DETERMINATION OF REVERTED PHOSPHORIC ACID 113 



methods, which have been given in the preceding pages, for its 

 determination must convince every careful observer that, as a 

 rule, each process is based on arbitrary standards, and can give 

 only concordant results when carried out under strictly unvary- 

 ing conditions. For this reason there can be no just comparison 

 between the results obtained by different methods, which vary 

 from each other only in slight particulars. When, on the other 

 hand, the processes are radically different, the deviations in data 

 become more pronounced. 



In such a condition of affairs the analyst is left to choose 

 between methods. He must be guided in his choice not only by 

 what seems to be the most scientific and accurate process, but 

 also, to a certain extent, by the general practice of his professional 

 brethren. For this country, therefore, it is strongly urged that 

 the methods adopted by the Association of Official Agricultural 

 Chemists be followed in every detail. 



By the phrase " reverted phosphoric acid " was originally 

 meant an acid once soluble in water, as CaH 4 (PO 4 ) 2 , and after- 

 wards changed to a form insoluble in water, but soluble in 

 ammonium citrate as Ca 2 H 2 (PO 4 ) 2 . But in practice this has never 

 been the true signification of the term. In the manufacture of 

 acid- and superphosphates there is formed, more or less of the 

 dicalcium phosphate, either directly or after a time, and this salt 

 which, in no sense can be called reverted, is entirely soluble in 

 r.mrnonium citrate. The iron and aluminum phosphates are also, 

 to a certain degree, soluble in the same reagent. When an acid 

 phosphate, containing various forms of calcium phosphate, is 

 applied to a soil containing iron and alumina, the soluble parts 

 of the compound tend to become fixed by union with those bases, 

 or by precipitation as Ca 2 H 2 (PO 4 ).,. But it is not alone reverted 

 phosphate formed in this way, which the analyst is called on to 

 determine in a fertilizer, although he may have occasion to treat 

 it in soil analysis. 



The expression "reverted phosphoric acid," therefore, in prac- 

 tice not only includes a dicalcium phosphate, which once may 

 have been the monocalcium salt, but also all of that salt origi- 

 nally existing in the superphosphate, and formed directly during 



