# 1911 Encyclopædia Britannica/Platinum

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​PLATINUM [symbol Pt, atomic weight 195·0 (O＝16)], a metallic chemical element. The name, derived from platina, the diminutive of Span. plata (silver), was first given to a mineral, platinum ore or native platinum, originally discovered in South America, from the resemblance to silver. Russia furnishes about 95% of the world’s annual supply of platinum.
Native platinum occurs usually in small metallic scales or flat grains, sometimes in the form of irregular nuggets, and occasionally, though rarely, in small crystals belonging to the cubic system. Grains of platinum have been found embedded, with chromite, in serpentine derived from an olivine-rock, the metal having probably separated out from an original basic magma. It is said to occur also in veins in syenitic and other rocks. Usually, however, platinum is found in detrital deposits, especially in auriferous sands, where it is associated with osmiridium (known also as iridosmine), chromite, magnetite, corundum, zircon, &c. The platinum has a steel grey or silver-white colour and a metallic lustre, is often magnetic, sometimes with polarity; has a hardness of about 4·5 and a specific gravity varying with its composition from 14 to 19. Native platinum usually contains more or less iron and copper, often gold, and invariably a small proportion of some of the allied metals—iridium, osmium, ruthenium, rhodium and palladium. From the associated metals it was named by J. F. L. Hausmann polyxene (Gr. πολύς, many, and ξένος, a guest), whilst from its occurrence as a white metal in auriferous alluvia it is sometimes known to miners as “white gold.”
Platina del Pinto was the name by which native platinum was first introduced into Europe from South America about the middle of the 18th century. Although it appears to have been known locally much earlier, the attention of scientific men in Europe was first directed to it by Antonio de Ulloa y Garcia de La Torre, a Spaniard who joined a French scientific expedition to Peru in 1735, and published in 1748 an account of his journey, in which he refers to platinum, though not under that name, as occurring with gold in New Granada (now Colombia). Sir William Watson, an English physicist, had, however, in 1741 received some grains of the mineral, probably from the ​ same locality, though brought by way of Jamaica; and it was he who first described it in 1750 as a new metal.
Native platinum was discovered in 1819 in the gold washings of Verkhniy-Isetsk, in the Urals, but it was not until 1822 that its true nature was recognized. The chief Russian localities are in the districts of Nizhne Tagilsk and Goroblagodatsk, where it is found in shallow drift deposits, containing pebbles of serpentine, which represent the original matrix. The Iset district has acquired importance in recent years. Although the platinum-bearing gravels usually contain a very small proportion of the metal, the average in 1895 being only 1 1/2 dwt. to the ton, rich discoveries have occasionally been made in the history of the workings, and nuggets of exceptional size have been unearthed. The largest recorded specimens are one of 310 oz. from Nizhne Tagilsk, and another of 72 1/2 oz. from the Goroblagodatsk district.
In 1831 platinum ore was recognized in the gold-bearing deposits of Borneo, where it had previously been regarded as worthless, being known to the natives as mas kodok (frog gold). Although recorded from various parts of the island, its occurrence seems to be definitely known only in Tanah-Laut, in the south-east of Borneo. In Australia platinum ore has been found near Fifield (near Condobolin), New South Wales; whilst in New Zealand it occurs in sands and gravels in the Thames gold-field, the Takaka River and the Gorge River flowing into Awarua Bay. Many localities in North America have yielded platinum, generally in beach sands or in auriferous alluvia, and in some cases the deposits are of commercial importance. The metal is found in Alaska, British Columbia, Oregon (Douglas county) and California (Butte county, Trinity county, Del Norte county). It has been recorded also from the states of New York and North Carolina. In a nickeliferous sulphide ore worked at Sudbury, in Ontario, platinum has been discovered in the form of an arsenide (PtAs$_2$), which has been called sperrylite by H. L. Wells, who analysed it in 1889, and named it after F. L. Sperry, of Sudbury It belongs to the pyrites group, and is interesting as being the only known mineral in which platinum occurs in combination except as alloy.
