4 METHODS OF ANALYSIS. 



(5) CITRATE-SOLUBLE PHOSPHOBIC ACID. 



The sum of the water-soluble and citrate-insoluble subtracted from the total 

 gives the citrate-soluble phosphoric acid. 



(b) OPTIONAL VOLUMETRIC METHOD. 



(1) PREPARATION OF REAGENTS. 



(a) Molybdate solution. To 100 cc of molybdate solution, prepared as 

 directed on page 2, add 5 cc of nitric acid, specific gravity 1.42. This solution 

 should be filtered each time before using. 



(6) Standard sodium or potassium hydroxid solution. Dilute 323.81 cc of 

 normal alkali, which has been freed from carbonates, to 1 liter. One hundred 

 cubic centimeters of the solution should neutralize 32.38 cc of normal acid ; 1 cc 

 is equal to 1 mg of P 2 O S (1 per cent of P 2 O B on a basis of 0.1 gram of substance). 



(c) Standard acid solution. The strength of this solution is the same as, or 

 one-half of, the standard alkali solution, and is determined by titrating against 

 that solution, using phenolphthalein as indicator. Any mineral acid may be 

 used. 



(d) Phenolphthalein solution. Dissolve one gram of phenolphthalein in 100 

 cc of alcohol. 



(2) TOTAL PHOSPHORIC ACID. 



(a) Methods of making solution. Dissolve according to methods (c 2 ), (a 5 ), 

 (a 6 ), or (a 7 ), (page 2), preferably by (a 5 ), when these acids are a suitable 

 solvent, and dilute to 200 cc with water. 



(b) Determination. (l^) For percentages of 5 or below use an aliquot cor- 

 responding to 0.4 gram of substance, for percentages between 5 and 20 use an 

 aliquot corresponding to 0.2 gram of substance, and for percentages above 20 

 use an aliquot corresponding to 0.1 gram of substance. Add from 5 to 10 cc 

 of nitric acid, depending on the method of solution (or the equivalent in am- 

 monium nitrate), nearly neutralize with ammonium hydroxid, dilute to from 

 75 to 100 cc, heat in water bath to from 60 to 65 C., and for percentages 

 below 5 add from 20 to 25 cc of freshly filtered molybdate solution. For per- 

 centages between 5 and 20 add from 30 to 35 cc of molybdate solution ; stir, let 

 stand about 15 minutes, filter at once, wash once or twice with water by de- 

 cantation, using from 25 to 30 cc each time, agitating the precipitate thoroughly 

 and allowing to settle; transfer to filter and wash with cold water until two 

 fillings of the filter do not greatly diminish the color produced with phenol- 

 phthalein by one drop of the standard alkali. Transfer precipitate and filter to 

 beaker or precipitating vessel, dissolve in small excess of standard alkali, add 

 a few drops of phenolphthalein solution, and titrate with standard acid. 



(6 2 ) Proceed as directed in (&,) with this exception : Heat in a water bath at 

 45 to 50 C., add the molybdate solution, and allow to remain in the bath with 

 occasional stirring for 30 minutes. 



(&,) Proceed as in (fcj to the point where the solution is ready to place in 

 the water bath. Then cool solution to room temperature, add molybdate solu- 

 tion at the rate of 75 cc for each decigram of phosphoric acid present, place the 

 stoppered flask containing the solution in a shaking apparatus and shake for 

 30 minutes at room temperature, filter at once, wash, and titrate as in preceding 

 method. 



(3) WATER-SOLUBLE PHOSPHOBIC ACID. 



Dissolve according to directions given under "Gravimetric method," (3), 

 page 3. To an aliquot portion of the solution corresponding to 0.2 or 0.4 





