22 



EXPERIMENTAL 



I. Methods of Determining Phosphoric Acid. 



The work about to be described necessitated a number of 

 phosphoric acid determinations. Hence it was thought advisible 

 in most cases to employ the Optional Volumetric Method of the 

 Association of Official Agricultural Chemists 1 . In order to test its 

 accuracy, this method was compared with the Official Gravimetric 

 Method 2 , on a variety of materials typical of those to be con- 

 sidered in the experiments following. The methods were carried 

 out as follows : 



Volumetric Method. The sample was dissolved in from 15 

 to 30 c.c. of strong hydrochloric acid and from 3 to 10 c.c. of nitric 

 acid. The solution was cooled, diluted to 250 c.c., and an aliquot 

 part taken for the determination. This portion was treated with 

 id c c. of nitric acid, nearly neutralized with ammonia, diluted 

 to about 100 c.c., and heated in the water bath to 65 degrees C. 

 Molybdic solution was then added (the amount depending on the 

 amount of P 2 O 5 present) the mixture stirred, allowed to stand 

 thirty minutes, and filtered on an asbestos filter. After thor- 

 ough washing with cold water, the yellow precipitate was re- 

 turned to the precipitating beaker, dissolved in standard potas- 

 sium hydroxide, a few drops of phenolphthalein added, and the 

 excess of alkali titrated with standard nitric acid. Each cubic 

 centimeter of the standard potassium hydroxide was equivalent to 

 r mg. P 2 5 . 



Gravimetric Method. The sample was dissolved in from 

 15 to 30 c.c. of strong hydrochloric acid and from 3 to 10 c.c. of 

 nitric acid. The solution was cooled diluted to 250 c.c., and an 

 aliquot part corresponding to 0.3 gram taken for analysis. The 

 solution was treated with loc.c. of nitric acid, nearly neutralized 

 with ammonia, diluted to about 100 c.c., and heated to 65 degrees 



1 U. S. Dept. Agr., Div. Chetn., Bui. No. 46, 13 (revised). 



2 Ibid., 10. 



