EXAMINATION OF COMMERCIAL FERTILIZERS 131 



cipitation in the manner above indicated, thus 

 determining the water-soluble phosphoric acid 

 directly, or it may be determined indirectly by 

 igniting the filter and contents and determining 

 the phosphoric acid insoluble in water by the 

 method prescribed for "total." In that case 

 use the whole solution from the residue for the 

 determination. The "water soluble" will then 

 be found by difference. 



Citrate Soluble Phosphoric Acid. Take 2 gms., 

 wash out the "water soluble" as before, and 

 then rinse the residue into a 200 cc. flask by use 

 of 100 cc. of absolutely neutral ammonium cit- 

 rate solution of sp.gr. 1 .09. Cork the flask, immerse 

 it in water at 65 and keep it at that tempera- 

 ture for exactly 30 minutes; then filter rapidly, 

 and wash with water of about the same tempera- 

 ture. The "reduced" phosphoric acid is by this 

 means removed. It will, however, be more con- 

 venient to determine it indirectly by igniting 

 the filter-paper and contents, and carrying 

 through the determination on the portion un- 

 dissolved, which really constitutes the " insoluble 

 phosphoric acid. 



Ammonium tartrate and also ammonium ox- 

 alate, have been used in place of the ammonium 

 citrate, but the results with those salts have been 

 found irregular and unsatisfactory, and their 

 use has been practically abolished. Especial 

 care is necessary to have the citrate solution 

 absolutely neutral a result not attained by dis- 



