160 AGRICULTURAL ANALYSIS 



preferably method 5, when these acids are a suitable solvent, 

 and dilute to 200 cubic centimeters with water. 



Determination. For percentages of phosphoric acid of five or 

 below use an aliquot corresponding to 0.4 gram substance ; for 

 percentages between five and 20 use an aliquot corresponding to 

 0.2 gram substance ; and for percentages above 20 use an aliquot 

 corresponding to o.i gram substance. Add from five to 10 cubic 

 centimeters of nitric acid, depending on the method of solution 

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

 ammonia, dilute to from 75 to 100 cubic centimeters, heat in a 

 water bath to from 60 to 65, and 'for percentages below five 

 add from 20 to 25 cubic centimeters of freshly filtered molybdic 

 solution. For percentages between five and 20 add from 30 to 

 35 cubic centimeters molybdic solution; stir, let stand about 15 

 minutes, filter at once', wash once or twice with water by decanta- 

 tion, using from 25 to 30 cubic centimeters each time, agitating 

 the precipitate thoroughly and allowing it to settle; transfer to 

 filter and wash five or six times, using enough water to make 

 with the decantation washings about 200 cubic centimeters. 

 Transfer precipitate and filter to beaker or precipitating vessel, 

 dissolve in small excess of standard alkali, add a few drops of 

 phenolphthalein solution and titrate excess of alkali with stand- 

 ard acid (nitric). 



Water-Soluble Phosphoric Acid. Dissolve the soluble acid ac- 

 cording to directions given under the official method for the grav- 

 imetric determination of water-soluble acid, paragraph 67. To an 

 aliquot portion of the solution corresponding to 0.2 or 0.4 gram, 

 add 10 cubic centimeters of concentrated nitric acid and then am- 

 monia until a slight precipitate is formed, dilute to 60 cubic 

 centimeters, and proceed as above described. 



Citrate-Insoluble Phosphoric Acid. (a) In Acidulated Sam- 

 ples. Heat 100 cubic centimeters of strictly neutral ammonium 

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

 a bath of warm water, keeping the flask loosely stoppered to pre- 

 vent evaporation. When the citrate solution in the flask has 

 reached 65, drop into it the filter containing the washed residue 

 from the water-soluble phosphoric acid determination, stopper 



