METHODS ADOPTED BY THE BRUSSELS CONGRESS 99 



When the mixture has cooled, add slowly 25 cubic centimeters 

 of the magnesia mixture, and then 42 cubic centimeters of the 

 ammonia. Keep the solution stirred by means of a closely 

 clipped feather which is pressed tightly against the sides of the 

 beaker; by this process the phosphate is precipitated after half 

 an hour in pure condition and completely without in the least 

 sticking to the wall of the beaker; filter, wash, and ignite, as 

 usually directed. 



(&) Insoluble PhosphoHc Acid. Moisten, in a porcelain dish, 

 10 grams of the powdered sample with water; add 50 cubic 

 centimeters of concentrated sulfuric acid, and heat for 15 min- 

 utes so that fumes of sulfuric acid will escape. When the mass 

 has cooled, wash it into a half-liter graduated flask, fill to 

 the mark, and shake well. After filtration, the clear filtrate may, 

 after some time, turn turbid by separation of calcium sulfate, but 

 as the ammonium citrate, which is afterwards added, again brings 

 the precipitate into solution, it is of no importance. Add to 50 

 cubic centimeters of the solution, corresponding to one gram of 

 the powdered sample, 20 cubic centimeters of the citric acid 

 solution, neutralize the mixture approximately, but not exactly, 

 by ammonia ; after cooling, add 25 cubic centimeters of magnesia 

 mixture; stir the fluid by means of a feather, as described, till 

 no more precipitate is formed, and finally add 33 cubic centi- 

 meters of ammonia while stirring for several minutes longer; 

 after half an hour the precipitate may be separated by filtration, 

 washed, and ignited, as usually directed. 



The above process is essentially the one used with basic slags. 

 When much organic matter is present, by continuing the heating 

 with sulfuric acid for some time it may be destroyed. 



92. Methods Adopted by the Brussels Congress, 1894. The re- 

 port of the committee on methods of analysis of phosphoric acid 

 requires the molybdate method to be used in all cases where the 

 quantity to be determined is very small. In other cases the citrate 

 method may be employed. 70 



(i) Soluble Phosphoric Acid. The soluble phosphoric acid is 

 determined by the method adopted at Brussels in the following 



70 L'Engrais, 1894, 9 : 928. 



