Platinum was discovered in South America independently by Antonio de Ulloa in 1735 and by N. Wood in 1741, but it had been in use by pre-Columbian Indians. The platinum metals are extremely rare elements; platinum itself is the most common, with an abundance in the Earth's crust of about a millionth of 1 percent, whereas the others of the group have abundances of about one ten millionth of 1 percent. Platinum occurs in nature as the pure metal and also in alloys with other metals of the group, principally in the alluvial deposits of the Ural Mountains, of Columbia, and of some parts of the western United States. In addition the element occurs in the mineral sperrylite and in the nickel bearing deposits of Ontario in Canada.
The large scale production of nickel makes it feasible to recover the small amounts of platinum (only 1 part of platinum to 2 million parts of ore). The recovery of the individual platinum metals from the natural alloys is a complex process that depends upon the distinct properties of the individual elements, even though all members of the group are generally similar in their chemical behavior and are quite unreactive. For example, when the natural alloy of platinum, palladium, osmium, and rhodium is digested with aqua regia, the palladium and platinum dissolve and are extracted as a solution of chloropalladic acid and chloroplatinic acid. Platinum is then precipitated from this solution as ammonium chloroplatinate.
Platinum is an attractive silvery white metal with a melting point of 1,774 deg C, a boiling point of 3,827 deg C and a density of 21.45 g/cu cm at room temperature. It has a coefficient of thermal expansion close to that of soda lime silica glass and is consequently used to make sealed electrodes in soft glass systems. The predominant oxidation states of platinum are +2 and +4. Platinum is chemically inert and will not oxidize in air at any temperature. It is resistant to acids and is not attacked by any single mineral acid but dissolves readily in aqua regia. The metal is rapidly attacked by fused alkali oxides and by peroxides and will react with fluorine and, at red heat, with chlorine. On heating, platinum combines directly with elemental phosphorus, silicon, lead, arsenic, antimony, sulfur, and selenium, a fact that influences the use of platinum laboratory equipment. In keeping with the other members of the platinum group, the metal shows a fairly strong tendency to form complex ions. The most common Pt(I) complexes have square planar structures and are diamagnetic.
Because of its inertness and attractive appearance, one of the major uses of platinum is in the manufacture of jewelry. Gold platinum alloys, referred to as white gold, are widely used in dentistry and in the making of jewelry. Platinum and its alloys are used in the manufacture of crucibles and evaporating dishes for chemical analyses. Other applications include the formation of thermocouple wires, electrical contacts, corrosion resistant apparatus, and the manufacture of platinum resistance thermometers used in the temperature control of furnaces. The alloy, consisting of 76.7% platinum and 23.3% cobalt by weight, forms an extremely powerful magnet. More recent applications of the metal involve the coating of missile cones and jet engine fuel nozzles. Along with palladium, the metal absorbs large volumes of hydrogen, retaining it at ordinary temperatures but desorbing it at red heat. The fact that the absorbed hydrogen is extremely reactive suggests that it is present either as atomic hydrogen or as a very reactive platinum hydrogen compound or complex. In finely divided form platinum is an excellent catalyst; about half of the annual production of the metal serves this purpose, most of it in the petrochemical industry. It is also used as a catalyst in the contact process for the manufacture of sulfuric acid from sulfur dioxide and oxygen. There is much current interest in the application of the metal as a catalyst in fuel cells and in catalytic converters as antipollution devices for automobiles. In the latter instance a suitable form of platinum will catalyze the oxidation of carbon monoxide to carbon dioxide and will convert nitric oxide to nitrogen and water. Fine platinum wire glows when placed in methyl alcohol vapor, where it acts as a catalyst, converting the alcohol to formaldehyde. This effect is used commercially to produce cigarette lighters and hand warmers. The price of platinum has varied widely relative to the price of gold. Although it is slightly more valuable today, it was once used as a gold adulterant.