130 MANUFACTURE OF FERTILIZING MATERIALS 



crystals, warm to 80 or 90 C. and add molyb- 

 date solution in the proportion of 50 cc. for 

 every 0.1 gm. of phosphoric acid assumed to be 

 present. For ordinary fertilizers which contain 

 less than 20 per cent phosphoric acid, 50 cc. will 

 be ample. Let stand warm, with frequent stirring, 

 until the precipitate settles readily when dis- 

 turbed; wash by decantation with cold dilute 

 solution of ammonium nitrate acidified with 

 nitric acid, allowing as little as possible of the 

 precipitate to get on the filter. Dissolve the pre- 

 cipitate in ammonia, and precipitate by mag- 

 nesia mixture, adding it slowly to the clear 

 solution, which is vigorously stirred. Let stand 

 cold for 30 minutes. 



Filter upon an ashless filter, wash with water 

 containing one-eighth its volume of ammonium 

 hydroxide, dry, ignite at first with gentle heat, 

 finally at red heat, in a porcelain crucible to 

 constant weight, and weigh as magnesium pyro- 

 phosphate. 



After ignition this precipitate should be 

 white or light gray in color. 



Water-soluble Phosphoric Acid. Place 2 gms. 

 of the well-ground and mixed sample in a 

 9-cm. filter and wash with successive portions 

 of water, say 15 to 20 cc. at a time, allowing 

 each lot to run off before the next is added, until 

 the washings measure 250 cc. If the washings 

 contain but little organic matter, one-fifth of 

 the filtrate may be used for the molybdate pre- 



