2IO AGRICULTURAL, ANALYSIS 



acid and the mixture evaporated to a thick sirup, smelling of hy- 

 drochloric acid. To the hot residue, from 1.5 to two cubic centi- 

 meters of hydrochloric acid, 1.12 specific gravity, are added, and 

 the mixture thoroughly stirred and dissolved in enough water to 

 make the volume 100 cubic centimeters. The phosphoric acid is 

 determined in 50 cubic centimeters of the filtrate by the direct 

 method. 



The process consists in adding 50 cubic centimeters of the 

 citrate-magnesia mixture to the same volume of the filtered citric 

 acid extract of the slag. The magnesia mixture contains 550 

 grams of magnesium chlorid and 700 grams of ammonium chlorid 

 dissolved in 3.5 liters of eight per cent, ammonia, and 6.5 liters 

 of water. The ammoniacal citrate solution for mixing with the 

 magnesia mixture, mentioned above, contains 2000 grams of citric 

 acid dissolved in 20 per cent, ammonia, and the volume made up 

 to 10 liters with the same reagent. Before use, equal parts of 

 the magnesia mixture and the ammoniacal citrate solution are 

 mixed together. 



184. Bottcher Method. The Bottcher modification of the direct 

 citrate method of determining the phosphoric acid dissolved from 

 basic slags by citric acid is as follows : 57 



In 50 cubic centimeters of the solution of a slag in citric acid 

 according to Wagner's method, the phosphoric acid is thrown 

 down in the prescribed manner by magnesium citrate solution, 

 the precipitate collected on a filter, washed several times with 

 five per cent, ammonia, and the moist filter ashed. The ash is 

 dissolved in warm hydrochloric acid, the dilute solution passed 

 through a small filter, washed with hot water, and the phosphoric 

 acid again precipitated with the citrate magnesia. 



The important point is that after the addition of the citrate of 

 magnesia, the mixture be immediately vigorously shaken and then, 

 without a moment's delay, filtered. Even standing for from 

 half an hour to an hour may cause serious annoyance and intro- 

 duce serious errors into the results. 



The direct precipitation of the phosphoric acid by ammoniacal 

 citrate of magnesia was adopted as the official method in the 

 57 Chemiker-Zeitung, 1897, 21 1: 168. 



