l82 AGRICULTURAL ANALYSIS 



Manipulation. Twenty-five or 50 cubic centimeters of a solu- 

 tion of the phosphate which has been made up to a standard vol- 

 ume and contains about one-tenth gram of phosphorus pent- 

 oxid, are placed in a beaker together with 100 cubic centimeters 

 of the molybdate solution and treated with as much ammonium 

 nitrate solution as will be sufficient to give the liquid a content 

 of 15 per cent, of ammonium nitrate. The contents of the beaker 

 are well mixed and warmed for about 20 minutes at from 60 to 

 80. After cooling, they are filtered and the precipitate washed 

 on the filter with cold water until a drop of the filtrate saturated 

 with ammonia does not become opaque on treatment with am- 

 monium oxalate. The filtrate is washed from the filter with 2.5 

 per cent, ammonia solution and precipitated slowly and with con- 

 stant stirring by the magnesia mixture. After standing for two 

 hours the ammonium magnesium phosphate is separated by filtra- 

 tion, washed with 2.5 per cent, ammonia until the filtrate contains 

 no more chlorin, and ignited. 



Conduct of the Citrate Method. The principle of this method 

 depends upon the fact that when a sufficient quantity of ammo- 

 nium citrate is added to phosphate solutions, iron, alumina, and 

 lime are retained in solution when, on the addition of the mag- 

 nesia mixture in the presence of free ammonia, the phosphoric 

 acid is completely precipitated as ammonium magnesium phos- 

 phate. 



Manipulation. From 10 to 50 cubic centimeters of the solu- 

 tion of the phosphate to be determined are treated with 15 

 cubic centimeters of the Joulie citrate solution avoiding warm- 

 ing. A few pieces of filter paper, the ash content of which is 

 known, are thrown in and, with stirring, 15 cubic centimeters 

 of magnesia mixture slowly added and if necessary also some free 

 ammonia. By the small pieces of filter paper the collection of 

 the precipitate against the sides of the vessel and on the stirring 

 rod is prevented and in this way the production of the precipitate 

 hastened. After standing from one-half an hour to two hours the 

 mixture is filtered, ignited, and weighed. If it be preferred to 

 estimate the phosphoric acid by titration, the precipitate is dis- 

 solved in a little nitric acid made slightly alkaline with ammonia, 



