100 AGRICULTURAL ANALYSIS 



manner: Five grams of the sample are rubbed to a powder in a 

 mortar, and then from 50 to 60 cubic centimeters of water 

 added. After allowing to settle for a few minutes the liquid por- 

 tion is decanted upon a filter. This operation is repeated three or 

 four times. Finally, the solid portions are washed upon the filter, 

 and the washing with water is continued until the filtrate amounts 

 to about three-quarters of a liter. A few drops of hydrochloric 

 acid are added until the filtrate is perfectly clear, and the volume 

 is then made up to one liter. Fifty cubic centimeters of the solu- 

 tion are then treated with 30 cubic centimeters of ammonium 

 citrate solution and one-third as much ammonia. Afterwards 

 30 cubic centimeters of magnesia mixture are added, drop by 

 drop, with constant stirring. 



For superphosphates containing more than 18 per cent, 

 of phosphoric acid only one gram of the sample is used, for ordi- 

 nary superphosphates two grams, and for compound fertilizers 

 four grams. The sample is first treated as above for soluble acid 

 until the filtrate amounts to 200 cubic centimeters, then clarified 

 with a drop of nitric acid, and made up to a quarter of a liter. 



(2) Reverted Phosphoric Acid. The filter containing the resi- 

 due is then introduced into a quarter liter flask and treated with 

 100 cubic centimeters of Petermann's alkaline ammonium citrate 

 solution, vigorously shaken, and left at room temperature for 

 15 hours. It is digested for an hour at 40 and filtered. Fifty 

 cubic centimeters of the filtrate are placed in a flask and, with 

 constant shaking, 35 cubic centimeters of magnesia mixture added. 

 The aqueous solution is treated in the same way. The precipi- 

 tate is collected, washed with two and one-half per cent, ammonia 

 until the chlorin disappears, ignited, weighed, and the number 

 obtained multiplied by 0.64 for phosphoric acid. The total acid is 

 determined in the usual way. 



93. Dutch Method for Citrate-Soluble Phosphoric Acid. The 

 reagents necessary are : 



(i) Citrate solution. Dissolve 165 grams of citric acid in 

 700 cubic centimeters of water, mix with 250 cubic centimeters 

 of ammonia of 0.92 specific gravity, and, aft?r co:ling, bring to 

 the volume of one liter. 



