A REVISION OF THE ATOMIC WEIGHT OF PHOSPHORUS. 177 



THE DETERMINATION OF SILVER IN SILVER PHOSPHATE. ' 



The platinum boat containing the silver phosphate was transferred to an 

 Erlenmeyer flask of "non-sol" glass of i liter capacity and treated with about 

 30 c.c, of 5 normal nitric acid. Solution took place rapidly. The solution was 

 not perfectly clear, however, owing to a very slight insoluble residue which 

 sometimes settled out on standing. The solution was then heated on a steam 

 bath until the residue dissolved completely. Upon the addition of about one 

 Hter of cold water a very sHght opalescence was produced, which was visible 

 only when the solution was carefully examined in a very favorable light. The 

 solution was again warmed until it became perfectly clear. The water and 

 nitric acid used in these processes did not give an opalescence \nsible in the 

 nephelometer when treated with silver nitrate. The nature of this residue ^vill 

 be discussed more in detail after describing the remainder of the analytical 

 process. 



About 800 c.c. of water was placed in a large glass-stoppered precipitating 

 flask and a very slight excess of hydrobromic acid was added from a burette. 

 The silver phosphate solution was then very carefully poured into the hydro- 

 bromic acid solution. This method of precipitation gives less opportunity for 

 the occlusion of silver phosphate or nitrate than the reverse method. The oc- 

 clusion of hydrobromic acid can do no harm. The flask was shaken for 20 

 minutes and was allowed to stand for several days until the precipitate had 

 completely settled. Then the precipitate was collected upon a weighed Gooch 

 crucible after many rinsings with pure water. In order to protect the mat of 

 the Gooch crucible from disintegration, it was covered by a circular disk of thin 

 platinum foil, perforated with many small holes. The precipitate was dried in 

 an electrically heated air bath for several hours at 90°, then for some time at 

 130°, and finally for at least eight hours at 180°. After the crucible containing 

 the precipitate had been weighed, the silver bromide was transferred to a por- 

 celain crucible and the loss on fusion determined. 



The presence of the platinum disk covering the mat makes it possible to trans- 

 fer very nearly all the silver bromide to the porcelain crucible without contam- 

 ination with asbestos and therefore it is unnecessary to correct the loss on 

 fusion for the small amount of silver bromide which is not fused. The loss on 

 fusion, which represents water remaining in the silver bromide, was subtracted 

 from the weight of the silver bromide. The asbestos shreds carried away by 

 the wash-waters and any silver bromide which may have escaped the Gooch 

 crucible were collected by passing the filtrate through a very small filter paper. 

 The paper was then burned and the residue, after treatment with a drop of 

 nitric and hydrobromic acids to convert any reduced silver into silver bromide, 

 was again gently heated and finally was weighed. The weight of the asbestos, 

 corrected for the ash of the paper, was added to the weight of the silver 

 bromide. 



