224 § 115. FERTILIZERS. 



II. 



COMMERCIAL CONCENTRATED MANURES. 



115. The determinations of phosphoric acid, potassa, and 

 nitrogen, are the most important in the examination of 

 these manures. 



a. The method first recommended by Pincus for esti- 

 mating phosphoric acid by means of a standard solution 

 of uranic acetate can be applied in the analysis of guanos, 

 bone-meal, bone-black, bone-ash, and most of the super- 

 phosphates, so long as but little ferric oxide or alumina is 

 present. The method deserves to be generally followed, 

 for it is quickly executed and sufficiently accurate, and 

 because it is desirable that all estimations of phosphoric 

 acid, in the numerous forms in which it is presented to the 

 public, should be made according to a common plan. 



The preparation of the standard solution and the man- 

 ner of conducting the analysis have been already describ- 

 ed (§ 61, c). 



For the determination, 50 c.c. of the solution to be test- 

 ed are generally taken, and 10 c.c. of the solution of sodic 

 acetate added. If the quantity of the precipitate that is 

 formed on the addition of this reagent and boiling is quite 

 small, it may be filtered out, washed, dried, and ignited ; 

 the residue may be regarded as ferric phosphate, FePO^, 

 containing 47.02°| „ of phosphoric acid (P„0 J. The volu- 

 metric process is carried out with the filtrate from this 

 precij)itate. If the precipitate of ferric phosphate is not 

 slight, a gravimetrical process is safer (§ 93, II). 



The volumetric process, under suitable circumstances, 

 gives results that are within 1 mgr. of the truth. 