Native platinum seems to be a mineral of rather wide distribution, but in very sparse quantity. The sands of the Rhine, derived from Alpine rocks, have been found to contain platinum in the proportion of 0·0004%. It has also been found in the sands of the Ivalo River in Lapland; it is recorded from Roros in Norway; and it was detected by W. Mallet in some of the gold-sands of the streams in Co. Wicklow, Ireland.
The table shows the official amount (in ounces Troy) of platinum produced in Russia for certain years, the actual amounts are much larger.—
(Rothwell's Mineral Industry, 1908)
Platinum is largely used for the manufacture of chemical apparatus, incandescent lamps, thermo-couples; in the manufacture of sulphuric acid by the contact process, in photography, and in jewelry. The price of the metal has risen considerably, not so much on account of the restricted supply, but chiefly because the sources of supply have passed into the hands of a few individuals The following data show the fluctuations in the average price of platinum ingot per ounce Troy:—
| | £ | s. | d. | | £ | s. | d. |
| 1874–1898: | 1 | 5 | 2 | to | 2 | 2 | 0 |
| 1899–1905: | 3 | 13 | 6 | " | 4 | 10 | 4 |
| 1906: | 4 | 15 | 2 | " | 7 | 19 | 8 |
| 1907: | 7 | 0 | 0 | " | 6 | 18 | 8 |
| 1908: | 5 | 2 | 6 | (average) price. | (average) price. | (average) price. | (average) price. |
Platinum may be extracted from its ore by both wet and dry processes. In the latter method, due to H. Sainte-Claire-Deville and H. J. Debray, the ore is smelted in a furnace constructed of two blocks of lime, and the metallic button so obtained is re-melted in a reverberatory furnace with galena or litharge, the lead platinum alloy being then cupeled, and the platinum fused into an ingot by re-smelting in a lime furnace (see Dingler's Polytech. Journ. 1859, 153, p. 38; 1859, 154, p. 383, 1862, 165, p, 205). The platinum so obtained is not pure. In Wollaston's wet method the ore is dissolved in aqua regia., the osmiridium, ruthenium and rhodium being left unattached, and the platinum precipitated as ammonium platinochloride by adding ammonium chloride in the presence of an excess of acid. The double chloride is then washed, dried and ignited, leaving a residue of metal. G. Matthey (Chem. News, 1879, 39, p. 175) obtains pure platinum from the commercial metal by fusing the latter with a large excess of lead. The lead alloy is then treated with a dilute nitric acid and the insoluble portion taken up in dilute aqua regia, From the solution so obtained lead is precipitated as sulphate, and platinum and rhodium as double ammonium chlorides. The rhodium ammonium chloride is converted by fusing with potassium and ammonium bisulphates into rhodium sulphate, which is then removed by extraction with water, when a residue of finely divided platinum remains. The German firm of Heraus (in Hanover) heat the raw ore with aqua regia and water under pressure, evaporate the solution to dryness, and heat the residue to 125° C. A clear aqueous extract of the residue is then acidified with hydrochloric acid and precipitated with ammonium chloride. The double chloride is ignited and the finely divided platinum so obtained is fused in the oxyhydrogen blowpipe.
Platinum is a greyish-white metal which is exceedingly malleable and ductile; the addition of a small quantity of iridium hardens it and diminishes its ductility very considerably. Its specific gravity is 20·85 to 21·71, and its mean specific heat from 0 to 100° C. is 0·0323 (J. Violle, Comptes rendus, 1877, 85, p 543); W. P. White (Amer. Journ. Sci., 1909, iv. 28, p. 334) gives the general formula S$_t$=0·03198+3·4×10$^{−6}$t. S$_t$ being the specific heat at t°C. Its temperature of fusion is in the neighbourhood of 1700 to 1800°C., various intermediate values having been obtained by different investigators (see J. A. Harker. Chem. News, 1005, 91, p. 262; C. Féry and C. Chéneveau, Comptes rendus, 1909, 148, p. 401; also C. W. Waidner and G. H. Burgess, ibid., 1909, 148, p. 1177). Its latent heat of fusion is 27·18 calories (Violle, loc. cit). The metal has been obtained in the crystalline condition by distillation in the electric furnace, or by decomposing its fluoride at a red heat (H. Moissan), Platinum, like palladium, absorbs large quantities of hydrogen and other gases, the occluded gas then becoming more “active”, for this reason platinum is used largely as a catalytic agent. Several forms of platinum, other than the massive form, may be obtained. Spongy platinum is produced when ammonium platinochloride is ignited; platinum black on the reduction of acid solutions of platinum salts; and colloidal platinum by passing an electric arc between two platinum wires under the surface of pure water (G. Bredig, Zeit. phys. Chem., 1901, 37, pp. 1, 323). Platinum is practically unoxidizable; it combines directly with phosphorus, arsenic, antimony, silicon, boron, and fluorine, and with almost all other metals. It is practically unattached by all acids, dissolving only in aqua regia or in mixtures which generate chlorine. When fused with alkaline hydroxides in the presence of air it forms palatinates. It is readily attacked by fused nitrates, and by potassium cyanide and ferrocyanide. All the platinum compounds when heated strongly decompose, and leave a residue of the metal. Of platinum salts, in the true sense of the word, none exist; there is no carbonate, nitrate, sulphate, &c; halide salts, however, are known, but are obtained in an indirect manner.
