Note 8 — Chapter XXIII · THE PLATINUM METALS

Nilson (1877), who investigated the platinochlorides of various metals subsequently to Bonsdorff, Topsöe, Clève, Marignac, and others, found that univalent and bivalent metals—such as hydrogen, potassium, ammonium … beryllium, calcium, barium—give compounds of such a composition that there is always twice as much chlorine in the platinic chloride as in the combined metallic chloride; for example, K2Cl2,PtCl4; BeCl2,PtCl4,8H2O, &c. Such trivalent metals as aluminium, iron (ferric), chromium, didymium, cerium (cerous) form compounds of the type RCl3PtCl4, in which the amounts of chlorine are in the ratio 3:4. Only indium and yttrium give salts of a different composition—namely, 2InCl3,5PtCl4,36H2O and 4YCl3,5PtCl4,51H2O. Such quadrivalent metals as thorium, tin, zirconium give compounds of the type RCl4,PtCl4, in which the ratio of the chlorine is 1:1. In this manner the valency of a metal may, to a certain extent, be judged from the composition of the double salts formed with platinic chloride.

Platinic bromide, PtBr₄, and iodide, PtI₄, are analogous to the tetrachloride, but the iodide is decomposed still more easily than the chloride. If sulphuric acid be added to platinic chloride, and the solution evaporated, it forms a black porous mass like charcoal, which deliquesces in the air, and has the composition Pt(SO₄)₂. But this, the only oxygen salt of the type PtX₄, is exceedingly unstable. This is due to the fact that platinum oxide, the oxide of the type PtO₂, has a feeble acid character. This is shown in a number of instances. Thus if a strong solution of platinic chloride treated with sodium carbonate be exposed to the action of light or evaporated to dryness and then washed with water, a sodium platinate, Pt₃Na₂O₇,6H₂O, remains. The composition of this salt, if we regard it in the same sense as we did the salts of silicic, titanic, molybdic and other acids, will be PtO(ONa)₂,2PtO₂,6H₂O—that is, the same type is repeated as we saw in the crystalline compounds of platinum tetrachloride with sodium chloride, or with hydrochloric acid—namely, the type PtX₄8Y, where Y is the molecule H₂O,HCl, &c. Similar compounds are also obtained with other alkalis. They will be platinates of the alkalis in which the platinic oxide, PtO₂, plays the part of an acid oxide. Rousseau (1889) obtained different grades of combination BaOPtO₂, 3(BaO)2PtO₂, &c., by igniting a mixture of PtCl₄ and caustic baryta. If such an alkaline compound of platinum be treated with acetic acid, the alkali combines with the latter, and a platinic hydroxide, Pt(OH)₄, remains as a brown mass, which loses water and oxygen when ignited, and in so doing decomposes with a slight explosion. When slightly ignited this hydroxide first loses water and gives the very unstable oxide PtO₂. Platinic sulphide, PtS₂, belongs to the same type; it is precipitated by the action of sulphuretted hydrogen on a solution of platinum tetrachloride. The moist precipitate is capable of attracting oxygen, and is then converted into the sulphate above mentioned, which is soluble in water. This absorption of oxygen and conversion into sulphate is another illustration of the basic nature of PtO₂, so that it clearly exhibits both basic and acid properties. The latter appear, for instance, in the fact that platinic sulphide, PtS₂, gives crystalline compounds with the alkali sulphides.