PLATINUM (Pt). The natural history of this metal has already been given. [PLATINUM, in NAT. HIST. DIV.] It has been very much used during the last fifty years in those experimental and manufacturing operations in which vessels are required that are not acted upon by hydrochloric, nitric, or sulphuric acids. Platinum knives, crucibles, capsules, sad dishes, are commonly employed ; while stills weighing from five hundred to ens thousand ounces are not unfrequently made use of for concentrating oil of vitriol.
The metallurgy of platinum has of late years received considerable attention, and been greatly improved by MM. Devine and Debray. Their memoir appears in the Annales de Chimie et de Physique' for August, 1859, and is abstracted in the Pharmaceutical Journal' and Chemical Newa ' : its title is, " On Platinum and its Associated Metals." The following ,table exhibits at a glance the composition of platinum ores from various sources ; for details concerning methods of analysis, see the paper above referred to.
The preparation of pure platinum from the ore is effected as follows. The ore is fused in a reverberatory furnace with an equal weight of galena, care being taken to avoid as far as possible any rapid oxidation of the lead. The mass is well mixed so as to convert all the iron into sulphide, and to produce an alloy of the platinum with lead ; some glass is then added, and sufficient lithargo to effect the decomposition of all the sulphides. The metal is then allowed to remain perfectly undisturbed for some time in order that the osmides may settle down and collect in the lower portion of the mass. The scoria is then removed ; the alloy of platinum and lead run off, except the lowermost portion which contains osmium, and the lead finally removed by the ordinary process of cupellation. The fusion and refining of the resulting platinum are effected in furnaces made of solid lime and heated by a large jet of oxygen and hydrogen gases.
Nearly all the platinum at present met with in commerce contains traces of osmium and a little silieium, but these may be perfectly removed by prolonged fusion in the oxyhydrogen furnace just mentioned.
Pure platinum is a white metal of the same softness as copper, and is susceptible of high polish. It slowly volatilises at a temperature considerably above its melting point, and, on quickly cooling, bubbles and spirts like silver. In tenacity and ductility it is superior to all metals except iron. Its specific gravity is 2115; or after hammering, 21-5. As a conductor of heat and electricity, it is inferior to gold and silver. It has the property of condensing gases on its surface, and when once heated to redness remains red hot for some time if a jet of cold hydrogen or coal gas be impinged upon it. Platinum is not tarnished by exposure to air, is unaffected by all acids except aqua regia, and Is not acted upon by dry chlorine. It very readily combines with other metals, so that nothing that contains, or is likely to yield, a metal as such, should be heated in vessels of platinum. Alkalies also corrode it through formation of a double oxide.
Chlorides of Platinum.—Platinum dissolves in a mixture of three ,arts hydrochloric acid and one part nitric acid. Such a mixture aqua regia) contains free chlorine, which unites with the platinum to 'orm bichloride of platinum (Pt CIO. The latter may be obtained in deliquescent priamatio crystals on evaporation. It is very 'soluble in water, alcohol, or ether. On heating it to 450* Fehr., halt the chlorine is expelled, and pro/ea/wide of p/afdnwie (Pt C1) remains. Both eblondris form crystalline double salts with alkaline chlorides. The potassio-cAloride of pier'inuat (KCI, Pt C1,1 is so sparingly soluble in water, that its formation is of considerable value as a qualitative test for potassium. Its insolubility in alcohol and ether admits of its being used as a means of quantitatively estimating potaasium. The PM mortise chloride of platin min (N 11,C1, Pt Cl„,) is isomorphous with the potassium salt, and is thrown down as a crystalline yellow precipitate on adding together tolerably strong solutions of blchloricle of platinum and chloride of ammonium.