[ARCHIBALD] ATOMIC WEIGHT OF POTASSIUM 51 



A quantity of silver, slightly in excess of the amount required to 

 precipitate the chlorine in the potassium chloride, had meanwhile been 

 weighed out and dissolved in nitric acid. Care was taken that none of 

 the silver should be carried away in the escaping vapours while solution 

 v/as taking place. When the silver had dissolved, the solution was 

 diluted to about 500 c.c. The silver chloride was then precipitated in 

 a dark room. The contents of the flask vigorously shaken for half an hour 

 and then set away for several hours or perhaps over night. A Gooch 

 crucible, fitted with an asbestos mat, is now prepared. The asbestos 

 used was carefully selected and thoroughly purified by treatment with 

 acids. The mat is washed on the filter pump with over a litre of water, 

 and dried in an electric oven for an hour, at a temperature of 135° C. 

 After standing in the desiccator for a sufficient length of time the 

 crucible is weighed, using another platinum crucible of the same weight 

 as a tare. 



In washing the precipitated silver chloride, the procedure was 

 about as follows : — After filtering through the mother liquor, and wash- 

 ing the precipitate twice by shaking it vigorously each time with about 

 150 c.c. of water (adding these washings to the main filtrate), about 

 300 c.c. of water was added, and this was shaken up vigorously and set 

 aside for several hours. This is then poured through the filter, keeping 

 this filtrate by itself. This treatment was repeated with about the 

 same amount of water, and the silver chloride then washed on to the 

 filter, dried for several hours, or perhaps over night, at a temperature 

 of about 125° C. in the electric drying oven, finally dried in a desiccator 

 for three or four hours, and then weighed with the same care that was 

 taken with the empty crucible. 



It should be stated that the filtering and washing above described 

 was carried out in either a subdued or a rul)y light, the silver chloride 

 being at all times protected from strongly actinic rays. As silver 

 chloride is slightly soluble in water, some of the precipitate must have 

 been dissolved by. the 600 cc. used in the final washing. To determine 

 this, the method worked out by Eichards was adopted. By means of 

 the nephelometer, a solution containing a known amount of silver was 

 compared with the unknown solution, hydrochloric acid being added to 

 the former and silver nitrate to the latter. The amount of silver 

 chloride found in the washing water was of course added to the observed 

 V/eight of the precipitate. 



Corrections were also applied where they were appreciable for the 

 asbestos carried away in the filtrate. This Avas determined by filtering 

 through an ashless filter, igniting, and weighing the residue. This 



