154 MANURES. VII. 



CaF 2 + H 2 SO 4 = CaS0 4 + 2HF 

 ALA+ 3H 2 S0 4 A1 2 (S0 4 ) 8 + 3H 2 



Fe 2 8 + 3H 2 S0 4 Fe 2 (S0 4 ) 8 + 3H 2 0. 



These various reactions consume a portion of the sulphuric 

 acid and, in many cases, are produced before the action upon 

 the calcium phosphate begins. 



In general the amount of sulphuric acid used is only suffi- 

 cient to liberate phosphoric acid from a portion of the calcium 

 phosphate, and a subsequent interaction then occurs between 

 the phosphoric acid so liberated and the unchanged tricalcium 

 phosphate 



Ca 8 P 2 H + 4H 8 P0 4 = 3CaH 4 P A 



thus producing monocalcium tetrahydrogen phosphate. 

 Superphosphates thus consist essentially of a mixture of 

 Calcium sulphate, CaS0 4 .2H 2 0, 

 Calcium tetrahydrogen phosphate, CaH 4 P 2 H , 

 and generally Tricalcium phosphate, Ca 8 P 2 O H . 



In many samples, aluminium sulphate and ferrous or ferric 

 sulphate are also present in small proportions. 



The important constituent is, of course, the monocalcium 

 phosphate, CaH 4 P 2 8 , which is soluble in water, the tricalcium 

 phosphate being of much less value. On keeping, many 

 superphosphates show a reduction in the amount of phosphate 

 soluble in water, and an increase in the insoluble phosphoric 

 acid. This may be caused in two ways 



(a) By the interaction of the monocalcium phosphate and the 



tricalcium phosphate leading to the formation of di- 

 calcium phosphate* 



CaH 4 P. 2 H + Ca 8 P A = 4CaHP0 4 



(or 2Ca 2 H 2 P 2 O s ). 



(b) By the formation of ferric and aluminium phosphate by the 



action of the monocalcium phosphate upon the iyon and 



aluminium sulphates 



A1 2 (S0 4 ) 3 + CaH 4 P A = 2 A1PO 4 + CaS0 4 + 2H 2 SO 4 

 Fe a (S0 4 ) + CaH 4 P 2 8 = 2FeP0 4 + CaS0 4 + 2H 2 S0 4 . 



* This substance is present even in fresh superphosphate, so that "reverted" 

 phosphate is to some extent a misleading name, inasmuch as some of the substance 

 so-called has never been present in a soluble form. The amount of phosphates of this 

 character is determined by taking advantage of their solubility in ammonium citrate 

 (?;. Chap. VIII.); the name "citrate-soluble" phosphorus pentoxide would therefore 

 be preferable to the names already mentioned. 