Platinum monoxide, PtO, obtained by heating the corresponding hydrate, is a dark-coloured powder which is easily reduced to the metal (L. Wohler, Ber., 1903, 36, p. 3475). The hydrated form, PtO·22H$_2$O, is obtained impure by precipitating the chloride with caustic soda, or by adding caustic soda to a boiling solution of potassium platinous chloride, K$_2$PtCl$_4$, the precipitate being rapidly washed and dried in vacuo (L. Wohler, Zeit. anorg. Chem., 1904, 40, p. 423). It is a black powder; when freshly prepared it is soluble in concentrated acids, but when dried it is insoluble. It is an acidic oxide, the dioxide being both acidic and basic. It behaves as a strong oxidizing and reducing agent. C. Engler and L. Wohler (Zeit. anorg. Chem., 1901, 29, p. 1) have shown that platinum black, containing occluded oxygen, is soluble in dilute hydrochloric acid and also liberates iodine from potassium iodide. and that the ratio between the amount of platinum dissolved and the amount of oxygen occluded agrees with the formation of a compound corresponding to the formula PtO. Platinum dioxide (platinic ​oxide), PtO$_2$·4H$_2$O, is obtained by adding an excess of caustic soda solution to a boiling solution of chlorplatinic acid, the hot solution being diluted and neutralized with acetic acid. It loses its water of hydration when heated, finally decomposing into platinum and oxygen. When freshly prepared it is soluble in dilute acids. Other hydrated forms of composition, PtO$_2$·3H$_2$O and PtO$_2$·2H$_2$O, have been described (E. Prost, Bull. soc. chem., 1886, 46, p. 156; H. Topsoe, Ber, 1870, 3, p. 462). The tetrahydrate may be considered as an acid, H$_2$Pt(OH)$_4$, for salts are known (namely the platinates) corresponding to it, those of the alkali metals being soluble in water, and possessing an alkaline reaction (M. Blondel, Ann. chim. phys., 1905 [viii.], 6, p. 81). A similar set of chlorine-holding compounds is also known, the chlorine replacing one or more hyldroxyl groups and giving rise to complexes of composition, H$_2$[PtCl$_5$(OH)], H$_2$[PtCl$_4$(OH)$_2$], H$_2$[PtCl$_2$(OH)$_4$] and H$_2$[PtCl(OH)$_5$]. The platinic salts (derived from PtO$_2$) are yellow or brown solids, which are readily reduced to the metallic condition. They give with sulphuretted hydrogen a dark brown precipitate, soluble in excess of ammonium sulphide. Potassium iodide gives a brown solution with gradual formation of a precipitate. They form characteristic precipitates with potassium and ammonium chlorides. The platinous salts are brown or colourless solids which, with sulphuretted hydrogen, give a dark brown precipitate of platinum sulphide, and with potassium iodide a gradual precipitation of platinic iodide, PtI$_2$. Platinum trioxide, PtO$_3$, is obtained as K$_2$O·3PtO$_3$, by electrolysing a solution of platinic hydroxide in potash, this compound with acetic acid giving the oxide as a brown, easily decomposable powder (L. Wohler and F. Martin, Ber., 1909, 42, P 3326).
