PLATINUM, a very heavy steel-grey metal of great useful ness in the chemical and mechanical arts.
Platinum was probably known in the impure state from very early times, but because of its physical properties and especially its high melting point it was impossible to work it by any of the arts then known and it therefore remained as an unnamed sub stance. (Symbol Pt, atomic number 78, atomic weight 195.23.) It was probably the metal alluded to in the early part of the i6th century by Scaliger, who refers to a metal incapable of being fused obtained from the mines in Mexico and Darien. About the middle of the 18th century it was introduced into Europe in small quantities and attracted the notice of various chemists. Scheffer, in a paper read before the Stockholm Academy entitled "On white gold or the seventh metal, termed in Spanish platina del Pinto" (1752), showed that it was insoluble in nitric acid but soluble in aqua regia, was precipitated from its solutions by mercury, and was infusible at the highest temperature of the furnace. He also dealt with some of its alloys and discovered that it was fusible when mixed with arsenic. Platinum was melted by Macquer and Baume in 1758 in the focus of a powerful burning glass. Count von Sickingen in 1772 prepared the metal in the form of foil and wire and showed that it was soluble in nitric acid when alloyed with a large quantity of silver. For the later history of platinum, its occurrence and separation from the other metals, see PLATINUM METALS and RUTHENIUM, RHODIUM, PALLADIUM, OSMIUM and IRIDIUM.
Platinum in the massive state is a greyish-white metal having a specific gravity of 21.4 and is exceedingly malleable and ductile. It melts at about 1,755° C and volatilizes readily in the electric furnace. Its latent heat of fusion is 27.18 calories. It is a bad conductor of heat and electricity. When pure it is a soft metal and on this account is not suitable for some purposes, but on alloying it with iridium in small amounts the hardness is greatly increased. It is capable of being welded at a temperature near its
melting point, but for easier working at a lower temperature gold is used as a solder in repairing damaged crucibles and other articles. It alloys easily with all the other metals of its own group, likewise with gold, silver and lead, but is scarcely acted upon by metallic mercury. It has been obtained in the crystalline condition by distillation in the electric furnace or by heating the fluoride to a red heat (Moissan). Spongy platinum is readily produced by ignition of the double chloride it pos sesses a much greater surface area than the massive metal and hence is more active when employed in catalysis. Platinum black is best prepared by warming a solution of any of the chlorides or double chlorides of platinum in caustic alkali with alcohol or some similar reducing agent, or by dissolving the lead in nitric acid from a not very rich lead–platinum alloy. It has an enor mous surface area and hence is the most usual form in which platinum is employed in catalytic reactions. The platinum black, for convenience in handling and also to minimize loss, is usually deposited upon some suitable material such as asbestos or barium sulphate. Colloidal platinum can be obtained in solution by passing an electric arc between platinum terminals under pure water.
Uses.—Platinum is used in the laboratory in the form of dishes, crucibles and weights, although a small amount of iridium is generally added to increase its hardness and hence its durability. Platinum vessels, however, should never be used for heating caustic alkalis, as they undergo corrosion producing platinates, and no metal of low melting-point should ever be heated in them. At one time platinum vessels were almost exclusively used in concentrating sulphuric acid (q.v.) made by the chamber process ; this use has been abandoned as sulphuric acid made by the contact process is already concentrated, and the chamber acid is now concentrated in cheaper vessels.