THE PLATINUM METALS 875 



separation is effected by means of ammonium chloride, which gives, with 

 platinio chloride, an insoluble yellow precipitate, PtCl 4 ,2NH 4 Cl, whilst 

 it forms soluble double salts with the lower chlorides RC1 2 and RC1 3 , 

 so that ammonium chloride precipitates the platinum only from the 

 solution obtained by the preceding method. These methods are 

 employed for preparing the platinum which is used for the manufacture 

 of platinum articles, because, having platinum in solution as calcium 

 platinochloride, PtCaCl 6 , or as the insoluble ammonium platinochloride, 

 Pt(NH 4 ) 2 Cl 6 , the platinum compound in every case, after drying or 

 ignition, loses all the chlorine from" the platinic chloride and leaves finely- 

 divided metallic platinum, which may be converted into homogeneous 

 metal by compression and forging, dr by fusion. 9 



s For the ultimate purification of platinum from palladium and iridium the metals 

 must be re-dissolved in aqua regia, -and the solution evaporated until the residue begins 

 to evolve chlorine. The residue is then re-precipitated with ammonium or potassium 

 chloride. The precipitate may still contain a certain amount of iridium, which passes 

 with greater difficulty from the tetraohloride, IxCl*, into the trichloride, IrCl 5 , but it will 

 be quite free from palladium, because the latter easily loses its chlorine and passes into 

 palladious chloride, PdCl 2 , which gives an easily-soluble salt with potassium chloride. 

 The precipitate, containing a small quantity of ioidium, is then heated with sodium 

 carbonate in a crucible, when the mass decomposes, giving metallic platinum and 

 iridium oxide. If potassium chloride has been employed, the residue after ignition is 

 washed with water and treated with aqua regia. The iridium oxide remains undissolved, 

 and the platinum easily passes into solution. Only cold and dilute aqua regia must be 

 used. The solution will then contain pure platinic chloride, which forms the starting- 

 point for the preparation of all platinum compounds. Pure platinum for accurate 

 researches (for instance, for the unit of light, according to Violle's method) may be 

 obtained (Mylius and Foerster, 1892) by Finkener's method, by dissolving the impure 

 metal in aqua regia (it should be evaporated to drive off the nitrogen compounds), and 

 adding NaCl so as to form a double sodium salt, which is purified by crystallising with a 

 email amount of caustic soda, washing the crystals with a strong solution of NaCl, and 

 then dissolving them in a hot 1 p.c. solution of soda, repeating the above and ultimately 

 igniting the double salt, previously dried at 120, in a stream of hydrogen ; platinum 

 black and NaCl are then formed. The three following are very sensitive tests (to 

 thousandths of a per cent.) for the presence of Ir, Eu, Rh, Pd (osmium is not usually 

 present in platinum which has once been purified, since it easily volatilises with Cl 

 and CO 2 , and in the first treatment of the crude platinum either passes off as Os0 4 

 or remains undissolved), Fe, Cu, Ag, and Pb : (1) the assay is alloyed with 10 parts of 

 pure lead, the alloy treated with dilute nitric acid (to remove the greater part of the 

 Pb), and dissolved in aqua regia; the residue will consist pf Ir and Ru; the Pb is 

 precipitated from the nitric acid solution by sulphuric acid, whilst the remaining 

 platinum metals are reduced from the evaporated solution by formic acid, and the 

 resultant precipitate fused with KHSO 4 ; the Pd and Rh are thus converted into soluble 

 salts, and the former is then precipitated by HgC 2 N 2 . (2) Iron may be detected by the 

 usual reagents, if the crude platinum be dissolved in aqua regia, and the platinum 

 metals precipitated from the solution by formic acid. (3) If crude platinum (as foil or 

 sponge) be heated in a mixture of chlorine and carbonic oxide it volatilises (with a 

 Certain amount of Ir, Pd, Fe, &c.) as PtCl 2 ,2CO (Note 11), whilst the whole of the Rh, 

 Ag, and Cu it may contain remains behind. Among other characteristic reactions for 

 the platinum metals, we may mention : (1) that rhodium is precipitated from the solution 

 obtained after fusion with KHS0 4 (in which Pt does not dissolve) by NH 3 , acetic and 

 formic acids ; (2) that dilute aqua regia dissolves precipitated Pt, but not Rh ; (3) thai 



