FERTILIZERS. 3 



(&) Determination. Take an aliquot portion of the solution prepared above 

 corresponding to 0.25 gram, 0.50 gram, or 1 gram, neutralize with ammonium 

 hydroxid, and clear with a few drops of nitric acid. In case hydrochloric or 

 sulphuric acid has been used as solvent, add about 15 grams of dry ammonium 

 nitrate or a solution containing that amount. To the hot solution add 50 cc 

 of molybdate solution for every decigram of phosphoric acid (P 2 O 8 ) that Is 

 present. Digest at about 65 C. for an hour, filter, and wash with cold water 

 or, preferably, ammonium nitrate solution. Test the filtrate for phosphoric 

 acid by renewed digestion and addition of more molybdate solution. Dissolve 

 the 'precipitate on the filter with ammonium hydroxid and hot water and 

 wash into a beaker to a bulk of not more than 100 cc. Nearly neutralize with 

 hydrochloric acid, cool, and add magnesia mixture from a burette; add slowly 

 (about 1 drop per second), stirring vigorously. After fifteen minutes add 

 12 cc of ammonium hydroxid solution, specific gravity 0.90. Let stand for 

 some time two hours is usually enough; filter, wash with 2.5 per cent am- 

 monia (NH S ) until practically free from chlorids ; ignite to whiteness or to a 

 grayish white and weigh. 



(3) WATER-SOLUBLE PHOSPHORIC ACID. 



Place 2 grams of the sample on a 9-cm filter, wash with successive small 

 portions of water, allowing each portion to pass through before adding more, 

 until the filtrate measures about 250 cc. If the filtrate be turbid, add a little 

 nitric acid. Make up to any convenient definite volume, mix well, use an 

 aliquot, and proceed as under total phosphoric acid. 



(4) CITRATE-INSOLUBLE PHOSPHORIC ACID. 



(a) Determination in acidulated samples. Heat 100 cc of strictly neutral 

 ammonium citrate solution of 1.09 specific gravity to 65 C. in a flask placed 

 in a warm-water bath, keeping the flask loosely stoppered to prevent evapora- 

 tion. When the citrate solution in the flask has reached 65 C., drop into it the 

 filter containing the washed residue from the water-soluble phosphoric acid 

 determination, close tightly with a smooth rubber stopper, and shake violently 

 until the filter paper is reduced to a pulp. Place the flask in the bath and 

 maintain it at such a temperature that the contents of the flask will stand at 

 exactly 65 C. Shake the flask every five minutes. At the expiration of exactly 

 thirty minutes from the time the filter and residue are introduced remove 

 the flask from the bath and immediately filter the contents as rapidly as pos- 

 sible. Wash thoroughly with water at 65 C. (er) Transfer the filter and its 

 contents to a crucible, ignite until all organic matter is destroyed, add from 

 10 to 15 cc of strong hydrochloric acid, and digest until all phosphate is dis- 

 solved ; or (&) return the filter with contents to the digestion flask, add from 

 30 to 35 cc strong nitric acid, from 5 to 10 cc strong hydrochloric acid, and boil 

 until all phosphate is dissolved. Dilute the solution to 200 cc. If desired, the 

 filter and its contents may be treated according to methods (a 2 ), (03), or (a 4 ) 

 under total phosphoric acid. Mix well, filter through a dry filter; take a defi- 

 nite portion of the filtrate and proceed as under total phosphoric acid. 



(&) Determination in nonacidulated samples. In case a determination of 

 citrate-insoluble phosphoric acid is required, in nonacidulated samples it is to 

 be made by treating 2 grams of the phosphatic material without previous wash- 

 ing with water, precisely in the way above described, except that in case the 

 substance contains much animal matter (bone, fish, etc.), the residue insoluble 

 in ammonium citrate is to be treated by any one of the processes described 

 under total phosphoric acid (a 2 ), (a 3 ), or (aj, page 2. 



