CRITICAL EXPERIMENTS FOR DETERMINATION OF POTASSIUM, ETC. 603 



with a measured volume of a standard solution of chloroplatinic acid prepared 

 from chemically pure metal by (in most cases) the chlorine process 

 (see page 564). 



Our final experiments were made in sets of, in general, four analyses of the 

 same kind of mixture. Before passing to these sets, we will shortly report on 

 a single experiment, made with the view of seeing how the Finkener process 

 works with relatively pure potassium salts. 



Analysis of a Mixture of Sodium and Potassium Chlorides, containing 981 



per cent of KC1. 



The solution analysed contained 71 mgs. of Na 2 S0 4 , = 5 -9 mgs. of NaCl, 

 and exactly 3701 grms. of KC1. It was mixed with sulphuric acid equal to 

 49 mgs. of H 2 S0 4 (to give the sodium a better chance of separating out as 

 sulphate), and then Finkenerised with 11 c.c. of a platinum solution, of which 

 10 c.c. would have sufficed by calculation.* Salt-mixture (^R 2 S0 4 + PtCl 6 K 2 ) 

 " recrystallised." Platinum obtained from the final chloroplatinate (by reduction 

 in the wet way)= 0*4855 grm. = 0*3696 grm. of KC1 : error, = —0*5 mg. 



The second ether-alcohol filtrate was worked up for potassium. It amounted 

 to 0*3 mg. of KC1. Assuming the first ether alcohol filtrate (which was not 

 analysed) to have contained the same quantity, we have — 



Total KC1 recovered, = 3696 + 0-6 - 0*3702, instead of 03701 grm. 

 The weight of platinum eliminated by " recrystallisation " was 31 mgs. 

 Hence, if this operation had been omitted, we should have had 

 0-4855 + 0-0031= 0-4886 grm. of platinum, equal to 0-3719 of KC1 ; i.e., a 

 positive error = 1*8 mgs. 



We now pass to the series of trials referred to.t 



I. Set of Experiments. 



Subject : — a " 95 per cent." salt.} Chloride of potassium operated upon in 

 each analysis, about 075 grm. Sulphuric acid required to replace every CI of 

 the mixture by -gS0 4 , = 10-9 c.c. ; actually added, = 12*0 c.c. The whole was 

 evaporated to dryness, and the residue ignited to expel the chlorine. Platinum 

 solution required by calculation, = 20 c.c, we added 21*5 c.c. i.e., a very small 

 excess, which, as we now know, was a mistake. The mixture of PtCl 6 K 2 and 



* See last line of this page. 



t The designations of the following sets of experiments do not in general indicate the order in which 

 they were carried out. 



% Meaning a mixture containing 95 parts of chloride of potassium in 100 of total anhydrous 

 salts. 



