Platinum (Sp. platina, little silver), a grayish white metal, distinguished by its great specific gravity and difficult fusibility, discovered by Wood, an assayer of Jamaica, in 1741. It occurs in the native state alloyed with palladium, rhodium, iridium, osmium, and ruthenium, and a little iron, in the form of small flattened grains; sometimes in larger nodules, alloyed with gold and traces of silver, and with copper, iron, and lead. It is found in alluvial districts, in the debris of the earliest volcanic rocks, on the slopes of the Ural mountains in Russia, in Brazil, Santo Domingo, Borneo, Ceylon, California, and Australia. In Nizhni Tagilsk, Ural mountains, it has been found with chromite in serpentine. Russia affords annually about 800 cwts. of platinum, which is nearly ten times the amount derived from Brazil, Colombia, Santo Domingo, and Borneo; the yield from Borneo is from 600 to 800 lbs. annually. It has been found in the sands of the Rhine; at St. Aray, valley of the Drac, in the French Alps; in the county of Wicklow, Ireland; in Honduras; in Rutherford co., North Carolina; and at St. Francois Beauce, in the province of Quebec. Although almost always in small grains, it has been found in masses of considerable size; one weighing 1,088 grains, and having a specific gravity of 18-94, was brought by Humboldt from South America and deposited in the Berlin museum.
In 1822 a specimen from Condoto was placed in the Madrid museum which was 2 1/3 in. in diameter and weighed 11,641 grains. Amass was found in 1827 in the Ural which weighed 10 9/16 Russian pounds, or 11.57 lbs. troy. The largest mass ever found weighs 21 lbs. troy, and is in the Demidoff cabinet. The grains of native platinum usually contain from 75 to 85 per cent, of the pure metal. - The extraction of platinum is somewhat difficult, the ordinary method being that devised by Wollaston, which is as follows. The lighter impurities are separated by washing, and the washed ore is first digested in nitric and then in hydrochloric acid, to remove the more easily oxidizable metals. It is then digested at a moderate heat in nitro-muriatic acid containing an excess of hydrochloric acid, and slightly diluted in order to dissolve as little iridium as possible. The yellowish red acid solution is then decanted and treated with sal ammoniac dissolved in five times its weight of water, which precipitates the metal in the form of a yellow double salt, ammonium platino-chloride, 2H4NC1, PtCl4, which, being washed in cold water, dried, and heated to redness, is reduced to the metallic state as a spongy mass.
It cannot be fused into a compact mass by a furnace heat alone, but as it possesses the property of welding like iron, it may be reduced to a compact state when hot by hammering, which is a part of Wollaston's process. Deville and Debray employ fusion in a lime crucible by the oxyhy-drogen blowpipe, to remove traces of osmium and silicon. They have also introduced an entirely new method of extraction, employing fusion instead of the wet way. A small re-verberatory furnace, having a cup-shaped bed of fire brick lined with clay, is heated to full redness and charged with a mixture of equal parts of platinum ore and galena, introduced in small quantities and stirred with an iron rod, while a little glass is thrown in to act as a flux, and afterward by degrees a quantity of litharge equal to that of the galena used. In this way the sulphur is oxidized and expelled, and the lead of the galena and litharge forms a fusible alloy with the platinum. The melted mass is now left to stand till the osmium and iridium fall to the bottom, they not being alloyed by the lead, when the lead and platinum alloy is cautiously taken out with iron ladles, and submitted to cupellation in the ordinary manner, after which the crude cupelled platinum is refined by the oxyhydrogen blowpipe on a bed of lime.
The alloy of platinum, iridium, and rhodium is harder and withstands a higher heat than pure platinum, and for that reason is better adapted for making crucibles. - The symbol of platinum is Pt; its atomic weight, 106.5; specific gravity, 21.5. It resists the highest heat of the forge, but melts in the voltaic arc and before the oxyhydrogen blowpipe, and may be volatilized with scintillations. Its crystalline form as found native is that of the octahedron, but all attempts to produce artificial crystals have failed. It expands less by heat than any other metal, and is much inferior to silver, gold, and copper as a conductor of electricity, ranking near iron, as is shown by the readiness with which a small wire is ignited by the galvanic current; this property is made use of in firing explosive compounds. It does not oxidize in the air at any temperature, nor is it attacked by any one acid; but if heated to redness in the air in contact with caustic alkalies or alkaline earths, a hydrated oxide is formed which combines with the alkaline base, in a similar manner to palladium. Platinum possesses the property of causing the union of oxygen with hydrogen and other combustible gases, even in the compact form, but more highly in the spongy state, and still more so as platinum black.
