CRITICAL EXPERIMENTS FOR DETERMINATION OF POTASSIUM, ETC. 565 



gas (purified by washing with water, and filtration through asbestos), the 

 stopper put on, and the whole allowed to stand. 



After about twelve hours the chlorine is mostly absorbed. The bottle is 

 then refilled with chlorine, again allowed to stand, &c, until the metal 

 is all, or mostly, dissolved. The solution is decanted off, again chlorinated 

 to make sure of the absence of platinous salt, evaporated on a water-bath 

 to expel the surplus chlorine and hydrochloric acid, and diluted to the 

 proper volume. Of course, if it is meant to be used as an exact standard 

 solution, the remnant of undissolved metal is collected, washed, ignited, 

 weighed, and allowed for. With a " chlorine Kipp " (see Dittmar's Exer- 

 cises in Quantitative Analysis, p. 137) at hand, the method is not so tedious 

 as it appears at first sight. We have long come to adopt it even for ordinary 

 laboratory purposes. When used in connection with work here reported on, 

 it is referred to as the " chlorine process." 



Chloride of Potassium. 



A few of our earlier experiments were made with a chloride obtained 

 from recrystallised chlorate, but in the majority of cases we prepared 

 our chloride of potassium from recrystallised jptfrchlorate. The perchlorate 

 is heated in a platinum basin until the bulk of the oxygen is removed. The 

 rest is then expelled by fusing the residue in a platinum crucible, and keeping 

 it at a dull-red heat until every trace of gas-bells has disappeared. The 

 fused salt is poured into a platinum basin, allowed to cool, broken up, and 

 used in this condition. The neutrality of the fused salt was made sure of 

 by dissolving 2 grms. in water, and titrating the "alkali" with very dilute 

 standard solutions of hydrochloric acid and caustic potash, in the presence of 

 aurine as an indicator. The "alkali" was found equivalent to 2io x iK 2 

 mgs. = 0*2 mg., or rather nil. 



Pure Sal- Ammoniac. 

 See the section on chloroplatinate of ammonium, page 627. 



Rubidium Chloride. 

 See the section on its chloroplatinate, page 618. 



Standard Solution of Nitrate of Silver. 

 As a rule, we prepared this reagent from pure nitrate, in which the water 

 had been determined immediately before use. A quantity, containing nxl7 

 grms. of dry nitrate, was dissolved to n litres. For the standardisation of the 

 solution a standard solution of chloride of potassium was made synthetically 

 by dissolving ^ KC1 grms. in water, diluting to 1 litre, and weighing the 



