magnetic metals such as iron and nickel are removed with powerful electromagnets; less dense impurities are removed by flotation methods in aqueous solution; volatile impurities are baked off at high temperatures; various acids dissolve away other metals. Pure platinum is obtained through additional chemical processes. The method for separating palladium from platinum is often determined by the type of ore being refined, but in general also involves a series of chemical processes to obtain the metal. Like platinum, iridium is separated by treating the...
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Uses of Platinum Metals (Encyclopedia of Global Resources)
Of the six platinum metals, palladium and platinum have the greatest economic importance. Both are fairly soft metals having a brilliant silvery appearance and are therefore widely used in the jewelry trade. When alloyed with palladium, gold takes on a silvery appearance (white gold) but will not tarnish as jewelry made from pure silver does. Palladium is also used in dentistry, surgical instruments, and electrical contacts. The mainsprings of many older wristwatches were fashioned from palladium. Powdered palladium is a good catalyst and is used for hydrogenation and dehydrogenation reactions. Platinum is used to make wires and vessels for laboratory use and as a coating on missile nose cones and jets, which are subject to very high temperatures. It is also used to make medical and dental alloys and electrical contacts. Finely divided platinum powder is an excellent catalyst that is used in the production of sulfuric acid and in petroleum refining.
The uses of the other four platinum metals are very limited. Their major use is in alloys, and most have some catalytic activity. All are fairly brittle metals and therefore difficult to machine into shapes when pure. Ruthenium, rhodium, and iridium are all used as hardening agents for softer platinum and palladium. Osmium is used to strengthen alloys where frictional wear must be minimized as in electrical switch contacts, ballpoint pen tips, phonograph needles, and...
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Further Reading (Encyclopedia of Global Resources)
Greenwood, N. N., and A. Earnshaw. “Nickel, Palladium, and Platinum.” In Chemistry of the Elements. 2d ed. Boston: Butterworth-Heinemann, 1997.
Heiserman, David L. Exploring Chemical Elements and Their Compounds. Blue Ridge Summit, Pa.: Tab Books, 1992.
Lide, David R., ed. CRC Handbook of Chemistry and Physics: A Ready-Reference Book of Chemical and Physical Data. 85th ed. Boca Raton, Fla.: CRC Press, 2004.
McDonald, Donald, and Leslie B. Hunt. A History of Platinum and Its Allied Metals. London: Johnson Matthey, 1982.
Weeks, Mary Elvira. Discovery of the Elements. 7th ed. New material added by Henry M. Leicester. Easton, Pa.: Journal of Chemical Education, 1968.
Natural Resources Canada. Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews. http://www.nrcan-rncan.gc.ca/mms-smm/busi-indu/cmy-amc/com-eng.htm
U.S. Geological Survey. Platinum-Group Metals: Statistics and Information. http://minerals.usgs.gov/minerals/pubs/commodity/platinum
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