The metal may be obtained in this latter form in several ways, of which the following is one of the most convenient. A solution of chloride of platinum is boiled with an excess of carbonate of soda, to which a quantity of sugar has been added, until the resulting precipitate becomes black. Chloride of sodium is formed, water and carbonic acid are produced by oxidation of the sugar, and the platinum is precipitated, so finely divided that it appears black. This powder is then collected on a filter, washed, and dried by a gentle heat, when it is found to have the power of condensing gases, especially oxygen, in its pores to a remarkable extent. It almost instantaneously converts alcohol into acetic acid, often with sufficient rise of temperature to cause combustion. It also converts wood spirit into formic acid. The unalterability of platinum at high temperatures, and its power of resisting the action of most chemical agents, render it useful for crucibles, evaporating dishes, forceps and foil for blowpipe experiments, and for other chemical and philosophical apparatus. Large platinum stills, sometimes weighing 2,000 oz., are used for the concentration of sulphuric acid. It was made into coin by the Russians to the amount of $2,500,000 between 1826 and 1864, when the coinage was discontinued.
It is sometimes used for the touch-holes of fowling pieces. Platinum forms alloys with most other metals, which are generally more fusible than the pure metal, the exceptions being with iridium and rhodium. Care must therefore be taken not to use platinum crucibles for melting other metals. Most of the platinum of commerce contains iridium, which increases its hardness and durability, without impairing its power of resisting chemical agents. Although when pure it is perfectly insoluble in nitric acid, when alloyed with 10 or 12 times its weight of silver it is with the latter metal easily dissolved by this acid. Hot oil of vitriol will dissolve out the silver. - Platinum forms two series of compounds,, the platinous, in which it is bivalent, and the platinic, in which it is quadrivalent, being similar to palladium. The dichloride, or platinous chloride, PtC12, is formed when platinic chloride, PtCl4, formerly called bichloride, is exposed in a dried and powdered condition to a heat of about 392° F., by which half of the chlorine is expelled. It is an olive-colored powder, insoluble in water, slightly soluble in nitric or sulphuric acid, but readily in hydrochloric acid when warmed. It is soluble in caustic potash, and in platinic chloride, forming with the latter a double salt.
The solution in hydrochloric acid, when treated with an alkaline chloride, deposits a double salt in fine red prismatic crystals, the potassium salt having the formula 2KCl,PtCl2. These double salts are called chloroplatinites or plati-nosochlorides. The tetrachloride, or platinic chloride, PtCl4, is formed by the action of ni-tromuriatic acid on metallic platinum in which the nascent chlorine has sufficient affinity to combine with the metal. The acid solution when evaporated to dryness yields a deliquescent red-brown residue, which is very soluble in water and in alcohol, the aqueous solution having a pure orange-yellow tint. Platinic chloride unites with many metallic chlorides, forming double salts, called chloroplatinates or pla-ti'nochlorides, the most important being those formed with the alkaline metals and ammonium. Potassium platinochloride, 2KCl,PtCl4, is formed whenever solutions of chlorides of potassium and platinum are mixed, as a bright yellow precipitate, and on this account platinic chloride is used in the laboratory as a test for potassium compounds. The other salts of platinum, as the iodides, bromides, and cyanides, are of less importance.
There are two oxides: a protoxide, prepared by the action of caustic potash on platinous chloride; and a dioxide, obtained by adding sodic carbonate to platinic nitrate. The quantity of platinum in a compound or mixture is estimated by causing the formation of double salts of chloride of platinum with potassium or ammonium. The addition of alcohol favors the formation of the precipitate.