94 AGRICULTURAL ANALYSIS 



tween the total phosphoric acid and that in the insoluble residue, 

 after treatment with ammonium citrate, as above, gives the 

 quantity of phosphoric acid soluble in citrate solution. The 

 difference between the total citrate-soluble and the water-soluble 

 gives the quantity of the reverted phosphoric acid. 



The ammonium citrate solution (Petermann's) used for the di- 

 gestion is made as follows : Two hundred and fifty grams of crys- 

 tallized citric is dissolved in half a liter of hot water, diluted 

 with 550 cubic centimeters of water, 276 cubic centimeters of 

 24 per cent, ammonia added, and finally, exactly neutralized by 

 adding, little by little, 50 per cent, citric acid solution. 



The Halle methods of separating the water and citrate-soluble 

 acids appear to be less complete and reliable than those in use 

 by the Official Agricultural Chemists of this country. The pre- 

 cipitation of basic phosphates, when large quantities of water are 

 used at once in separating soluble acid, must tend to diminish 

 the quantity obtained, while the lack of care in assuring the neu- 

 trality of the citrate solution might lead to varying results. 



(4) Double Superphosphates. In the case of double super- 

 phosphates, which sometimes contain large quantities of pyro- 

 phosphate, the solution is made in the usual way so that in 100 

 cubic centimeters there will be contained two grams of the sub- 

 stance. Usually 10 grams are used and the volume made up 

 to half a liter. Twenty-five cubic centimeters of the filtrate are 

 diluted with 75 cubic centimeters of water and the pyro- 

 converted to orthophosphoric acid by heating with 10 cubic centi- 

 meters of strong nitric acid on a sand-bath. The heating should 

 be continued until the volume is reduced to 25 cubic centimeters. 

 The strongly acid liquid is made alkaline with ammonia and after- 

 wards slightly acid with nitric acid, and the rest of the process 

 is carried on in the usual way. 



(5) Phosphoric Acid in the Residue of Superphosphate Manu- 

 facture. In the mixture of superphosphates and gypsum, the res- 

 idue of the manufacture of double superphosphates, the phos- 

 phoric acid is estimated in the following manner: Five grams 

 of the substance are placed in a dish, rubbed up with absolute al- 

 cohol, and washed into a 250 cubic centimeter flask. The flask is 



