Platinum (sym. Pt; atom. wt. 197.4; sp. gr. 21.48 to 21.50) is one of the 'noble metals.' It is found only in the native state, usually occurring in small glistening granules of a steel-gray colour, which always contain, along with some gold, copper, iron, and sand, an admixture, in varying proportions, of several metals—iridium, rhodium, palladium, osmium, ruthenium—most of which are rarely found except in association with platinum. Sometimes, however, it is found in masses of the size of a pigeon's egg, and pieces weighing ten or even twenty pounds have occasionally been found. It was originally found in the Spanish gold-mines of Darien, but is now chiefly obtained from the Ural Mountains, though it has also been found in Brazil, Colombia, San Domingo, California, Oregon, Canada, and Borneo.
There are two modes of obtaining platinum in the form of ingots from the crude native metal, both of which require notice. The earlier method—Cocks's, usually attributed to Wollaston (c. 1800-28)—was to precipitate a solution in aqua regia by a solution of sal ammoniac; the precipitate on incineration gave spongy platinum, which was pressed into form and hammered hot, so as to form an ingot, which it happens to be able to do, since platinum welds like wrought-iron when not too highly polished.
Deville and Debray's method is first to form a fusible alloy of this metal with lead, by exposing the platinum ore—2 cwt. being used in a single experiment, with equal weights of galena and litharge gradually added, and a little glass to act as a flux—to full redness in a reverberatory furnace lined with clay. The sulphur of the galena is oxidised and expelled, and the liquid alloy of lead and platinum is allowed to rest for some time, to allow the osmide of iridium, which is not affected by the preceding operations, to sink to the bottom. The upper portions of the alloy are then decanted, and cast into ingot-moulds, which are submitted to cupellation; and the metallic platinum which is left after the cupellation is melted and refined in a furnace of lime—which is employed in consequence of its being a very bad conductor of heat—by means of the oxyhydrogen blowpipe. The platinum obtained in this manner is nearly pure, and very ductile and malleable.
Platinum exhibits a bluish-white metallic lustre; it is exceedingly malleable and ductile, and is very infusible, melting only before the oxyhydrogen blowpipe, or in a very powerful blast-furnace, such as that used by Deville and Debray (fusing-point, 1779° C.). It expands less by heat than any other metal, and it is the heaviest form of matter known, with the exception of iridium (sp. gr. = 22.42) and osmium (22.48). It is unaffected by atmospheric action, and does not undergo oxidation in the air at even the highest temperatures. It is not acted on by nitric, hydrochloric, sulphuric, or hydrofluoric acid, or, in short, by any single acid; but in aqua regia it slowly dissolves, and forms a soluble tetrachloride. In consequence of its power of resisting the action of acids it is of great service in experimental and manufacturing chemical processes, platinum spatulas, capsules, crucibles, &c. being employed in every laboratory. Platinum is, however, corroded if heated with the alkalies or alkaline earths, and especially with a mixture of nitrate of potash and hydrated potash, an oxide being formed which combines with the alkaline bases. In consequence of its infusibility and its non-oxidisability by atmospheric action, it is in great demand for electrical as well as for chemical apparatus; and the recent introduction of platino-type processes in Photography (q.v.) has caused such a run upon platinum that its price has gone up from about 35s. per oz. in 1880 to 55s. in 1889, and 80s. in 1890.
The form of the metal known as spongy platinum has been already noticed. The metal may, however, be obtained in a state of subdivision much finer than that in which it is left on heating the double chloride of platinum and ammonium—viz. in the state known as Platinum Black. In this form it resembles soot. It may be prepared in various ways, of which one of the simplest is to boil a solution of bichloride of platinum, to which an excess of carbonate of soda and a quantity of sugar have been added, until the precipitate formed after a little time becomes perfectly black, and the supernatant liquid colourless. The black powder is then collected on a filter, washed, and dried by a gentle heat. In its finely comminuted state, either as spongy platinum or platinum black, it possesses a remarkable power of condensing and absorbing gases, one volume of platinum black being able to absorb more than 100 volumes of oxygen. This absorption appears to be accompanied by a conversion of some or all of the oxygen into the modification known as Ozone (q.v.), since the metal becomes capable of exerting the most energetic oxidising action, even at ordinary temperatures. For example, it can kindle a jet of hydrogen, can oxidise sulphurous acid into sulphuric acid, ammonia into nitric acid, and alcohol into acetic acid, the rise of temperature in the last case being often sufficiently great to cause inflammation. Platinum in the compact form, as foil or wire, possesses similar powers, but in a far lower degree.
Platinum may be easily alloyed with most of the metals, the alloys being in general much more fusible than pure platinum. Hence care must be taken not to heat the oxides of metals of easy reduction, such as lead and bismuth, in platinum crucibles, as, if any reduction took place, the crucible would be destroyed by the fusion of the resulting alloy. An alloy of platinum, iridium, and rhodium is found, by the investigations of Deville and Debray, to be harder and capable of resisting a higher temperature than the pure metal, and hence is admirably adapted for the formation of crucibles, &c.; but it is not now in the market.
There are two oxides of platinum, a protoxide, PtO, and a binoxide, PtO2. The sulphides and chlorides correspond in number and composition to the oxides. Of these compounds the tetrachloride, PtCl4, alone requires notice. It is formed by dissolving platinum in aqua regia, and evaporating the solution to dryness; and it is obtained as a deliquescent, reddish-brown mass, which forms an orange-coloured solution in water, from which, on evaporation, it crystallises in prisms. It is also freely soluble in alcohol and ether. A solution of this salt is much used for the recognition and determination of potash and ammonia.