A REVISION OF THE ATOMIC WEIGHT OF PHOSPHORUS. 175 



more and more add as the precipitation proceeds, and as the precipitation is 

 therefore far from complete, the concentrations of the two hydrophosphate 

 ions gradually approach a very considerable value. At no stage could the solu- 

 tion become alkaline by hydrolysis. It should be noticed that the procedure 

 differs from that used in preparing Sample N in that the precipitate is formed 

 in the presence of an excess of silver nitrate instead of an excess of phosphate, 

 and that this difference in the method of mixing greatly changes the conditions 

 of precipitation. 



The precipitate, which was designated Sample R, coagulated and settled 

 quite readily. The washing and drying were completed as usual. 



It will be shown that samples of silver phosphate prepared under these vari- 

 ous conditions have nearly, if not exactly, the same composition. Further 

 proof of the absence of acid phosphate in these samples is given by experiments 

 to be described later which show that no water is given off when this material 

 is fused. 



An attempt to prepare a sample by pouring silver nitrate into disodium am- 

 monium phosphate yielded unsatisfactory results. Since the disodium am- 

 monium phosphate solution was alkaline, owing to hydrolysis, it contained free 

 ammonia, which prevented the precipitation of silver phosphate at first. Nearly 

 one quarter of the silver nitrate was added before a permanent precipitate was 

 produced. At the end of the precipitation the solution was of course essentially 

 neutral. Even after standing for 4 days the precipitate had not appreciably 

 settled. Since the coagulation of the precipitate seems to occur much more 

 readily in the presence of excess of silver, a considerable amount of silver 

 nitrate in solution was added. The precipitate coagulated and settled immedi- 

 ately. It was washed and dried as usual. This sample was somewhat darker in 

 color than the other samples and gave a large amount of insoluble residue when 

 treated -with dilute nitric acid. The analysis showed that it contained about 

 0.02 per cent too much silver. This method of preparation is evidently unsatis- 

 factory. 



Three unsuccessful attempts were made to prepare silver phosphate from 

 trisodium phosphate. The samples obtained in this way did not appear homo- 

 geneous after being dried and contained considerable sodium in spite of pro- 

 tracted washing. Two of these samples were found by analysis to contain, 

 respectively, 4.4 and 4.1 per cent less silver than pure trisilver phosphate. The 

 third of these samples was so unsatisfactory in appearance and in its behavior 

 during its preparation that it was not analyzed. This method of preparing 

 silver phosphate is evidently not suitable for our purpose. Time was lacking 

 to investigate further this anomalous behavior. 



