138 AGRICULTURAL ANALYSIS 



If the operations which precede have been well conducted, the 

 total phosphoric acid contained in the material under examina- 

 tion is found upon the filter paper, except the small portion which 

 remains adhering to the beaker in which the precipitation has 

 been made. The determination of the phosphoric acid comprises 

 the following operations : First, solution of the ammonium mag- 

 nesium phosphate, and second, titration by means of a standard 

 solution of uranium. 



124. Solution of the Ammonium Magnesium Phosphate. The 

 phosphate which has been collected upon the filter is dissolved by 

 a 10 per cent, solution of pure nitric acid. This solution is caused 

 to pass into the beaker in which the precipitation was made in 

 order to dissolve the particles of phosphate which remain adherent 

 to its sides ; and this solution is then thrown upon the filter. The 

 filtrate is then received in a flask of about 150 cubic centimeters 

 capacity, marked at 75 cubic centimeters. After two or three 

 washings with the acidulated water, the filter itself is detached 

 from the funnel and introduced into the vessel which contains 

 the solution. 



The whole of the filtrate being collected in the flask, it is sat- 

 urated by one-tenth ammoniacal water until a slight turbidity is 

 produced. One or two drops of dilute nitric acid are now added 

 until the liquor becomes limpid, and the flask is placed upon a 

 sand-bath in order to carry the liquid to the boiling-point. After 

 ebullition there is added five cubic centimeters of acid sodium 

 acetate so as to cause the free nitric acid to disappear, and 

 immediately the titration, by means of a standard solution of 

 uranium, is undertaken. 



125. Acid Sodium Acetate. The acid sodium acetate is prepared 

 as follows : Crystallized sodium acetate, 100 grams ; glacial acetic 

 acid, 50 cubic centimeters ; distilled water, enough to make one 

 liter. 



126. Standard Solution of Uranium Nitrate. A solution of ura- 

 nium is to be prepared as follows : Pure uranium nitrate, 40 

 grams ; distilled water, about 800 cubic centimeters. Dissolve 

 the uranium nitrate in the distilled water and add a few drops of 

 ammonia until a slight turbidity is produced, and then a sufficient 



