128 MANUFACTURE OF FERTILIZING MATERIALS 



when the term "available" phosphoric acid is 

 employed. English analysts usually apply the 

 term "available" as a synonym for "water sol- 

 uble," and, on the other hand, the term "sol- 

 uble" is also sometimes used when the sum of 

 No. 1 and No. 2 is meant. 



No. 3 is usually called "insoluble," but to 

 express it more exactly it has also been called 

 "citrate insoluble." 



Much of this confusion of terms has arisen 

 from the diversity of opinion as to the utility 

 of the different forms in which the phosphoric 

 acid may be combined. 



The sum of Nos. 1, 2, and 3 is called "total 

 phosphoric aciti." 



Total Phosphoric Acid. The phosphoric acid 

 is usually separated as the molybdate compound, 

 which is dissolved in ammonia, and precipitated 

 for weighing by magnesium mixture. 



As the organic matter of the fertilizer would 

 interfere with the complete separation of the 

 phospho-molybdate, it must be destroyed (usu- 

 ally by ignition, with or without the addition 

 of some nitrate). Hydrochloric acid is the best 

 solvent for ignited phosphates, but the molyb- 

 date precipitation is best made in a nitric acid 

 solution, so that, although after ignition hydro- 

 chloric acid must be used to effect solution, 

 nitric acid and nitrates should largely predom- 

 inate when the molybdate separation is effected. 



Weigh out 2 gms. of the sample in a platinum 



