PLATINUM

(Pt), chemical element, the best known and most widely used of the six platinum metals of Group VIII, Period 6, of the periodic table. A very heavy, precious, silver-white metal, platinum is soft and ductile and has a high melting point and good resistance to corrosion and chemical attack. For example, its surface remains bright after being brought to white heat in air, and, though it readily dissolves in aqua regia, it is scarcely attacked by simple acids. Small amounts of iridium are commonly added to give a harder, stronger alloy that retains the advantages of pure platinum. Platinum and its alloys are indispensable in the chemical laboratory for electrodes and for crucibles and dishes in which materials can be heated to high temperatures. Platinum is used for electrical contacts and sparking points because it resists both the high temperatures and chemical attack of electric arcs. Jewelry and dental alloys account for much of its use; platinum-iridium is used for surgical pins. The prototype international standard kilogram of mass was made from an alloy of 90 percent platinum and 10 percent iridium. The electrical resistivity of platinum is relatively high and depends markedly upon the temperature; the international temperature scale from -183 to 630 C (-297 to 1,166 F) is defined in terms of a resistance thermometer made with platinum wire. As a catalyst, platinum has many applications, notably in automotive catalytic converters and in petroleum refining. The Italian-French physician Julius Caesar Scaliger alluded (1557) to a refractory metal, probably platinum, found between Darin and Mexico. The first certain discovery was in the alluvial deposits of the Ro Pinto, Colombia. The Spaniards called the new metal platina del Pinto for its resemblance to silver. The world's most important deposits occur in the Transvaal of South Africa. Other deposits are found in Russia, Finland, Ireland, Borneo, New South Wales, New Zealand, Brazil, Peru, and Madagascar. In North America native platinum is found in Alaska, California, and Oregon, in British Columbia, and in Alberta. Placer deposits are the most productive sources of the native element. The ordinary variety of native platinum is called polyxene; it is 80 percent to 90 percent platinum, with 3 percent to 11 percent iron, plus the other platinum metals, and gold, copper, and nickel. For mineralogical properties, see native element (table). Platinum is also found in the very rare native alloy platiniridium. Platinum occurs combined with arsenic as sperrylite (PtAs2) in the coppernickel-mining district near Sudbury, Ont., and with sulfur as cooperite (PtS) in the Transvaal. For information about the mining, recovery, and production of platinum, see Industries, Extraction and Processing: Platinum group. Additional details on the application of pure platinum and its alloys are also provided in the same article. Platinum forms an important series of compounds with the oxidation states of +2 and +4. Many of these compounds contain coordination complexes in which chloride ion (Cl-), ammonia (NH3), or other groups are bonded to a central platinum atom. Among the transition metals, platinum has one of the greatest tendencies to form bonds directly with carbon. Natural platinum is a mixture of six isotopes: platinum-190 (0.0127 percent), platinum-192 (0.78 percent), platinum-194 (32.9 percent), platinum-195 (33.8 percent), platinum-196 (25.3 percent), and platinum-198 (7.21 percent). All are stable except platinum-190 and platinum-192, which have been reported as long-lived alpha emitters. atomic number 78 atomic weight 195.09 melting point 1,769 C (3,216 F) boiling point 3,827 C (6,920 F) specific gravity 21.45 (20 C) valence 2, 4 electronic config. 2-8-18-32-17-1