Platinum

PLATINUM, symbol Pt, a metal of the platinum subgroup of group VIII of the periodic system; atomic weight 195.2, atomic number 78, specific gravity ~21.5, melting point 1,770°. Platinum is easily forged and drawn into thin wire. Platinum is softer than gold, but some of its alloys (with iridium) are distinguished by great hardness. The coefficient of thermal expansion of platinum is almost the same as that of glass, which is why it can be fused into it. Platinum is almost non-volatile when heated up to 1,300°. In nature, platinum is found in a native state, but mixed with other platinum metals. The main deposits of platinum are located in the Middle Urals. When ore is treated with aqua regia, platinum passes into solution, forming H₂PtCl₆. From a solution of its salts, platinum precipitates under the action of reducing agents (e.g., formaldehyde) in the form of a fine black powder, platinum black. Platinum, especially in the form of platinum black or spongy platinum, possesses strong catalytic properties in many reactions. For example, if platinum black is introduced into a mixture of oxygen and hydrogen, these gases combine rapidly. Hydrogen peroxide is rapidly decomposed by platinum black or sponge. When heated in air, platinum does not oxidize at all. Only a few elements and compounds act chemically on platinum: chlorine, carbon (e.g., in the sooty flame of a burner), sulfur and hydrogen sulfide, phosphorus, caustic alkalis, and compounds containing the CN group. Arsenic, antimony, and lead form low-melting alloys with platinum. In its compounds, platinum is divalent and tetravalent (for example, PtCl₄). Platinum has a sharply expressed tendency to enter into the composition of complex anions, e.g., (PtCl₆)". Application of Platinum. Some of the most important technical processes are based on the catalytic properties of platinum (platinized asbestos), for example, the contact method for producing sulfuric acid. Of greatest importance for technology are such properties of platinum as its chemical inertness, combined with malleability and a high melting point. Their application is the basis for: 1) the use of platinum vessels in laboratories and factories (concentration of sulfuric acid); when working with platinum vessels, one should keep in mind the destructive effect of the above-mentioned substances; 2) the use of platinum in the electrical industry for electrodes, thermocouples, etc.; 3) the use of platinum for various technical and jewelry alloys; an alloy of platinum with silver for artificial teeth has the same coefficient of expansion as dental mass; 4) among platinum salts, H₂PtCl₆ is an important reagent in quantitative analysis for K' and NH₄' ions, and for organic bases. Screens for X-ray rooms are impregnated with the fluorescent salt BaPt(CN)₄·4H₂O. At one time, attempts were made to use platinum for syphilis in the form of intravenous injections of small quantities of its double salts, but due to its high toxicity, treatment with platinum was abandoned.

A. Balandin.