874 PRINCIPLES OF CHEMISTRY 



To obtain pure platinum, the ore is treated with aqua regia in which 

 only the osmium and iridium are insoluble. The solution contains the 

 platinum metals in the form RC1 4 , and in the lower forms of chlorina- 

 tion, RC1 3 and RC1 2 , because some of these metals for instance, 

 palladium and rhodium form such unstable chlorides of the type RX 4 

 that they partially decompose even when diluted with water, and pass 

 into the stable lower type of combination j in addition to which the 

 chlorine is very easily disengaged if it comes in contact with substances 

 on which it can act. In this respect platinum resists the actiop of 

 heat and reducing agents better than any of its companions that is, 

 it passes with greater difficulty from PtCl 4 to the lower compound 

 PtCl 2 . On this is based the method of preparation of more or less 

 pure platinum. Lime or sodium hydroxide is added to the solution in 

 aqua regia until neutralised, or only containing a very slight excess of 

 alkali. It is best to first evaporate and slightly ignite the solution, in 

 order to remove the excess of acid, and by heating it to partially con- 

 vert the higher chlorides of the palladium, &c., into the lower. The 

 addition of alkalis completes the reduction, because the chlorine held 

 in the compounds RX 4 acts on the alkali like free chlorine, converting 

 it into a hypochlorite. Thus palladium chloride, PdCl 4 , for example, 

 is converted into palladious chloride, PdCl 2 , by this means, according 

 to the equation PdCl 4 + 2NaHO=PdCl 2 + NaCl+NaC10 + H 2 0. In 

 a similar manner iridic chloride, IrCl 4 , is converted into the trichloride, 

 IrCl 3 , by this method. When this conversion takes place the platinum 

 still remains in the form of platinic chloride, PtCl 4 . It is then possible 

 to take advantage of a certain difference in the properties of the higher 

 and lower chlorides of the platinum metals. Thus lime precipitates the 

 lower chlorides of the members of the platinum metals occurring in 

 solution without acting on the platinic chloride, Pt01 4 , and hence the 

 addition of a large proportion of lime immediately precipitates the 

 associated metals, leaving the platinum itself in solution in the form 

 of a soluble double salt, PtCl 4 ,CaCl 2 . A far better and more perfect 



the platinum (and other platinum tnetals) owing to Its being able to form a very charac- 

 teristic alloy containing PtPb. If an alloy of the two metals be left exposed to moiet 

 air, the excess of lead is converted into carbonate (white lead) in \he presence of the 

 water and carbonic acid of the air, whilst the above platinum alloy remains unchanged. 

 The white lead may be extracted by dilute acid, and the alloy PtPb remains unaltered. 

 The other platinum metals also give similar alloys with lead, The fusibility of these, 

 alloys enables the platinum metals to be separated from the gangue of the ore, and they 

 may afterwards be separated from the -lead by subjecting the alloy to oxidation in 

 furnaces furnished with a bone ash bed, because the lead is then oxidised and absorbed 

 by the bone ash, leaving the platinum metals untouched. This method of treatment 

 was proposed by H. Sainte-Claire Deville in the sixties, and is also used in the analysis ofi 

 those metals (see further on). 