Platinum bichloride, PtCl$_2$, is obtained by heating chlorplatinic acid to 300-350° C. (J. J. Berzelius), or, mixed with more or less platinic chloride, by passing chlorine over spongy platinum at a temperature of 250° (P. Schutzenberger, Comptes rendus, 1870, 70, pp. 1134, 1287). It may also be obtained by the decomposition of the compound HCl·PtCl$_2$·2H$_2$O (see below) at 100° C., this method giving a very pure product (L. F. Nilson, Journ. prak. Chem., 1877 (2), 1 5, p. 260). It is a brown or greyish green coloured solid, which is soluble in hydrochloric acid. It decomposes into its constituent elements when heated. It combines with many chlorides to form characteristic double salts. Platinum bichloride combines with carbon monoxide, yielding compounds of composition, PtCl$_2$.CO, PtCl$_2$·2CO, 2PtCl$_3$·3CO (P. Schutzenberger, Ann. chem. phys., 1870 (4), 21, p. 350). Hydrogen platinochloride or chlorplatinous acid, H$_2$PtCl$_4$, is only known in solution, and as such is obtained when platinum bichloride is dissolved in hydrochloric acid, or by decomposing the barium salt with sulphuric acid, or the silver salt with hydrochloric acid. Its salts, the platinochlorides or chlorplatinites, are obtained by reducing the chlorplatinates or directly from the acid itself. They are mostly soluble in water giving red solutions. They are readily oxidized, and nascent hydrogen reduces them to metallic platinum Potassium platinochloride or chlorplatinite, K$_2$PtCl$_4$, is prepared by reducing hydrogen platinichloride with sulphur dioxide or potassium platinichloride with potassium oxalate in the presence of iridium (Klason, Ber., If904, 37, p. 1360); or by adding potassium chloride to a solution of platinum bichloride in hydrochloric acid. It crystallizes in dark red prisms, is readily soluble in water, but insoluble in alcohol. The solution of the free acid when concentrated in vacuo leaves a residue of Hcl PtCl$_2$ 2H$_2$O. When the tree acid is reduced by alcohol, or when ethylene is passed into a solution of platinum bichloride in hydrochloric acid, PtCl$_2$ C$_2$H$_4$ is obtained as a brown amorphous mass which decomposes when heated. When the bichloride is, heated in a current of carbon monoxide, a sublimate of platinomonocarbonyl dichloride, PtCl$_2$CO, dicarbonyl dichloride, PtCl$_2$(CO)$_2$, and tricarbonyl tetrachloride, Pt$_2$Cl$_4$(CO)$_2$, is obtained. The first forms bright yellow needles and the second white acicular crystals. The bichloride also combines with phosgene to form PtCl$_2$.2COCl$_2$.
Platinum chloride, PtCl$_4$, is obtained when chlorplatinic acid is heated in a current of dry hydrochloric acid gas to 165° C. (W. Pullinger, Journ Chem Soc., 1892, 61, p. 422) or in a current of dry chlorine at 275° C. (A. Rosenheim and W. Lowenstamm, Zeit. anorg. Chem, 1903, 37, p. 394). It forms a reddish brown crystalline mass which is very hygroscopic. Numerous hydrates are known. The chloride is characterized by the readiness with which it forms double salts with the metallic chlorides and with the hydro chlorides of most organic bases Chlorplatinic acid, H$_2$PtCl$_6$·6H$_2$O, is obtained by dissolving platinum in aqua regia containing an excess of hydrochloric acid, or by the action of chlorine (dissolved in hydrochloric acid) on platinum sponge It crystallizes in needles, which are very deliquescent and dissolve easily in water. It melts in its own water of crystallization at 70° C, and when heated in vacuo to 100° C. it leaves a residue of composition HCl PtCl$_4$ 2H$_2$O. The potassium and ammonium salts and the salts it forms with organic bases are characterized by their exceedingly small solubility in water. The aqueous solution of the acid reddens litmus and decomposes the metallic carbonates Its salts may be prepared by the direct action of the acid on the metallic hydroxides or carbonates, and are usually of an orange or yellow colour and are mostly soluble in water. Potassium chlorplatinate, K$_2$PtCl$_6$, is obtained, in the form of a yellow crystalline precipitate, when a concentrated solution of a potassium salt is added to a solution of chlorplatinic acid. It crystallizes in octahedra which are scarcely soluble in water, and practically insoluble in absolute alcohol. It decomposes at a red heat into platinum, chlorine and potassium chloride. The corresponding sodium salt, Na$_2$PtCl$_6$·6H$_2$O, is much more soluble in water and in alcohol. The ammonium salt, (NH$_4$)$_2$PtCl$_6$, resembles the potassium salt in its solubility in water and in alcohol. Corresponding bromo- and iodo- compounds are known. Platinum bifluoride and tetrafluoride, PtF$_2$ and PtF$_4$, were obtained simultaneously by H Moissan (Ann. chem. phys., 1894 (6), 24, p. 282) by the action of fluorine on platinum at 500-600° C. They may be separated by taking advantage of their different solubilities in water.
