8 THE ROYAL SOCIETY OF CANADA 



nitrites, and the platinum is then precipitated with ammonium 

 chloride. 



Electrolytic methods of separation can be used under certain 

 conditions. Classen^ electrolyses a hydrochloric acid solution with a 

 current density of ND, oo = 05 amperes and a potential difference of 

 1 -2 volts. The platinum is deposited, while these conditions will not 

 bring down the iridium. Smith^ favours the use of a solution contain- 

 ing an alkali phosphate and phosphoric acid; these reagents preventing 

 the deposition of the iridium but not hindering that of the platinum. 

 A silver plated platinum dish was used as cathode. 



In the preparation of some pure platinum salt, for use in another 

 investigation, from a quantity of platinum ore, it appeared to one of 

 us^ that the best separation of platinum from iridium was obtained 

 by precipitating the platinum, as ammonium chloroplatinate, by the 

 addition of ammonium chloride to a solution of the chlorides slightly 

 acid with hydrochloric acid; even with one precipitation an almost 

 quantitative separation appeared to be reached. As the relative 

 solubility of the iridium and platinum compounds determines, to a 

 large extent, the efficiency of this method of separation, it was thought 

 worth while to study the solubility of the ammonium chloriridate 

 and chloroplatinate particularly, as the values given by Crookes* 

 for the solubility of the ammonium chloroplatinate appeared to be 

 far from correct. At the same time, some measurements have also 

 been made of the solubility of ammonium bromoplatinate and of all 

 these compounds in the presence of an excess of the precipitating 

 reagent. 



Preparation of Materials. 



Ammonium chloroplatinate: — In preparing this salt, about 25 

 grams of platinum scrap were boiled in concentrated hydrochloric 

 acid. The platinum was then scoured with emery and again treated 

 with concentrated acid. After washing away this acid with water 

 the scrap was dissolved in aqua regia and the solution evaporated to 

 dryness. Concentrated hydrochloric acid was now added and the 

 evaporation continued. This process was repeated until it was con- 

 sidered that the last trace of nitric acid had been removed. That 

 it is very necessary to remove the last trace of nitric acid, if one is to 

 obtain a good separation, has been pointed out in several cases; 



* Classen, Quantitative Analysis by Electrolysis, p. 218. 

 2 Smith, Am. Chem. Jour., 14, 435 (1892). 

 8 Proc. Royal Soc. Edin., 29, 721, (1900). 

 « Chem. News, 9. 37. 



