BAXTER AND JONES. — ATOMIC WEIGHT OF PHOSPHORUS. 145 



first half of the silver nitrate. The concentration of the silver ion is 

 kept very low by the excess of phosphate and, therefore, little occlu- 

 sion of the acid salts is to be expected in spite of the fact that the 

 solution contains appreciable concentrations of the monohydrophos- 

 phate and dihydrophosphate ions. The precipitate during this stage 

 is very finely divided and does not settle well and, therefore, no 

 attempt was made to collect it separately. 



During the addition of the second half of the silver nitrate the 

 solution becomes slightly acid and the solubility of the silver phos- 

 phate increases rapidly. The precipitate settles readily. During the 

 second stage the conditions are more favorable for the occlusion of 

 the acid phosphate, but only a small amount of silver phosphate is 

 precipitated during this stage. 



After standing a short time the mother liquor was decanted from 

 the precipitate, and exactly the calculated amount of redistilled 

 ammonia, diluted to one liter, was added to neutralize the excess 

 of acid and complete the precipitation. Since this sample was evi- 

 dently produced from a solution which was slightly acid at the be- 

 ginning of the precipitation, although very nearly neutral at the end, 

 and since it contained a considerable amount of silver, the conditions 

 were favorable for the formation of acid salts. 



Both precipitates were transferred to a large platinum dish and 

 washed many times by decantation with the purest water. This 

 washing was prolonged over more than twenty-four hours in order 

 to give time for all soluble matter to be leached out. When the 

 precipitates were tested for nitrate with diphenylamine, negative 

 results were obtained. Sodium was found to be absent by spectro- 

 scopic tests. The precipitates were drained as far as possible in a 

 platinum centrifugal machine, and the drying was completed by heat- 

 ing in platinum crucibles in an electric air bath for several hours, first 

 at 90° and finally at about 130°. The dried lumps of silver phosphate 

 were then gently ground in an agate mortar. The samples were pre- 

 served in platinum crucibles over sulphuric acid in the dark. All of 

 the operations were performed in a dark room. 



The sample prepared by pouring silver nitrate into disodium phos- 

 phate is designated Sample N, and the sample prepared by adding 

 ammonia to the mother liquors is designated Sample 0. 



Sample P. A 0.03 normal solution of disodium ammonium phos- 

 phate was prepared by dissolving a weighed amount of disodium hy- 

 drogen phosphate and then adding the calculated amount of redistilled 

 ammonia. The solution was then slowly poured into a 0.03 normal 

 solution of silver nitrate. By this method of precipitation the solu- 

 vol. xlv. — 10 



