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Rh oxide), PtO2·4H2O, 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, PtO2·3H2O and PtO2·2H2O, 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, H2Pt(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, H2[PtCl5(OH)], H2[PtCl4(OH)2], H2[PtCl2(OH)4] and H2[PtCl(OH)5]. The platinic salts (derived from PtO2) 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, PtI2. Platinum trioxide, PtO3, is obtained as K2O·3PtO3, 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, PtCl2, 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·PtCl2·2H2O (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, PtCl2.CO, PtCl2·2CO, 2PtCl3·3CO (P. Schutzenberger, Ann. chem. phys., 1870 (4), 21, p. 350). Hydrogen platinochloride or chlorplatinous acid, H2PtCl4, 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, K2PtCl4, 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 PtCl2 2H2O. When the tree acid is reduced by alcohol, or when ethylene is passed into a solution of platinum bichloride in hydrochloric acid, PtCl2 C2H4 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, PtCl2CO, dicarbonyl dichloride, PtCl2(CO)2, and tricarbonyl tetrachloride, Pt2Cl4(CO)2, is obtained. The first forms bright yellow needles and the second white acicular crystals. The bichloride also combines with phosgene to form PtCl2.2COCl2.

Platinum chloride, PtCl4, 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, H2PtCl6·6H2O, 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 PtCl4 2H2O. 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, K2PtCl6, 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, Na2PtCl6·6H2O, is much more soluble in water and in alcohol. The ammonium salt, (NH4)2PtCl6, resembles the potassium salt in its solubility in water and in alcohol. Corresponding bromo- and iodo- compounds are known. Platinum bifluoride and tetrafluoride, PtF2 and PtF4, 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, PtS2, 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 Pt2S3 and Pt5S6 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.H2SO4.H2O, has been prepared by M. Blondel (Ann. chem. phys., 1905, (8), 6, p. 81) by the solution of the hydrate H2Pt(OH)6, i.e. PtO2·4H2O, 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(SO4)2.2H2O, 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 M2Pt(NO2)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 M2Pt(C2O4)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 H2Pt(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 M2Pt(CN)4.Cl2. x H2O. By the decomposition of the barium salts of this type, addition products of the free acid, of composition H2Pt(CN)4Cl2.4H2O and H2Pt(CN)4.Br2, have been obtained (C. Blomstrand, Ber., 1869, 2, p. 202). They) are deliquescent solids which are exceedingly soluble in water. Potassium platinocyanide, K2Pt(CN)4.3H2O, 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