566 PROFESSOR DITTMAR AND MR JOHN M' ARTHUR ON 



solution produced before the last mixing. 50 (or 100) c.c. of this solution were 

 weighed into a tared phial, 51 (or 101) c.c. of silver solution added, the mixture 

 reweighed to obtain the weight of the silver solution added, then shaken, and 

 the chloride of silver allowed to settle in the dark. The supernatant liquor 

 was decanted off, iron-alum added, and the exact quantity of dissolved silver 

 determined by to-and-fro titration with decimal {i.e. centi-normal) solutions 

 of sulphocyanate of ammonium and nitrate of silver. These solutions were 

 standardised only by volume, and measured off in burettes. From the result 

 and the specific gravity of the stronger silver solution, it was easy to calculate 

 the exact number of grammes of the latter which precipitate 1 3 ff KCl = 74 - 59 

 grms. of chloride of potassium. This number was recorded as the " weight- 

 equivalent " of the reagent, and utilised in the calculation. In a perfectly 

 similar manner we proceeded in our determination of chlorine by titration. The 

 given solution of chloride was diluted to a known weight. A small fraction, 

 weighed out, served for a preliminary titration of the chlorine in which the 

 reagents""" were merely measured by volume. In a larger aliquot part then 

 the exact weight of the chlorine was determined by a method of gravimetric 

 titration closely analogous to the one explained for the standardisation of 

 the silver solution. This particular modification of Volhard's method having 

 been worked out by one of us (long ago) for the determination of the chlorine 

 in sea- water (see "Challenger" Memoirs, Physics and Chemistry, vol. i. in 

 Dittmar's Report on the Composition of Ocean Water), it will be referred to 

 as the Challenger method. 



In most of our analyses we determined the chlorine by means of this titri- 

 metric method ; sometimes we checked the results by gravimetric determina- 

 tions in other aliquot parts of the respective solutions. In a few cases, when 

 we had only little material at our disposal, we combined Volhard's method 

 with the ordinary gravimetric method. The chlorine was precipitated, by 

 means of a known sufficient weight of the standard silver solution, and the 

 chloride of silver collected and weighed. In the filtrate and wash -waters the 

 still unprecipitated silver was then determined by Volhard's method, with 

 volumetrically adjusted decimal (centi-normal) solutions. As each such dupli- 

 cate analysis was preceded by a preliminary titration of the chlorine, we had 

 no difficulty in so adjusting the weight of silver solution to be added, that the 

 excess of silver left unprecipitated could be conveniently determined in the way 

 explained. 



After this introduction, we will proceed next to report on a series of 

 experiments for determining the composition of chloroplatinate of potassium 

 as produced under conditions similar to those prevailing in certain forms of the 

 corresponding analytical method. 



* The silver solution and a volumetric deci-normal solution of sulphocyanate. 



