94 AGRICULTURAL CHEMISTRY 



limited amount of air, it forms phosphorus trioxid, P 2 O 3 , 

 which after long standing dissolves in water, forming 

 phosphorous acid. In fertilizer, soil, and food analysis, 

 the amount of phosphorus is expressed in terms of P 2 O 5 . 



108. Phosphoric Acid and Phosphates. Ordinary 

 phosphoric acid is produced by the action of H 2 SO 4 upon 

 bone ash. 



3 H 2 S0 4 + Ca 3 (P0 4 ) 2 == 2H 3 P0 4 + 3 CaSO 4 . 



Salts of ortho or ordinary phosphoric acid are the most 

 common forms of the acid derivatives. Since this acid 

 contains three replaceable H atoms, three salts are formed, 

 as : Na 3 PO 4 , normal sodium phosphate ; Na 2 HPO 4 , di- 

 sodium phosphate ; and NaH 2 PO 4 , monosodium phos- 

 phate. The three calcium salts of phosphoric acid are : 



CaH 4 (PO 4 ) 2 , monocalcium phosphate. 



Ca 2 H 2 (PO 4 ) 2 , dicalcium phosphate. 



Ca 3 (PO 4 ) 2 , tricalciurn phosphate. 



In addition to the ordinary phosphoric acid, there are 

 other derivatives, as : 



H 3 PO 4 = H 2 O-hHPO 3 (metaphosphoric acid). 



2H 3 PO 4 = H 2 O + H 2 P 2 O 7 (pyrophosphoric acid). 



109. Phosphate Fertilizers. In deposits of phosphate 

 rock, the phosphoric acid is mainly in combination with 

 calcium as Ca 3 (PO 4 ) 2 , and is of little value as plant 

 food until it is treated with H 2 SO 4 and converted into 

 monocalcium phosphate, which is soluble and available 

 as plant food. Ca 3 (PO 4 ) 2 + 2H 2 SO 4 = CaH 4 (PO 4 ) 2 + 

 2CaSO 4 . Large amounts of phosphates undergo this 

 treatment in the manufacture of commercial fertilizers. 



