152 AGRICULTURAL ANALYSIS 



process is based on the separation of the phosphoric acid as the 

 ammonio-manganous salt and the subsequent oxidation of the 

 manganese to peroxid acid. 12 The. quantity of the peroxid is de- 

 termined by the titration with hyposulfite of soda of the iodin 

 liberated from potassium iodid. After a study of the methods of 

 preparing the ammonio-manganous salt according to the proced- 

 ures of Otto, Heintz and Gibbs, the following process was adopted 

 as the most suitable : To 50 cubic centimeters of a solution of so- 

 dium phosphate, containing about 100 milligrams of phosphoric 

 acid, are added 10 cubic centimeters of a 20 per cent, solution of 

 ammonium chlorid, 10 cubic centimeters of ammonia, 25 cubic 

 centimeters of a solution of ammonium citrate (150 grams citric 

 acid, 500 cubic centimeters ammonia and 1000 cubic centimeters 

 water), an excess of a manganous salt, and 25 cubic centimeters 

 of a 2.5 per cent, solution of magnesium sulfate. 



This mixture shows an immediate yellow coloration, increasing 

 little by little to a greenish brown, but preserves its complete lim- 

 pidity. After 24 hours the sides of the vessel are found coated 

 with colorless brilliant crystals of ammonio-manganous phosphate, 

 but even after several days the separation of the phosphoric acid 

 is not complete. 



If the reagents employed be hot, a different series of phenomena 

 are presented. The yellowish color at first more pronounced, soon 

 disappears and while the liquid is boiling, if it be stirred without 

 striking the walls of the vessel, there are immediately separated 

 brilliant crystals of the salt of a pale rose color. At the end of 

 a few minutes the total phosphoric acid is precipitated. The 

 analysis is conducted as follows : After covering the liquid, in 

 which the phosphoric acid has been separated as above described, 

 the contents of the vessel are thrown on a filter and the precipitate 

 washed with 100 cubic centimeters of a dilute solution of ammo- 

 nium chlorid (0.5 per cent.). The filtration and washing should be 

 made rapidly to avoid danger of solution of the crystals, and the 

 whole operation should last only a few minutes. 



Insoluble phosphates are first dissolved in an acid and the phos- 

 phoric acid thrown out by ammonium molybdate, the yellow 



" Lindeman and Motteu, Bulletin de la Societe* chimique de Paris, 

 1895. [3], 13 :523- 



