156 AGRICULTURAL ANALYSIS 



Thomson has shown that of the three hydrogen atoms in 

 phosphoric acid, two must be saturated with alkali before the 

 reaction with phenolphthalein is neutral. 14 Therefore, when the 

 yellow precipitate is broken up by an alkali, according to the 

 reaction to follow, only four of the six molecules of ammonium 

 are required to form a neutral ammonium phosphate as determined 

 by the indicator employed. The remaining two molecules of 

 ammonium unite with the molybdenum, forming also a salt neu- 

 tral to the indicator. 



Phenolphthalein is preferred because, as has been shown by 

 Long, its results are reliable in the presence of ammonium salts 

 unless they be present in large quantity, and if the solution be 

 cold and the indicator be used in sufficient quantity. 15 To pre- 

 pare the indicator for this work, one gram of phenolphthalein is 

 dissolved in 100 cubic centimeters of 60 per cent, alcohol. At 

 least one-half of a cubic centimeter of the solution is used for 

 each titration. 



The advantages claimed for the method are its speed and 

 accuracy. Much time is saved by avoiding the necessity for the 

 removal of the silica by evaporation. The results of analyses 

 with and without the removal of the silica are practically identical. 

 When the silica is not removed it is noticed that the filtrate 

 from the yellow precipitate has a yellow tint. 



The reaction is represented by the following formula : 



(NH 4 ) 6 (P0 4 ) 2 (Mo0 3 ) 24 -4- 4 6KOH=(NH 4 ) 4 (HP0 4 ) 2 + 

 (NH 4 ) 2 MoO 4 -r-23K 2 MoO 4 -f22H 2 O. 



From this reaction it is seen that the total available acidity of 

 one molecule of the yellow precipitate titrated against phenol- 

 phthalein is equivalent to 23 molecules of potassium hydroxid. 



Calculation of Results. The standard alkali is of such strength 

 that one cubic centimeter is equal to one per cent, of phosphoric 

 acid when one gram of material is employed and one-tenth of it 

 taken for each determination. If in a given case one gram of a 

 sample and one-tenth of the solution are used, and 50 cubic cen- 

 timeters of alkali added to the yellow precipitate, it requires 



14 Chemical News, 1883, 47 : 127, 186. 



15 American Chemical Journal, 1889, 11 : 84. 



