VII. SPECIAL MANURES 145 



Superphosphate is made by treating the mineral with sulphuric 

 acid, when a replacement of phosphoric acid by sulphuric acid takes 

 place, calcium sulphate and free phosphoric acid being formed : 



Ca 3 P 2 8 + 3H 2 S0 4 = 3CaS0 4 + 2H 3 P0 4 . 



The sulphuric acid also acts upon calcium carbonate, calcium 

 chloride or fluoride, and oxides of iron and aluminium, if these be 

 present, evolving carbon dioxide, hydrochloric or hydrofluoric acid : 



CaCO 3 + H 2 S0 4 = CaS0 4 + H 2 O + C0 2 



CaCl 2 + H 2 SO 4 = CaS0 4 + 2HC1 



CaF 2 + H 2 S0 4 = CaS0 4 + 2HF 



A1 2 3 + 3H 2 S0 4 = ALXS0 4 ) 3 + 3H 2 



Fe 2 O 3 + 3H 2 SO 4 = Fe 2 (S0 4 ) 3 + 3H 2 0. 



These various reactions consume a portion of the sulphuric acid and, 

 in many cases, are completed before the action upon the calcium 

 phosphate begins. 



In general, the amount of sulphuric acid used is only sufficient 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 3 P 2 8 + 4H 3 P0 4 = 3CaH 4 P 2 8 



thus producing monocalcium tetrahydrogen phosphate. 

 Superphosphates thus consist essentially of a mixture of 



calcium sulphate, CaSO 4 .2H 2 0, 



calcium tetrahydrogen phosphate, CaH 4 P 2 O 8 , 



and, generally, tricalcium phosphate, Ca 3 P 2 O 8 . 



In many samples, aluminium sulphate and ferrous or ferric 

 sulphate are also present in small proportions. 



The important constituent is, of course, the monocalcium phos- 

 phate, 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 tri- 

 calcium phosphate leading to the formation of dicalcium phosphate 1 



CaH 4 P 2 8 + Ca 3 P 2 8 = 4CaHP0 4 

 (or 2Ca 2 H 2 P 2 O 8 ). 



(b)- By the formation of ferric and aluminium phosphates by the 

 action of the monocalcium phosphate upon the iron and aluminium 

 sulphates : 



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

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

 stance so-called has never been present in a soluble form. The amount of phos- 

 phates of this character is determined by taking advantage of their solubility in 

 ammonium citrate (vide Chap. IX) ; the name " citrate-soluble " phosphorus pent- 

 oxide would therefore be preferable to the names already mentioned. 



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