174 RESEARCHES UPON ATOMIC WEIGHTS. 



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-Hquor was decanted from the pre- 

 cipitate, and exactly the calculated amount of redistilled ammonia, diluted to 

 I Hter, was added to neutralize the excess of acid and complete the precipitation. 

 Since this sample was evidently produced from a solution which was slightly 

 acid at the beginning 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 24 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 spectroscopic tests. 

 The precipitates were drained as far as possible in a platinum centrifugal 

 machine, and the drying was completed by heating 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 preserved 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 phosphate is 

 designated Sample N, and the sample prepared by adding ammonia to the 

 mother-liquors is designated Sample O. 



Sample P. — A 0.3 normal solution of disodium ammonium phosphate was 

 prepared by dissolving a weighed amount of disodium hydrogen 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 solution is maintained as nearly neutral as is possible, be- 

 cause the excess of silver prevents the concentration of phosphate in solution 

 from exceeding a very small value, so that neither can the solution become 

 alkaline by hydrolysis nor can the concentration of hydrophosphate attain an 

 appreciable value. The absence of the hydrophosphate ions would be expected 

 to prevent the formation and occlusion of acid silver phosphate in this sample 

 whereas in Sample N the same result is probably brought about by the absence 

 of the silver ion. Unfortunately both of these favorable conditions can not be 

 combined in one precipitation, as will be shown later. This precipitate 

 settled readily. The washing, testing, and drying were carried out as already 

 described for Samples N and O. This sample is designated Sample P. 



Sample R. — A 0.03 normal solution of sodium ammonium hydrogen phos- 

 phate was slowly poured into a similar solution of an equivalent amount of 

 silver nitrate. Under these conditions the solution contains an excess of silver, 

 which tends to produce occlusion of acid phosphates, since the solution becomes 



