DETERMINATION OF PHOSPHORIC ACID 109 



sent are carried down with it. A trace of phosphoric acid has 

 been found in the iron and alumina subsequently separated from 

 the solution. The precipitate obtained is dissolved in aqua regia, 

 made strongly ammoniacal and an excess of ammonium hydrosul- 

 fid added. The iron and the alumina are thrown out by this 

 treatment. After standing for an hour the precipitate is separat- 

 ed by filtration, washed with the ammonium sulfid to remove 

 the last traces of tin and the phosphoric acid is separated from 

 its filtrate as ammonio-magnesium salt. Following is the second 

 method of conducting the analysis as described by Crookes: 78 



The phosphate should be dissolved in nitric acid, and any 

 chlorin present be expelled by repeated evaporations with the 

 solvent. Finally, to the evaporated mass the strongest nitric 

 acid is added. Pure tin foil is added and heat applied. The phos- 

 phoric acid is precipitated by the stannic acid formed. The 

 quantity of tin used should be from four to five times as great as 

 that of the phosphoric acid present. The preliminary heating 

 is indispensable, since in the cold the metal is apt to become pas- 

 sive in which state it resists the action of the acid. 



The precipitate is collected on a filter, washed and dissolved 

 in caustic potash. The solution is saturated with hydrogen sul- 

 fid, and on adding acetic acid in slight excess the tin sulfid is 

 separated and removed by filtration. The whole of the phos- 

 phoric acid, supposed to be almost free of tin, is now found in 

 the filtrate. The filtrate is concentrated to small bulk and any 

 tin sulfid present separated by filtering, and the phosphoric acid 

 finally removed from the ammoniacal filtrate by precipitation 

 with magnesia mixture. The chief difficulties of this method 

 are to be found, on the one hand, in the retention of some of the 

 phosphoric acid by the iron and alumina which may be present, 

 and on the other, in the presence of some tin in the final mag- 

 nesium pyrophosphate. If the tin be all removed as sulfid, the 

 latter source of error will be avoided. It is difficult to secure pure 

 metallic tin, and this is another disturbing element in the process. 



78 Select Methods in Chemical Analysis, 4th Edition, 1905 : 497. 



