THE FRENCH OFFICIAL METHOD 73 



4. Superphosphates, precipitated (reverted) phosphates, am- 

 monia-magnesium phosphates. 



5. Phosphatic slags. 



In the first class the phosphoric acid is determined by direct 

 precipitation by the citrate method. 



In the second and third classes previous to the separation of the 

 phosphoric acid the organic matter is destroyed after treating 

 with some slaked lime to prevent the organic matter from reduc- 

 ing any phosphate. After the reduction of the organic matter 

 the process is continued as in the first class. 



In the fourth class the precipitated (reverted) phosphoric acid 

 is dissolved in ammonium citrate. About 0.75 gram of the sample 

 is rubbed in a mortar with a few drops of the citrate solution 

 and the paste washed into a flask of 150 cubic centimeters capac- 

 ity with 60 cubic centimeters of the citrate of ammonia solution 

 and digested with frequent shaking for 12 hours. The flask is 

 subsequently filled to the mark and, after shaking, the contents 

 are poured on a filter, and in 100 cubic centimeters of the filtrate, 

 representing 0.5 gram of the sample, the phosphoric acid is sepa- 

 rated as above. 



The total phosphoric acid is determined as in the first in 

 stance. 



In the fifth class the phosphoric acid is separated as usual 

 after solution in hydrochloric acid and not nitric. When the 

 slags are very rich in lime it is advisable to dissolve first in acetic 

 acid and separate the greater part of the lime as oxalate before 

 dissolving the phosphoric portion of the slag in hydrochloric acid. 



The molybdate method is also used officially by the French 

 chemists in harmony with the usual directions. 



The methods employed are so nearly like those already described 

 that their repetition is not deemed necessary. The determination 

 of the degree of fineness of the sample is properly regarded by 

 the French chemists as of great importance. 



In the case of natural phosphates and slags it is advised that 

 the sample be separated by a sieve of 0.17 millimeter mesh. At 

 least 90 per cent, of the sample should pass such a sieve. 



