178 RESEARCHES UPON ATOMIC WEIGHTS. 



In order to determine the soluble silver bromide, the filtrate was evaporated 

 until most of the excess of nitric acid was driven off. The precipitating flask 

 and all the flasks which had held the filtrate were rinsed with strong ammonia 

 and the rinsings added to the evaporated wash-water. Enough ammonia was 

 added to make the solution alkaHne and it was then diluted to 100 c.c. in a 

 graduated flask. The amount of silver bromide present was determined by 

 comparison in the nephelometer with a very similar solution containing a 

 known amount of silver bromide. Both precipitates were dissolved in ammonia 

 and reprecipitated at the same time and under precisely similar conditions in 

 the nephelometer tubes by a slight excess of nitric acid. The amount found in 

 this way was added to the weight of the silver bromide. 



In order to determine whether silver phosphate is occluded by silver chloride, 

 about 6 gm. of silver phosphate were dissolved in nitric acid and the solution 

 was diluted and poured into an excess of hydrochloric acid. After standing until 

 the supernatant hquid was clear, the precipitate was washed very thoroughly 

 with water and then dissolved in redistilled ammonia. The solution was diluted 

 to I liter and the silver chloride was reprecipitated with nitric acid. The precipi- 

 tate was filtered out and the filtrate evaporated in a platinum dish until concen- 

 trated. A little sodium carbonate was added and the dish was heated to expel 

 all volatile ammonium salts. The residue was dissolved in about 3 c.c. of water, 

 and treated with an excess of ammonium molybdate reagent with gentle warm- 

 ing. After standing for 3 days', not the slightest precipitate ot yellow color had 

 appeared, showing that no phosphate had been occluded by the silver chloride. 

 Although not tested experimentally, it is reasonable to suppose that silver 

 bromide also does not possess the property of occluding appreciable quantities 

 of silver phosphate or phosphoric acid. 



INSOLUBLE RESIDUE. 



The presence of a slight residue or opalescence, after dissolving the dried 

 silver phosphate in dilute nitric acid, proved the most perplexing difficulty 

 which was encountered. The effort to discover the nature of this insoluble 

 matter and eliminate it consumed a large part of the time devoted to this 

 research. In an effort to make sure that it was not due to some unknown 

 impurity, nineteen different samples of silver phosphate were prepared, the 

 source of material, method of purification, and method of precipitation being 

 varied. Disodium phosphate, trisodium phosphate, and sodium ammonium 

 phosphate were carefully purified and converted into silver phosphate under 

 varying conditions without appreciable effect upon the amount of the residue. 

 Phosphorus oxychloride was twice fractionally distilled, converted into phos- 

 phoric acid, and then into disodium phosphate by means of sodium hydroxide 

 made from sodium amalgam. The product was crystallized three times. Silver 

 phosphate made from this material gave a slight residue, very similar to that 

 obtained from the best samples made in other ways. Unfortunately, it was 



