198 AGRICULTURAL ANALYSIS 



five grams of slag are used the mean volume of gypsum in the 

 solution is about two cubic centimeters. 



170. Estimation of Total Acid. In the determination of total 

 phosphoric acid in a slag, 25 cubic centimeters of the strongest 

 sulfuric acid are placed in an erlenmeyer having a wide neck, 

 and with careful shaking five grams of the fine slag meal grad- 

 ually added. The flask is heated over a naked flame until solu- 

 tion is complete. When the mass is cold it is washed into a quar- 

 ter liter flask ; again allowed to cool, filled with water to the mark, 

 and two cubic centimeters of water, corresponding to the volume 

 of gypsum undissolved, are added, well mixed, and filtered. In 

 50 cubic centimeters of the filtrate, the phosphoric acid is deter- 

 mined by either the molybdate or citrate methods already de- 

 scribed. 



171. Alternate Method. The following method may also be 

 used : Ten grams of the substance are heated with 50 cubic cen- 

 timeters of concentrated sulfuric acid until white vapors have 

 been evolved for some time. The operation lasts for about 15 

 minutes and can be carried on in a half-liter flask or in a por- 

 celain dish. Without regarding the undissolved material, the 

 volume of the liquid is now made up to half a liter and filtered. 

 The filtered liquid becomes turbid after some time through the 

 separation of calcium sulfate, but this turbidity should not be 

 regarded. To 50 cubic centimeters of the solution, correspond- 

 ing to one gram of substance, 20 cubic centimeters of citric acid 

 (500 grams citric acid to the liter) are added, and it is after- 

 wards nearly neutralized by the addition of 10 per cent, am- 

 monia and the liquid, which is warmed by this operation, cooled. 

 There are added 25 cubic centimeters of the ordinary magnesium 

 chlorid mixture and the solution stirred until turbidity is pro- 

 duced, one-third of its volume of 10 per cent, ammonia added, 

 and again stirred for about a minute to promote precipitation. 



Instead of the addition of the citric acid and ammonia, am- 

 monium citrate prepared as follows may be added: Fifteen hun- 

 dred grams of citric acid are dissolved with water, made up to 

 three liters, five liters of 24 per cent, ammonia and seven liters 



