74 CHEMICx\L MANUEES. 



(1) 8Ca3(PO,)-^ + (3H,S0, + 12HoO = 4H,P0^ + Ca.tPO^)^ + 6(CaS04 + 2H2O) 

 Calcium Sulphuric ,„ Phosphoric Tribasic GvDsum 



phosphate. acid. Watei. ^d^^ phosphate. "^^ 



■(2) 4(H,P0J + Ca,(P0J2 + SH^O = 3[CaH4(P04)2 + HoO] 



Phosphoric Tribasic „, , Monocalcic ^^r , 



• 1 ■^ u i. Water. . 1 . Water, 



acicl. phosphate. phosphate. 



One can demonstrate the soundness of this equation by treating 

 after equation (1) 310 parts of tribasic phosphate of hme with 196 

 parts of monohydrated sulphuric acid and 90 parts of water, dikit- 

 ing the mass rapidly with water and filtering quickly. Two-thirds 

 of the phosphoric acid are then present in the filtrate in the free 

 state. But if the mass be only diluted after it has set, all the 

 phosphoric acid exists in the filtrate as monocalcic phosphate. The 

 explanation is that phosphoric acid is a much weaker acid, and 

 acts much more slowly than sulphuric acid, and that is why the 

 reaction of equation (2) is accomplished more slowly than equation 

 (1). If an insufficient amount of sulphuric acid be used for con- 

 version, only bicalcic phosphate is obtained. 



Ca,(PO,)^ + H,SO, + 6H2O = Ca,H2(POJ2,4H,0 + CaS042H20 



In this conversion, a portion of the tribasic phosphate of lime is 

 transformed first into monocalcic phosphate CaH^(P04)^, but that 

 re -combines with the remainder of the tribasic phosphate of lime. 



CaH,(PO,)^H.,0 + Ca..(P0J2 + 7H.0 = 2Ca.,Ho(PO,)22H.O 

 2o2 ^ 310 126 



(5s,s 688 



But the free phosphoric acid makes the superphosphate damp, 

 pasty, and unfit for distribution (passing through the drill, etc.) in 

 that condition. If the raw phosphate be rendered soluble, by 

 hydrochloric acid or by nitric acid, no free phosphoric acid is 

 obtained, but merely acid phosphate of lime, very soluble in water. 

 Dry superphosphates can be obtained in that way. But such acids 

 are less abundant and much dearer than sulphuric acid. However, 

 hydrochloric acid when it is to be had cheap, is sometimes added 

 in the proportion of 10 per cent to the sulphuric acid. But the 

 phosphate is not rendered soluble by pure hydrochloric acid, as 

 that would give rise to calcium chloride, which is highly deliques- 

 cent, and cause the superphosphate to be damp and unsaleable. 

 Moreover, calcium chloride is injurious to vegetation when present 

 to any great extent. In presence of sulphuric acid, the calcium 

 chloride is again converted into calcium sulphate and hydrochloric 

 .acid, which assists in dissolving the phosphate. 



