594 PROFESSOR DITTMAR AND MR JOHN M'ARTHUR ON 



The mixture is dissolved in a little water, mixed with more of chloro- 

 platinic acid than necessary for the conversion of both metals into chloro- 

 platinates, and evaporated to a magma on a water-bath. The residue, after 

 cooling, is digested in 30 c.c. of alcohol of 80 per cent, by volume, the liquid 

 decanted off through a small filter, and the precipitate washed with the same 

 alcohol, until the last runnings give only an opalescence with nitrate of silver. 

 The washed precipitate is dried on the filter, dissolved off with boiling water, 

 the solution evaporated in a tared crucible to dryness, dried exhaustively at, 

 first 105° C, then at 130°, and weighed. From the weights, the potassium is 

 calculated (as K 2 0) in order to see what the exact, but unreasoning, applica- 

 tion of the method would lead to. 



But the precipitate can neither be presumed to contain the whole of the 

 potassium, nor to be pure ; hence — 



. Firstly, the crude precipitate is reduced in hydrogen, the platinum weighed, 

 and from the aqueous solution of the chloride of potassium, the latter 

 recovered by a renewed application of Feesenius' method. 



Secondly, the filtrate from the original chloroplatinate (which contains the 

 sodium and excess of chloroplatinic acid), is freed from its alcohol, by 

 evaporation to dryness, the residue reduced in hydrogen, the alkalies are 

 extracted with water, and made into normal sulphates. In these, the 

 potassium which escaped precipitation is recovered by Finkener's method 

 (sal-ammoniac form). The platinum obtained in the reduction of this small 

 quantity of chloroplatinate of potassium is weighed, and the chloride of 

 potassium reconverted into chloroplatinate by Fresenius' process, to be 

 weighed as such. 



The pure chloride of potassium used for preparing the mixtures was made 

 from recrystallised chlorate.* For the preparation of potash-free chloride of 

 sodium, we used two methods, of which the following gave the best results : — 

 Ordinary " pure " sulphate of soda is dissolved in water, and the solution 

 saturated with hydrochloric acid gas, to precipitate about one-half of the 

 alkali metal as chloride, which is collected on a funnel stopped up with a round 

 glass bead fitting pretty closely into the neck of the funnel, and washed with 

 fuming hydrochloric acid. The salt is then dried, redissolved in water, and 

 reprecipitated by hydrochloric acid gas. The dried product contained a mere 

 trace of sulphate, which was neglected. To test the salt for potassium, a 

 large quantity of it was made into sulphate, and 23 grms. of this subjected to the 

 Finkener process, sal-ammoniac form. The potassium extracted was determined 

 in the way just described, and its chloroplatinate identified by microscopic 

 inspection. It amounted to 038 mg. of K 2 calculated from the platinum, and 

 to 43 mg. calculated from the chloroplatinate. From the mean, 0*40 mg., we 



* We subsequently came to prefer the ^erchlorate for obvious reasons. 