Platinum monosulphide, PtS, is obtained by the direct union of platinum and sulphur; by heating ammonium chlorplatinate with sulphur; or by the action of sulphuretted hydrogen on the chlorplatinites. It is a dark coloured powder which is almost insoluble in aqua regia. It decomposes when heated strongly leaving a residue of metallic platinum, the same reduction taking place at comparatively low temperatures when it is heated in a current of hydrogen. Platinic sulphide, PtS$_2$, is formed when the chlorplatinates are heated with sulphuretted hydrogen to 60° C. The precipitate must be rapidly washed and dried in vacuo, since it oxidizes rapidly on exposure to air. It is a black powder, which when heated strongly in air decomposes and leaves a residue of platinum, but if heated in absence of air leaves a residue of the monosulphide. It is scarcely affected by acids and is little soluble in solutions of the alkaline sulphides. Sulphides of composition Pt$_2$S$_3$ and Pt$_5$S$_6$ have been described (R. Schneider, Pogg. Ann, 1869, 138, p. 604; 1873, 148, p. 633; 1873, 149, p. 381). A salt of composition, Pt(OH)$_4$.H$_2$SO$_4$.H$_2$O, has been prepared by M. Blondel (Ann. chem. phys., 1905, (8), 6, p. 81) by the solution of the hydrate H$_2$Pt(OH)$_6$, i.e. PtO$_2$·4H$_2$O, in dilute sulphuric acid (1.1) at 0° C. On the addition of cold concentrated sulphuric acid to the solution so obtained, the above salt is precipitated in the form of minute needles, which readily decompose in the presence of water. A platinum sulphate, Pt(SO$_4$)$_2$.2H$_2$O, has been obtained by L. Stuchlik (Ber., 1904, 37, p. 2913) by the action of sulphuric acid (s.g. 1.84) on platinum under the influence of an alternating current. A crystalline precipitate is obtained, which is soluble in water and is very hygroscopic.
The platinonitrites of composition M$_2$Pt(NO$_2$)$_4$ are mostly obtained by double decomposition from the potassium salt, which is formed by adding a warm aqueous solution of potassium nitrite to one of potassium chlorplatinate. They are mostly colourless or pale yellow solids which are more or less soluble in water (L. F. Nilson, Ber., 1876, 9, p. 1722). The corresponding platino-oxalates M$_2$Pt(C$_2$O$_4$)$_2$ were first obtained by J. W. Dobereiner (Pogg. Ann., 1833, 104, p. 180) and their constitution was determined by H. G. Soderbaum (Ber., 1888, 21, p. 567 R; Zeit. anorg. Chem., 1894, 6, p. 45). The sodium salt, from which the others are obtained by double decomposition, is formed by adding a warm solution of oxalic acid to sodium platinate. On recrystallization from alkaline solutions the salts are obtained in yellow or orange crystals (see M. Vezes, Bull. soc. chem., 1898 (3), 19, p. 875). These salts are scarcely soluble in water and decompose explosively when suddenly heated. The free acid is obtained by decomposing the silver salt with hydrochloric acid, the indigo blue solution so obtained on concentration in vacuo yielding a red crystalline mass, which dissolves in water with an indigo blue colour, changing to yellow on dilution.
