Platinum is highly prized because of its pleasing appearance, rarity, and resistance to tarnishing. Platinum also has many other uses, one of the most important of which is as a catalyst. Finely-divided platinum catalysts are used to produce pharmaceuticals, vitamins, and pollution control devices such as engine mufflers. Platinum or its alloys are used in resistance thermometers, electric furnaces, laboratory vessels, electrodes, electrical contacts, printed circuits, hearing aids, and self-winding watches.
Almost all platinum occurs in the native, or uncombined, state. It is usually found in basic, or alkaline, rocks, particularly those rich in the mineral olivine. Platinum is also found in stream beds and in sands that have been formed by the erosion of rocks. Such deposits occur in Alaska, Colombia, and the Soviet Union. Large amounts are obtained as a by-product of copper and nickel mining in Canada and of gold mining in South Africa. The world's leading producers of platinum are Canada, the Soviet Union, and South Africa.
Platinum is a soft silvery-white metal that can easily be drawn into wire and hammered into various shapes. Pure platinum is too soft for many uses, and it must be hardened by the addition of iridium or another metal of the platinum group. Platinum is one of the heaviest of the metals. Its density is 21.45 grams per cubic centimeter at 20° C.
Chemically, platinum is relatively unreactive. It does not oxidize in air and is not affected by most strong acids. However, it dissolves in hot aqua regia, a mixture of nitric and hydrochloric acids, and is attacked by sulfur, cyanides, caustic alkalies, such as sodium hydroxide, and halogens, such as chlorine and bromine.
Melting point 1772° C (3222° F) Boiling point 3827° C (6920° F) Oxidation numbers +2 and +4 Atomic weight 195.09 Atomic number 78
The most important platinum compound is chloroplatinic acid, H2PtCl6, which is formed by dissolving platinum in aqua regia. It is used in electroplating and for many other purposes. Other platinum compounds include platinic chloride, PtCl4, which is used in analytical chemistry, and platinic ammonium chloride (NH4)2PtCl6, used for electroplating and for producing porous platinum sponge.