PHOSPHORIC ACID. 9 



gives no precipitate with barium chloride, gives a yellow precipitate 

 with silver nitrate, and does not coagulate albumen. 



Phosjohoric Acid Anhydride, PqO-, is obtained by the combustion 

 • of phosphorus in a current of very dry air. It forms imponderable, 

 highly deliquescent, white flocks. It dissolves in water with the 

 hissing noise of red-hot iron. Hence its use to dry gases at temper- 

 atures at which sulphuric acid would be decomposed, and to prepare 

 by dehydration certain acid anhydrides or certain organic com- 

 pounds, nitriles for example. Heated to redness with charcoal it 

 yields carbonic oxide and phosphorus. 



Metaphosplioric Acid. HPO3, is monobasic. It is prepared by 

 hydrating the cold anhydride, by heating orthophosphoric acid to 

 redness, or by calcining ammonium phosphate at a red heat. It is 

 a vitreous substance, volatile at a red heat, very soluble in water, 

 with which it combines to reproduce orthophosphoric acid. It is 

 used to dry gases. The metaphosphates prepared by calcining 

 monometallic orthophosphates are only partially reduced by char- 

 coal. 



Pyrophosphoric Acid is dibasic. It is prepared by subjecting 

 •orthophosphoric acid to prolonged heat at 200° C. Calcined, it gives 

 metaphosphoric acid ; with water it reproduces orthophosphoric 

 acid. Pyrophosphates, prepared by calcining bi-metallic orthophos- 

 phates, or by heating phosphoric acid with an oxide, are reducible 

 by charcoal. 



As far as the manufacture of chemical manures is concerned, the 

 •only important modification to be borne in mind is the ordinary or 

 orthophosphoric acid. However, pyrophosphoric acid is sometimes 

 met with in phosphates that have been roasted to facilitate grinding, 

 and metaphosphoric acid in superphosphates dried at too high a 

 temperature. 



OrtkophospJioric Acid, HgPO^. — This, the ordinary phosphoric 

 acid, is tribasic. It is obtained mixed with phosphorus, in 

 the slow oxidation of phosphorus in moist air. It is prepared in 

 ■the laboratory by oxidizing red phosphorus by the aid of heat with 

 fifteen times its weight of nitric acid of 20 B. The reaction may be 

 expressed thus : 3P -f- 0HNO3 + 2Hp = 3H.3PO, + 5N0. The phos- 

 phorus is only added gradually and in small portions at a time. 

 Orthophosphoric acid is produced, which remains in the retort, and 

 a little nitrogen dioxide, which carries over some nitric acid. Care 

 is therefore taken to redistil " cohobitate," until the whole of the 

 phosphorus has disappeared; the residue is then evaporated in a 

 platinum basin to expel all nitric acid and to convert any phos- 

 phorous acid which may have been formed into phosphoric acid, 

 taking care not to exceed 188° C, otherwise pyrophosphoric acid 

 will be obtained. Orthophosphoric acid is also produced by the 

 ■action of hot water on metaphosphoric acid, by igniting hypophos- 