Platinum cyanide, Pt(CN)$_2$, is formed by the addition of mercuric cyanide to a solution of a chlorplatinite, or by the decomposition of mercury or ammonium platinocyanide by heat. It is an amorphous powder which is insoluble in water, acids or alkalis, but is soluble in a solution of hydrocyanic acid. It burns when heated. The platinocyanides are derived from the acid H$_2$Pt(CN)$_6$, which is formed by the decomposition of the mercury or copper-salt with sulphuretted hydrogen, or of the barium salt with sulphuric acid. It crystallizes from wafer in cinnabar-red prisms which contain five molecules of water of crystallization; in the anhydrous condition it is of a yellowish green colour. It decomposes carbonates. Its salts, which are characterized by the property of polychroism, may be prepared by the usual methods, or by the solution of metallic platinum in the alkaline cyanides or alkaline earth cyanides under the influence of an alternating current (A. Brochet and J. Petit, Ann. chem. phys., 1904 (8), 3, p. 460; M. Berthelot, Comptes rendus, 1904, 138, p. 1130). Those of the alkali and alkaline earth metals are soluble in water. Many combine with the halogen elements to form complex salts of the type M$_2$Pt(CN)$_4$.Cl$_2$. x H$_2$O. By the decomposition of the barium salts of this type, addition products of the free acid, of composition H$_2$Pt(CN)$_4$Cl$_2$.4H$_2$O and H$_2$Pt(CN)$_4$.Br$_2$, have been obtained (C. Blomstrand, Ber., 1869, 2, p. 202). They) are deliquescent solids which are exceedingly soluble in water. Potassium platinocyanide, K$_2$Pt(CN)$_4$.3H$_2$O, is obtained by dissolving platinum bichloride in potassium cyanide; by heating potassium ferrocyanide with spongy platinum; or by heating ammonium chlorplatinate with potassium cyanide. It crystallizes in needles which effloresce readily. The dry salt is exceedingly hygroscopic and is very soluble in water. When boiled with aqua regia it forms the chlorine addition ​product, K$_2$Pt(CN)$_4$.Cl$_2$.2H$_2$O. It combines directly with iodine. Barium platinocyanide, BaPt(CN)$_4$.4H$_2$O, is prepared by the action of baryta water on the copper salt, by dissolving platinum in barium cyanide under the influence of an alternating current; by the addition of barium cyanide to platinum bichloride, or by the simultaneous action of hydrocyanic and sulphurous acids on a mixture of baryta and chlorplatinic acid (P. Bergsoe, Zeit. anorg. Chem., 1899, 19, p. 318) it crystallizes in yellow monoclinic prisms and is soluble in hot water. It is employed for the manufacture of fluorescent screens used for the detection of X-rays.
The platinum salts combine with ammonia to form numerous derivatives which can be considered as salts of characteristic bases. The first compound of this type was isolated in 1828 by Magnus, who obtained a green salt by the action of ammonia on platinum bichloride. Two series of these salts are known, one in which the metal corresponds to bivalent platinum, the other in which it corresponds to tetravalent platinum. The general formulae of the groups in each series are shown below, the method of classification being that due to Werner
In the above table X represents a monovalent acid radical and R a monovalent basic radical. For methods of preparation of salts of these series see P. T. Cleve, Bull. soc. chim. 1867 et seq; S. M. Jörgensen, Journ. prak. Chem. 1877 et seq; C. W. Blomstrand, Ber. 1871 et seq, and A. Werner, Zeit. anorg. Chem. 1893 et seq. A very complete account of the method of classification and the general theory of the metal ammonia compounds is given by A. Werner, Ber. 1907, 40, p. 15.
Platinum also forms a series of complex phosphorus compounds. At 250° finely divided platinum and phosphorus pentachloride combine to form PtCl$_2$.PCl$_3$, as dark claret-coloured crystals. With chlorine this substance gives PtCl$_3$.PCl$_4$ as a yellow powder, and with water it yields phosphoplatinic acid, PtCl$_2$.P(OH)$_3$, which may be obtained as orange-red deliquescent prisms.
The atomic weight of platinum was determined by K. Seubert (Ann. 1888, 207, p. 1; Ber. 1888, 21, p. 2179) by analyses of ammonium and potassium platinochlorides, the value 194.86 being obtained.