122 AGRICULTURAL ANALYSIS 



DIRECT WEIGHING OF THE PHOSPHOMOLYBDATE 

 PRECIPITATE 



no. Method of Hanamann. It has already been stated that 

 many attempts have been made to determine the phosphoric acid 

 by direct weighing as well as by titration, as in the Pemberton 

 method. The point of prime importance in such a direct de- 

 termination is to secure an ammonium phosphomolybdate mix- 

 ture of constant composition. Unless this can be done no direct 

 method, either volumetric or gravimetric, can give reliable re- 

 sults. Hanamann proposes to secure this constant composition 

 by varying somewhat the composition of the molybdate mixture 

 and precipitating the phosphoric acid under definite conditions. 90 

 The molybdate solution employed is prepared as follows : 



Molybdic acid 100 grams. 



Ten per cent, ammonia I .o liter. 



Nitric acid ( 1.246 sp.gr.) i .5 liters. 



The precipitation of the phosphoric acid is conducted in the 

 cold with constant stirring. It is complete in half an hour. The 

 ammonium phosphomolybdate is washed with a solution of ammo- 

 nium nitrate and then with dilute nitric acid, dried, and ignited 

 at less than a red heat. It should then have a bluish black color 

 throughout. Such a body contains 4.018 per cent, of phosphoric 

 anhydrid. 



Twenty-five cubic centimeters of a sodium phosphate solution 

 containing 50 milligrams of phosphoric acid (P 2 O 5 ), treated as 

 above, gave a bluish black precipitate weighing i .249 grams, which 

 multiplied by 0.04018, equaled 50.018 milligrams of phosphorus 

 pentoxid. The method should be tried on phosphates of various 

 kinds and contents of phosphorus pentoxid before a definite judg- 

 ment of its merits is formed. The method is applicable to phos- 

 phates containing a large percentage of phosphoric acid as well as 

 to compounds having very little. In the former case only a small 

 aliquot of the solution is subjected to precipitation, while in the 

 latter, all or large portions are used. In one mixture 20 grams of 

 superphosphate were dissolved in one liter of water and 10 cubic 

 centimeters of the solution poured into 35 cubic centimeters of the 

 90 Chemiker-Zeitung, 1895, 19 : 553. 



