A REVISION OF THE ATOMIC WEIGHT OF PHOSPHORUS. 171 



buttons were then scrubbed with sea-sand and thoroughly cleansed with am- 

 monia and nitric acid. They were then dissolved in redistilled nitric acid, in a 

 platinum dish. After the silver nitrate solution had been evaporated on a steam 

 bath imtil saturated, an equal volume of redistilled nitric acid was added and the 

 solution was cooled. The precipitated silver nitrate was very completely drained 

 in a centrifugal machine, provided with platinum Gooch crucibles to retain the 

 salt.^ A similar recrystallization followed. The final product was preserved 

 in Jena glass vessels under a bell- jar. 



Disodium Phosphate. — One kilogram of Merck's best disodium phosphate was 

 dissolved in hot water in a porcelain dish and hydrogen sulphide was passed into 

 the solution for several hours. After standing for 24 hours, the solution was 

 again heated, saturated with hydrogen sulphide and filtered. The filtrate was 

 slightly green, owing to the presence of iron. The solution was boiled to expel 

 the hydrogen sulphide and a small amount of green precipitate filtered out. The 

 filtrate was still distinctly green. The sodium phosphate was then crystallized 

 fifteen times, five times in porcelain with centrifugal drainage of the crystals 

 in a large porcelain centrifugal machine, ten times in platinum vessels with cen- 

 trifugal drainage of the crystals in platinum Gooch crucibles. The green color 

 concentrated in the first mother-Uquor, When tested by means of the Marsh 

 test, this material was found to contain only a mere trace of arsenic, which was 

 estimated to be o.oi mg. in 10 gm. of the salt. This small amount could have no 

 effect on the analytical results, especially since the percentage of silver in silver 

 arsenate is nearly the same as in silver phosphate. By means of the nephelo- 

 meter it was proved that this material contained no chloride or other substances 

 which could be precipitated by silver nitrate in the presence of dilute nitric 

 acid. 



Sodium Ammonium Hydrogen Phosphate. — The best commercial micro- 

 cosmic salt was recrystallized four times in platinum vessels. It was tested for 

 arsenic by Marsh's method with negative results and gave no opalescence visi- 

 ble in the nephelometer when tested with silver nitrate and dilute nitric acid. 



PREPARATION OF TRISILVER PHOSPHATE. 



Silver phosphate was prepared by mixing dilute solutions of silver nitrate 

 with solutions of sodium and ammoniima phosphates. Since it is not feasible 

 to purify silver phosphate by recrystallization, the conditions of precipitation 

 must be so chosen that a pure product will be obtained at once. 



In order to avoid inclusion and occlusion of silver nitrate, sodium nitrate, 

 sodium phosphate, or monosilver or disilver phosphate, all of the solutions 

 for precipitation were made about 0.03 N. All samples after precipitation were 

 thoroughly washed and allowed to stand in water for at least 24 hours, in order 

 to convert occluded acid phosphates into trisilver phosphate. Qualitative tests 



* Baxter: Jour. Amer. Chetn. Soc, 30, 286 (1908). 



