I] FERTILIZERS 3 



practically free from chlorin, dry the filter and precipitate and transfer the latter 

 to a weighed porcelain crucible. Ignite the filter separately and add its ash to the 

 precipitate in the crucible. Ignite to whiteness or to a grayish white, weigh, and 

 calculate to phosphoric acid (P2O5). 



Volumetric Method. — Official. 



7 REAGENTS. 



(a) Molyhdate solution. — To 100 cc. of molybdate solution prepared as directed 

 in 4 (a), add 5 cc. of nitric acid (sp. gr. 1.42). This solution should be filtered im- 

 mediately before using. 



(b) Standard sodium or potassium hydroxid solution. — Dilute 323.81 cc. of N/1 

 alkali, free from carbonates, to 1 liter. 100 cc. of the solution should neutralize 

 32.38 cc. of N/1 acid; 1 cc. is equivalent to 1 mg. of P2O5 (1% of F2O5 on a basis of 

 0.1 gram of substance). 



(C) Standard acid solution. — Prepare an acid solution corresponding to the, 

 or to one-half of the, strength of (b), and standardize by titration against that 

 solution, using phenolphthalein as indicator. Hydrochloric or nitric acid may be 

 used. 



(d) Phenolphthalein solution. — Dissolve 1 gram of phenolphthalein in 100 cc. of 

 alcohol. 



8 PREPARATION OF SOLUTION. 



Dissolve according to 5 (b), (e), (f), or (g), preferably by (e), when these acids 

 are a suitable solvent, and dilute to 200 cc. with water. 



9 DETERMINATION. 



(a) For percentages of 5 or below use an aliqifot corresponding to 0.4 gram of 

 substance, for percentages between 5 and 20 use an aliquot corresponding to 0.2 

 gram of substance, and for percentages above 20 use an aliquot corresponding to 

 0.1 gram of substance. Add 5-10 cc. of nitric acid, depending on the method of 

 solution (or the equivalent in ammonium nitrate), nearly neutralize with ammonium 

 hydroxid, dilute to 75-100 cc, heat in a water bath to 60°-65°C., and for percentages 

 below 5 add 20-25 cc. of freshly filtered molybdate solution. For percentages be- 

 tween 5 and 20 add 30-35 cc. of molybdate solution. For percentages greater than 

 20 add sufficient molybdate solution to insure complete precipitation. Stir, let 

 stand in the bath about 15 minutes, filter at once, wash once or twice with water by 

 decantation, using 25-30 cc. each time, agitate the precipitate thoroughly and 

 allow to settle; transfer to the filter and wash with cold water until the filtrate from 

 2 fillings of the filter yields a pink color upon the addition of phenolphthalein and 1 

 drop of the standard alkali. Transfer the precipitate and filter to a beaker or pre- 

 cipitating vessel, dissolve the precipitate in a small excess of the standard alkali, 

 add a few drops of phenolphthalein solution, and titrate with the standard acid. 



(b) Proceed as in (a) with this exception: Heat in a water bath at 45°-50''C., 

 add the molybdate solution, and allow to remain in the bath with occasional stirring 

 for 30 minutes. 



(C) Proceed as in (a) to the point where the solution is ready to place in the 

 water bath. Then cool the solution to room temperature, add molybdate solution 

 at the rate of 75 cc. for each decigram of phosphoric acid present, place the stoppered 

 flask containing the solution in a shaking apparatus and shake for 30 minutes at 

 room temperature, filter at once, wash, and titrate as in (a). 



