THE PLATINUM METALS 379 



Platinous chloride, PtCl 2 , is formed when hydrogen platinoghloride, 

 PtH 2 Cl 6 , is ignited at 300, or when potassium is heated at 230 in a 

 fitream of chlorine. The undecomposed tetrachloride is extracted from 

 the residue by washing it with water, and a greenish-grey or brown 

 insoluble mass of the dichloride (sp. gr. 5'9) IB then obtained. It is 

 soluble in hydrochloric acid, giving an acid solution of the composition 

 PtCl 8 ,2HCl, corresponding with the type of double salts PtR 2 Cl 4 . 

 Although platinous chloride decomposes below 500, still it is formed to 

 a small extent at higher temperatures. Troost and Hautefeuille, an<$ 

 Seelheim observed that when platinum was strongly ignited in a stream of 

 chlorine, the metal, as it were, slowly volatilised and was deposited in 

 crystals j a volatile chloride, probably platinous- chloride, was evidently 

 formed in this case, and decomposed subsequently to its formation, 

 depositing crystals of platinum. 



The properties of platinum above- described are repeated more or less 

 distinctly, or sometimes with certain modifications, in the above-men- 

 tioned associates and analogues of -this metal. Thus although palladium 

 fortns PdCl 4 , this form passes into PdCl a with extreme ease. 9 Whilst 



treated with acetic acid, the alkali combines with the latter, and a platinio hydroxide^ 

 Pt(OH) 4 , remains as a brown mass, which loses water and oxygen when ignited, and in 

 BO doing decomposes with a slight explosion. When slightly ignited this hydroxide first 

 loses water and gives the very unstable oxide Pt0 2 . Piatinio sulphide,. PtS 2 , 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 ia 

 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 basio 

 nature of PtO 2 , so that it clearly exhibits both basio and acid properties. The latter 

 appear, for instance, in the fact that platiuic sulphide, PtS 2 , gives crystalline compounds 

 with the alkali sulphides. 



* In comparing the characteristics of the platinum metals, it must be observed that 

 palladium in its form of combination PdXg gives saline compounds of considerable) 

 stability. Amongst them palladoua chloride is formed by the direct action of chlorine 

 or aqua regia (not in excess or in dilute solutions) on palladium. It forms a brown 

 solution, which gives a black insoluble precipitate of palladoua iodide, PdI 2 , with 

 solutions of iodides (in this respect, as in many others, palladium resembles mercury in 

 the mercuric compounds HgX 8 ). With a solution of mercuric cyanide it gives a yellowish 

 white precipitate, pallodous cyanide, PdC 8 N 2 , which is soluble in potassium cyanide, and 

 gives other double salts, M 2 PdC 4 N 4 . 



That portion of the platinum ore which dissolves in aqua regia and is precipitated 

 by ammonium or potassium chloride does not contain palladium. It remains in solu- 

 tion, because the palladic chloride, PdClj, is decomposed and the palladoua chloride 

 formed is not precipitated by ammonium chloride; the same holds good for all the other 

 lower chlorides of the platinum metals. Zinc (and iron) separates out all the unprecipi. 

 tated platinum metals (and also copper, &c.) from the solution. The palladium is found 

 to these platinum residues precipitated by einc. If this mixture of metals be treated 

 with aqua regia, all the palladium will pass into solution as palladous chloride, with, 

 some platinic chloride. By this treatment the main portion of (he irtdium, rhodium, &c. 

 remains almost undtseolved, the platinum is separated from the mixture of palladia 

 tod platinto chlorides by a solution of ammonium chloride, and the eolation of 



