114 AGRICULTURAL, ANALYSIS 



its manufacture, as well as any iron and aluminum phosphates 

 present which are soluble in ammonium citrate. It also includes 

 any tricalcium phosphate, such as that existing in bones, which 

 may pass into solution under the influence of ammonium citrate. 

 The expression "citrate-soluble" is, therefore, to be preferred to 

 "reverted" phosphoric acid. 



102. Theory of Reversion. In the reversion of the phosphoric 

 acid in superphosphates the iron plays a far more important role 

 than the aluminum sulfate. It was formerly supposed that the 

 reversion took place as indicated in the following formula: 

 2CaH 4 (PO 4 ) 2 +Fe 2 O 3 =2(CaHPO 4 .FePO 4 )+3H 2 O, while Wag- 

 ner affirms that the reverted acid compounds consist of vary- 

 ing quantities of ferric oxid, aluminum oxid, phosphorus pent- 

 oxid, and calcium oxid, in various states of combination. 83 

 The more probable reaction is the following: 3CaH 4 (PO 4 ) 2 -f- 

 Fe 2 ( S0 4 ) 3 +4H 2 0=2 ( FePO 4 ,2H 3 PO 4 ,2H 2 O ) -f-3CaSO 4 . This 

 reaction can be demonstrated by adding to a superphosphate 

 solution one of a ferric salt. In addition to free phosphoric 

 acid, iron phosphate is separated, which gradually passes into 

 an insoluble form by the abstraction of water due to the crystal- 

 lization of the gypsum. The alumina present in a superphos- 

 phate seems to have no direct influence on the process of re- 

 version. Its phosphate salt is not acted on by the acid calcium 

 phosphate. Even when a superphosphate solution is treated with 

 alum no precipitation is produced, except on warming, and this 

 disappears when the mass is again cold. 



It is therefore not necessary in the process of manufacture 

 to separate the alumina by digestion with a hot soda-lye before 

 treating the mass with sulfuric acid. 



In order to avoid the reversion of the phosphoric acid several 

 plans have been proposed. One of the best is to use a little 

 excess of sulfuric acid in the manufacture. This tends to hold 

 the phosphoric acid in soluble form, but is objectionable on 

 account of drying, handling, and shipping the fertilizer. During 

 the digestion, moreover, it is important that the temperature do 

 not rise above 120. Another method consists in adding to the 

 M Lehrbuch der Diingerfabrikation. 



