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platinum[plat´unum] Pronunciation Key, metallic chemical element; symbol Pt; at. no. 78; at. wt. 195.08; m.p. 1,772°C; b.p. 3,827±100°C; sp. gr. 21.45 at 20°C; valence +2 or +4. Pure platinum is a malleable, ductile, lustrous, silver-white metal with a face-centered cubic crystalline structure. Chemically inactive, it is unaffected by common acids but dissolves in aqua regia, forming chloroplatinic acid (H2PtCl6). It is attacked by the halogens, sulfur, or caustic alkalies. It does not combine with oxygen even at high temperatures. Like palladium, it absorbs large quantities of hydrogen, which it releases at red heat. Platinum is found in nature alloyed with the other metals of the so-called platinum group, found in group VIII of the periodic table; the other five metals in this group are iridium, osmium, palladium, rhodium, and ruthenium. These metals are found in alluvial deposits in Russia, South Africa, Colombia, and Alaska. Platinum and the related metals are recovered commercially as a byproduct of the refining of nickel ores mined near Sudbury, Ont., Canada; from gold mines in South Africa; and from the alluvial deposits in Russia. There is no routine method for separating platinum from other metals; it is usually recovered by complex chemical methods. Platinum has many uses, especially in alloys. Most important of the alloys are those with iridium. Platinum and its alloys are used in surgical tools, laboratory utensils, electrical resistance wires, contact points, and standard masses. (The International Prototype Kilogram, kept at SEvres, France, is a cylinder of platinum-iridium alloy.) Because its thermal coefficient of expansion is nearly equal to that of glass, platinum is used to make electrodes sealed in glass. It is used extensively in jewelry and in dentistry. A platinum-cobalt alloy is used to make very powerful magnets. Platinum is specially prepared for use as a catalyst. Finely divided, the metal is platinum black, a powder. It also may be used as platinum sponge, formed when platinic ammonium chloride, (NH4)2PtCl6, is ignited, or as platinized asbestos, prepared by heating asbestos after dipping it in chloroplatinic acid. Platinum catalysts are used in the contact process for producing sulfuric acid, in the Ostwald process for the production of nitric acid, and in petroleum cracking, as well as in a variety of other reactions. Although natural platinum alloys may have been used several centuries before Christ, modern knowledge of the metal dates from about 1736, when its existence in South America was reported by A. de Ulloa. Some of this "platina" was taken to England, and soon thereafter many leading chemists published reports on it. A process discovered about 1803 by W. H. Wollaston for making the metal malleable made possible its commercial use for laboratory apparatus and other purposes. Although platinum was used as an adulterant for gold over a century ago, it is now considered the more valuable of the two.
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