CRITICAL EXPERIMENTS FOR DETERMINATION OF POTASSIUM, ETC. 567 



I. Experiments on the Composition of Chloroplatinate of Potassium. 



(First Series. ) 



The general modus operandi was as follows : — A known weight of chloride 

 of potassium (weighed out in the form of a gravimetrically standarised solution) 

 was mixed with, in general, a slight or moderate excess of standard chloro- 

 platinic acid solution, whose weight was determined and recorded likewise. 

 After production of the chloroplatinate (in some way or other), the mother- 

 liquor was decanted and filtered off, the chloroplatinate washed (as a rule 

 first with small instalments of water, then with alcohol), dried at a certain 

 temperature, and weighed. The mother-liquor, after removal of the alcohol 

 from the alcoholic portion, was diluted with water, placed in an Erlenmeyer 

 flask, and the platinum reduced out by purified hydrogen in the wet way at 

 80 to 90° C. After complete reduction, the hydrogen was displaced by 

 carbonic acid (to avoid explosions), the platinum filtered off, ignited, and 

 weighed. The filtrate and wash-waters were diluted to a certain weight, and 

 aliquot weighed parts used for the repeated determination of the fixed 

 chlorine, i.e., the chlorine present as KC1. For this purpose the respective 

 liquid was evaporated to dryness, the residue dried further at 130°, then 

 moistened with water, the solution re-evaporated, and the residue heated 

 again to 130°, so that the free hydrochloric acid could be assumed to be away, 

 which point, however, was always made sure of by the application of a thread 

 of delicate litmus-paper to the last solution. In the chloride of potassium thus 

 isolated, the chlorine was determined ; generally by means of the " Challenger" 

 method. In the later experiments a very small quantity of fixed chlorine 

 present in the chloroplatinic acid used was determined in a large quantity of 

 the respective reagent, and allowed for. By deducting the weight of platinum 

 and chloride of potassium found in the mother-liquor from the weights 

 originally taken, we obtained the weights that had passed into the precipitate. 



In addition thereto, the precipitated chloroplatinate, as a rule at least, was 

 analysed, more or less completely. Our method of analysis, in the earlier 

 experiments, was to reduce the chloroplatinate in a boat standing within a 

 combustion-tube, in hydrogen gas. To avoid loss the out-going gas was made 

 to bubble through a little water contained in a bulbed U-tube. The contents of 

 the U-tube were evaporated to dryness, the residue united with the aqueous 

 extract of the ignited mass and the washings of the combustion-tube, the 

 whole evaporated to dryness, the residue re-dissolved in water, and the small 

 quantity of platinum, which invariably separated out (even from perfectly 

 clear liquors) collected and weighed. In one case the chloride of potassium 

 thus obtained was weighed directly ; as a rule, we relied for it on the 



