1891.] PUBLIC DOCUMENT — No. 33. 273 



previously been washed with oxahite of ammonia, washing 

 thoroughly with water. Dissolve the phosphates of iron and 

 alumina on the filter with warm dilute hydrochloric acid, and 

 wash into a beaker containing a small quantity of powdered 

 tartaric acid. When the latter has gone into solution, mix 

 with the filtrate from the oxalate of ammonia. The phos- 

 phoric acid is precipitated with magnesia mixture, and 

 treated as in (1). 



Soluble phosphoric acid : Weigh out 2 grams into a 

 beaker, cover with 10 to 15 cubic centimetres of water, and 

 allow it to stand for fifteen minutes, stirring three times at 

 equal intervals. Decant the solution through a filter into a 

 graduated cylinder. Add another like quantity of water, 

 and let it stand fifteen minutes more, stirring as before. 

 Filter the solution into the cylinder, and wash the residue on 

 the .filter until the filtrate amounts to 200 cubic centimetres. 

 The phosphoric acid is determined in an aliquot part of the 

 solution as under total phosphoric acid. 



Insoluble phosphoric acid : Add 100 cubic centimetres of 

 neutral ammonia citrate (sp.gr. 1.09) to the l)eaker in which 

 the digestion with water has been made. Put in a water- 

 bath and heat to 65° C. Drop in the filter containing the 

 residue from the above operation, and digest for thirt}^ 

 minutes, stirring every five minutes. Filter and wash 

 thoroughly, using the suction pump. Dry, and burn. The 

 ash is then treated as under total phosphoric acid. 



Reverted phosphoric acid : The sum of the soluble and 

 insoluble subtracted from the total gives the reverted or 

 citrate-soluble phosphoric acid. 



Reagents : The reagents used in the estimation of phos- 

 phoric acid are prepared according to directions given in the 

 "Proceedings of the Association of Official Agricultural 

 Chemists," 1889, pages 225 and 226. 



For ammonium citrate, 370 grams of citric acid are dis- 

 solved in 1,500 cul)ic centimetres of water, nearly neutralized 

 with crushed carbonate of ammonia, heated to expel carbonic 

 acid, exactly neutralized with ammonia, and brought to a 

 specific gravity of 1.09. 



The molybdic solution is prepared by dissolving 100 grams 

 ofmolybdic acid in 417 cubic centimetres of ammonia of 



