A REVISION OF THE ATOMIC ^VEIGHT OF PHOSPHORUS. 



i8i 



DETERMINATION OF MOISTURE IN THE DRIED SILVER 



PHOSPHATE. 



In order to find out how efficient the drying of the silver phosphate had been, 

 experiments were made to determine the amount of water retained by silver 

 phosphate which had been dried for analysis as described above. (See page 

 176.) The water was determined by fusing the dried phosphate in a current 

 of dry air and collecting the moisture set free in a weighed phosphorus pentoxide 

 tube. Since the melting point of pure silver phosphate is considerably above 

 the softening point of hard glass, it was found advantageous to lower the melt- 

 ing point of the phosphate by the use of silver chloride as a flux. 



About 15 gm. of silver phosphate were placed in one end of a large silver 

 boat and in the other end about 12 gm. of previously fused silver chloride. 

 The boat was then inserted in a hard-glass tube and dried under the same con- 

 ditions as prevailed in preparing the samples for the determination of the silver 

 content. After the silver phosphate had been heated for seven hours in a current 

 of purified air dried by phosphorus pentoxide, the air passing over the boat in 

 the furnace was conducted through a weighed U-tube containing resub limed 

 phosphorus pentoxide for 30 minutes. This was done to make sure that all the 

 water which had been liberated from the silver phosphate without fusion had 

 been swept out of the apparatus. In no case was there a gain in weight during 

 this process of more than 0.05 mg., which is about the limit of error in weighing 

 the phosphorus pentoxide tubes. The backward diffusion of moisture was pre- 

 vented by a second tube containing pentoxide. 



The carefully weighed phosphorus pentoxide tube was again attached to the 

 tube containing the silver boat with its charge of silver phosphate and silver 

 chloride. The latter tube was then heated hot enough to fuse the silver chlo- 

 ride, which flowed down to the silver phosphate and readily caused the entire 

 charge to fuse completely. The liberated water was swept into the phosphorus 

 pentoxide tube by a current of dry air for about 30 minutes. The tube was 

 then reweighed to determine the water evolved by the fusion of silver phosphate. 

 The pentoxide tube was weighed by substitution for a very similar counter- 

 poise tube, one stopcock of each tube being open during the weighing. Before 

 being weighed both tubes were wiped with a damp cloth and allowed to stand 

 near the balance for at least 30 minutes. 



The following table gives the results of these experiments: 



