PHOSPHATIC MANURES 215 



phosphate into the soluble monocalcic compound accord- 

 ing to the equation : 



Ca 3 (P0 4 ) 2 + 2H 2 S0 4 = 2CaS0 4 + CaH 4 (P0 4 ) 2 



Tricalcic Sulphuric Calcium Monocalcic 



phosphate. acid. sulphate. phosphate. 



As the calcium sulphate is not removed, the super- 

 phosphate necessarily contains a smaller proportion of 

 phosphoric acid than the untreated phosphate. 



If a smaller quantity of sulphuric acid were used, 

 dicalcic phosphate would be formed, thus 



Ca 3 (P0 4 ) 2 + H 2 S0 4 = CaS0 4 + 2CaHP0 4 



Tricalcic Sulphuric Calcium Dicalcic 

 phosphate. acid. sulphate. phosphate. 



If a larger quantity of sulphuric acid were employed, 

 the whole of the lime would be removed and phosphoric 

 acid would be formed, thus 



Ca 3 (P0 4 ) 2 + 3H 2 S0 4 = 3CaS0 4 + 2H 3 P0 4 



Tricalcic Sulphuric Calcium Phosphoric 

 phosphate. acid. sulphate. acid. 



Tricalcic phosphate is the normal " phosphate of lime," 

 and is the substance commonly alluded to by that name. 

 Dicalcic phosphate is insoluble in water, and though more 

 readily soluble in various reagents than the tricalcic 

 form, it does not exhibit the characteristic properties of 

 a superphosphate. Monocalcic phosphate is soluble in 

 cold water, but if the solution be heated, or even on 

 standing in the cold, it changes into dicalcic phosphate 

 and phosphoric acid, thus 



CaH 4 (P0 4 ) 2 = CaHP0 4 + H 3 P0 4 



Monocalcic Dicalcic Phosphoric 



phosphate. phosphate. acid. 



