MANUFACTURE OF SUPERPHOSPHATES 277 



cess above described, and afterwards the phosphoric acid 

 obtained by evaporating to dryness with 4 a little sulfuric acid, 

 taking the residue up with water, reducing the vanadic with sul- 

 furous acid and precipitating the phosphoric acid with molyb- 

 date solution as described above. 



CHEMISTRY OF SUPERPHOSPHATE MANUFACTURE 

 237. Chemical Changes in the Manufacture of Superphosphates. 

 In this country the expressions "acid" and "super" as applied 

 to phosphates are used interchangeably. A more correct use of the 

 terms would designate by "acid" the phosphate formed directly 

 from tricalcium phosphate by the action of sulfuric acid, while by 

 "super" would be indicated a similar product formed by the action 

 of free phosphoric acid on the same materials. In Germany the lat- 

 ter compound is called "double phosphate." 



The reaction which takes place in the first instance is repre- 

 sented by the following formula : 



3 Ca 3 (P0 4 ) 2 +6H 2 S0 4 +i2H 2 0=4H 3 P0 4 +Ca 3 (P0 4 ) 2 -f- 

 6(CaSO 4 .2H 2 O) ; 



and 4 H3P0 4 -fCa 3 (P0 4 ) 2 -f3H 2 0=3[CaH 4 (P0 4 ) 2 .H 2 0]. 

 A simpler form of the reaction is expressed as follows : 

 Ca 3 (P0 4 ) 2 +2H 2 S0 4 + 5 H 2 

 =CaH 4 (PO 4 ) 2 .H 2 O+2[CaSO 4 .2H 2 O]. 



If 310 parts, by weight, of finely ground tricalcium phosphate 

 be mixed with 196 parts of sulfuric acid and 90 parts of water, 

 and the resulting jelly be quickly diluted with a large quantity 

 of water, and filtered, there will be found in the filtrate about 

 three-quarters of the total phosphoric as free acid. If, however,, 

 the jelly, at first, formed as above, be left to become dry and hard, 

 the filtrate, when the mass is beaten up with water and filtered, 

 will contain monocalcium phosphate, CaH 4 (PO 4 ) 2 . 



If the quantity of sulfuric acid used be not sufficient for com- 

 plete decomposition, the dicalcium salt is formed directly accord- 

 ing to the following reaction : 



Ca 3 (PO 4 ) 2 +H 2 SO 4 +6H 2 O 

 = Ca 2 H, ( PO 4 ) 2 .4H 2 O+ CaSO 4 .2H 2 O. 

 This arises, doubtless, by the formation, at first, of the regular 



