CRITICAL EXPERIMENTS FOR DETERMINATION OF POTASSIUM, ETC. 563 



regia, and eliminated from the solution by means of pure hydrogen in the wet 

 way. The metallic platinum was then filtered off, was washed successively 

 with hot water, hot hydrochloric acid, and again with water, dried, and ignited 

 in a porcelain crucible. When 23 -338 grms. of this metal were dissolved in 

 dilute aqua regia, with the view of preparing a standard solution, a black, 

 sooty-looking residue remained, which was removed by filtration, ignited, and 

 weighed. It amounted to 25'7 mgs. The solution was freed from nitric acid 

 by repeated evaporation on a water-bath with hydrochloric acid, the ultimate 

 residue dissolved in water, diluted to 471 c.c. in a tared bottle, and the solution 

 weighed. It weighed 509*969 grammes, and, taking the part soluble in aqua 

 regia as pure platinum, contained 23 - 3164 grms. of this metal. For roughly 

 quantitative work, it was assumed to contain 49*5 mgs. of platinum = £ x 198 

 mgs. per c.c. 



Strictly speaking, we had no right to assume that all the metal contained in 

 this solution was pure platinum; but as the solution exhibited a normal colour \ 

 and gave light yellow precipitates with chlorides of potassium and ammonium^ 

 we thought it might safely be used as pure chloroplatinic acid of the above 

 titre, the more so as the atomic weight of iridium is only a little less than that 

 of platinum, and a small admixture of the former metal in a chloroplatinate 

 could not appreciably affect the percentage of metal in it. We accordingly did 

 use this solution in some of our experiments. 



Matthey' s Platinum. — To examine this metal for impurities, 10 grms. of it were 

 dissolved in hydrochloric acid, with the aid of the least sufficiency of nitric. 

 The solution was very light in colour, and no visible residue remained. The 

 solution was transferred to a large platinum basin, mixed with a large excess 

 of caustic soda,* boiled for 20 minutes, the hypochlorite destroyed by alcohol, 

 and the resulting liquid acidified strongly with hydrochloric acid. No 

 permanent precipitate was produced. The solution was precipitated with 

 sal-ammoniac, the chloroplatinate (which was light yellow) filtered off, and 

 the filtrate concentrated to a small volume, and allowed to stand to recover 

 a second instalment of chloroplatinate. The filtrate from this was evaporated 

 to dryness, the sal-ammoniac burned off, the residue washed with water, 

 ignited, and weighed. It weighed 123 mgs. On treatment with aqua regia, a 

 white residue (silica ?) remained, which weighed 326 mgs. after ignition. Hence 

 metal in solution = 90 4 mgs. This solution was mixed with a weighed 

 quantity of a standard solution of chloride of potassium, containing 64*79 mgs. 

 of KC1 (corresponding to about 0*95 time 90*4 mgs. of platinum), the mixture 

 evaporated to a very small volume, and mixed, after cooling, with absolute 

 alcohol, to precipitate the " chloroplatinate " produced. The precipitate, after 



* Specially made from pure (Trommsdorff's) carbonate-crystals, in a nickel basin. 



