346 PHOSPHORUS. 



PHOSPHORIC ACID. 



Ey. 892.28, or 7 1.44; PO 5 ; forms three hydrates and three 

 classes of salts : 

 Monobasic phosphate of water, or metaphosphate 



ofwater . . . . . HO + PO 5 



Bibasic phosphate of water., or pyrophosphate of 



water. ..... 2HO + PO 5 



Tribasic phosphate of water, or phosphate of water 3HO -f PO 5 



Preparation. To obtain this acid in a state of purity, the 

 most convenient process is to set fire to about a drachm of 

 phosphorus upon a little metallic capsule, placed in the centre 

 of a large stone- ware plate, and immediately cover it by a dry 

 bell jar of the largest size. The phosphorus is converted into 

 white flakes of phosphoric acid which are retained, with very 

 little loss, within the bell-jar, and fall upon the plate like snow. 

 The dry phosphoric acid is distinguished by the same shade of 

 white, absence of crystallization, and perfect opacity, as solid 

 carbonic acid. Exposed for a few minutes to the air, it deli- 

 quesces ; and when the solid acid is collected in a wine-glass, and 

 a few drops of water are thrown upon it, it is converted into a 

 hydrate with explosive ebullition, from the heat evolved. The 

 anhydrous acid is perfectly fixed, unless in the presence of 

 aqueous vapour, when it sublimes away, probably in the state 

 of a hydrate. 



Phosphorus may likewise be oxidated by means of nitric 

 acid. In this operation, the fuming nitric acid should be 

 diluted with an equal bulk of water, to avoid accidents from the 

 violent action of the acid, which may cause the phosphorus to 

 be projected in a state of ignition ; the diluted acid is boiled 

 upon the phosphorus, and being afterwards evaporated to dry- 

 ness, it yields a hydrated phosphoric acid. 



Phosphoric acid is also obtained in large quantity from cal- 

 cined bones, which are reduced to a fine powder and mixed 

 with 4-5ths of their weight of oil of vitriol, previously diluted 

 with 4 or 5 times its bulk of water, as in the preparation of 

 phosphorus (page 339). Carbonate of ammonia is then added 

 to the filtered solution of phosphoric acid and the resulting 

 phosphate of ammonia being evaporated to dryness and heated 

 to low redness in a platinum crucible, a hydrated phosphoric 



