74 AGRICULTURAL ANALYSIS 



82. Swedish Official Method for Determination of Phosphoric 

 Acid. 52 The Swedish chemists determine phosphoric acid in fer- 

 tilizers both by the molybdate and the citrate methods. These 

 methods carefully conducted according to the directions given 

 below, give very concordant results. In doubtful cases the former 

 method is taken as the deciding one, it having proved by long 

 practice to give very satisfactory results. 



Reagents for the Molybdate Method. i. Molybdic Solution. 

 Prepared by dissolving 100 grams of finely powdered molybdic 

 acid with heat in 400 grams of eight per cent, ammonia of 0.967 

 specific gravity and pouring the solution into 1500 grams of nitric 

 acid of one and two-tenths specific gravity ; or else by dissolv- 

 ing 150 grams ammonium molybdate in one liter of hot water, and 

 pouring the solution into one liter of nitric acid of 1.2 specific 

 gravity. Prepared in this way, the molybdic solution will con- 

 tain, in the former case, five per cent., in the latter case, from five 

 to six per cent, of molybdic acid, and 100 cubic centimeters of it 

 are required for precipitating one-tenth gram of phosphorus 

 pentoxid. 



2. Magnesia Mixture. Prepared with no grams of crystal- 

 lized magnesium chlorid, 140 grams of ammonium chlorid, 700 

 grams of eight per cent, ammonia of 0.967 specific gravity and 

 1300 grams of distilled water. The mixture is filtered after a few 

 days, if necessary ; 10 cubic centimeters are required for precipi- 

 tating one-tenth gram of phosphorus pentoxid. 



3. Ten per cent, ammonia of 0.959 specific gravity. 



Determinations: (a) Water-Soluble Phosphoric Acid. i. 

 Preparation of the Aqueous Solution. Of superphosphates and 

 other fertilizers containing water-soluble phosphoric acid, 20 

 grams are treated with water in a mortar ; lumps are crushed 

 lightly but completely with the pestle without pulverizing 

 finer; the whole mass is then washed into a graduated flask hold- 

 ing one liter, which at once is filled up to the mark. The volume 

 taken up by the residue insoluble in water is left out of considera- 

 tion in the calculation. After standing in the flask ( which is 



51 Official Swedish Methods Translated for the Author by F. W. Woll. 



