PHOSPHORUS 191 



phosphate formed from monocalcium phosphate by reaction with 

 tricalcium phosphate: 



CaH 4 (P0 4 ) 2 + Ca 3 (P0 4 ) 2 = 2 Ca^PO,),. 



On long standing, this sort of reaction evidently takes place if an 

 excess of tricalcium phosphate was left in the original product, and 

 consequently the percentage of water-soluble phosphorus may be 

 greater in fresh acid phosphate than in that which has been stored 

 for some time, the dicalcium phosphate, or " reverted," being sol- 

 uble in citrate solution, but not in water. 



If the raw phosphate rock contains 12 per cent of phosphorus, 

 the acid phosphate made from it will contain about 6 per cent of 

 phosphorus. The most common grade is known as 14 per cent acid 

 phosphate, which the fertilizer agent would say means that the 

 acid phosphate con tains 14 per cent of " phosphoric acid," by which, 

 however, is meant not 14 per cent of true phosphoric acid, H 3 PO 4 , 

 but 14 per cent of phosphorus pentoxid, P 2 O 5 , which is equivalent 

 to 6.1 per cent of the element phosphorus, corresponding to 122 

 pounds of phosphorus per ton of acid phosphate, which sells at 

 about $15 a ton. 



Where 250 pounds of phosphorus cost $7.50 in fine-ground 

 natural rock phosphate, the same amount of phosphorus will 

 usually cost $30 in the two tons of acid phosphate. 1 



Double superphosphate. Double superphosphate consists chiefly 

 of monocalcium phosphate, CaH 4 (PO 4 ) 2 , and a moderate amount of 

 impurities. It is richer in phosphorus than any other fertilizer 

 material. It is made (i) by treating low-grade phosphate rock 

 with an excess of sulfuric acid, by which true liquid phosphoric 

 acid is liberated. This is leached out of the mass, and (2) this true 

 phosphoric acid is applied to high-grade phosphate rock, thus: 



(1) Ca 3 (P0 4 ) 2 + 3 H 2 S0 4 = H 6 (P0 4 ) 2 + 3 CaSO 4 ; 



(2) Ca 3 (P0 4 ) 2 + 2 H 6 (P0 4 ) 2 = 3 CaH 4 (P0 4 ) 2 . 



1 Both acidulated bone and acid phosphate are sometimes called superphos- 

 phate; and in England "super" (meaning literally over or higher) is the common 

 term for acid phosphate, somewhat as photographers use the term "hypo" (mean- 

 ing under or lower) for sodium thiosulfate, formerly incorrectly called hyposulfite 

 of soda. 



