4 METHODS OF ANALYSIS [Chap. 



WATER-SOLUBLE PHOSPHORIC ACID. 



10 Gravimelric Method. — Official. 



Place 2 grams of the sample on a 9 cm. filter, wash with successive small portions 

 of water, allowing each portion to pass through before adding more, until the filtrate 

 measures about 250 cc. If the filtrate is turbid, add a little nitric acid. Make up 

 to any convenient volume, mix well, use an aliquot, and proceed as under 6. 



1 1 Volumetric Method. — Official. 



Treat the sample as directed under 10. To an aliquot of the solution cor- 

 responding to 0.2 or 0.4 gram, add 10 cc. of concentrated nitric acid and ammonium 

 hydroxid until a slight permanent precipitate is formed, dilute to 60 cc, and pro- 

 ceed as directed under 9. 



CITRATE-INSOLUBLE PHOSPHORIC ACID.— OFFICIAL. 



12 REAGENTS. 



In addition to the reagents given under 4 and 7 prepare ammonium citrate solu- 

 tion by one of the following methods: 



Ammonium citrate solution. — (1) Dissolve 370 grams of commercial citric acid 

 in 1500 cc. of water; nearly neutralize with commercial ammonium hydroxid; cool; 

 add ammonium hydroxid until exactly neutral (testing with litmus or azolitmin 

 paper), and dilute sufficiently to make the specific gravity 1.09 at 20°C. The vol- 

 ume will be about 2 liters;- or, 



(2) To 370 grams of commercial citric acid add commercial ammonium hydroxid 

 until nearly neutral; reduce the specific gravity to slightly more than 1.09 at 20°C. 

 and make exactly neutral, testing as follows: Prepare a solution of fused calcium 

 chlorid, 200 grams to the liter, and add 4 volumes of strong alcohol. Make this 

 solution exactly neutral, using a small amount of freshly prepared corallin solution 

 as preliminary indicator, and test finally by withdrawing a portion, diluting with 

 an equal volume of water, and testing with cochineal solution; 50 cc. of this solution 

 will precipitate the citric acid from 10 cc. of the citrate solution. To 10 cc. of the 

 nearly neutral citrate solution add 50 cc. of the alcoholic calcium chlorid solution, 

 stir well, filter at once through a folded filter, dilute with an equal volume of 

 water, and test the reaction with neutral solution of cochineal. If acid or alkaline, 

 add ammonium hydroxid or citric acid, as the case may be, to the citrate solution, 

 mix, and test again, as before. Repeat this process until a neutral reaction is 

 obtained. Add sufficient water to make the specific gravity 1.09 at 20°C. 



13 DETERMINATION. 



(a) Acidulated samples. — Heat 100 cc. of strictly neutral ammonium citrate 

 solution (sp. gr. 1.09) to 65°C. in a 250 cc. Erlenmeyer flask placed in a warm water 

 bath, keeping the flask loosely stoppered to prevent evaporation. The level of the 

 water in the bath should be above that of the citrate solution in the flask. When 

 the citrate solution has reached 65°C., drop into it the filter containing the washed 

 residue from the water-soluble phosphoric acid solution in 1 0, close tightly with 

 a smooth rubber stopper, and shake violently until the filter paper is reduced to 

 a pulp, relieving the pressure by momentarily removing the stopper. Place the 

 flask in the bath and maintain its contents at exactly 65°C. Shake the flask every 

 5 minutes. At the expiration of exactly 30 minutes from the time the filter and the 

 residue are introduced, remove the flask from the bath and immediately filter the 

 contents as rapidly as possible through a quick-acting filter paper. Wash with 



