CITRATE-INSOLUBLE PHOSPHORIC ACID 6 1 



until the filtrate measures about 250 cubic centimeters. If the 

 filtrate be turbid, add a little nitric acid. Make up to any con- 

 venient definite volume; mix well; take any convenient portion 

 and proceed as under total phosphoric acid. 



68. Citrate-Insoluble Phosphoric Acid. The official method ap- 

 plied to samples previously acidulated is as follows: Heat 100 

 cubic centimeters of strictly neutral ammonium citrate solution of 

 1.09 specific gravity to 65 in a flask placed in a bath of warm 

 water, keeping the flask loosely stoppered to prevent evaporation. 

 When the citrate solution in the flask has reached 65, drop 

 into it the filter containing the washed residue from the water- 

 soluble phosphoric acid determination, close tightly with a 

 smooth rubber stopper ; and shake violently until the filter paper 

 is reduced to a pulp. Place the flask again in the bath and 

 maintain the water in the bath at such a temperature that the 

 contents of the flask will stand at exactly 65. Shake the flask 

 every five minutes. At the expiration of exactly 30 minutes 

 from the time the filter and residue are introduced, remove the 

 flask from the bath and immediately filter as rapidly as possible. 

 It has been shown by Sanborn in his investigations, that 

 the filtration is greatly facilitated by adding asbestos pulp. 

 Wash thoroughly with water at 65. Transfer the filter and its 

 contents to a crucible, ignite until all organic matter is destroyed, 

 add from 10 to 15 cubic centimeters of strong hydrochloric 

 acid, and digest until all phosphate is dissolved ; or return the fil- 

 ter with contents to the digestion flask, add from 30 to 35 

 cubic centimeters of strong nitric, and from five to 10 cubic 

 centimeters of strong hydrochloric acid, and boil until all the phos- 

 phate is dissolved. Dilute the solution to 200 cubic centimeters. 

 If desired, the filter and its contents can be treated according to 

 methods i, 2, or 3, paragraph 61, under preliminary treatment of 

 samples containing organic matter. Mix well; filter through a 

 dry filter; take a definite portion of the filtrate and proceed as 

 under total phosphoric acid, paragraph 64. 



In case a determination of citrate-insoluble phosphoric acid be 

 required in non-acidulated goods it is to be made by treating two 

 grams of the phosphatic material, without previous washing 



