PHOSPHORUS. 97 



absorbs the latter, forming orthophosphoric acid; by evapo- 

 rating the latter sufficiently, metaphosphoric acid is reformed. 



Metaphosphoric acid is a monobasic acid which coagulates 

 albumen (pyro- and ortho-phosphoric acids do not) and gives a 

 white precipitate with ammonio-silver nitrate. It acts as a 

 poison, whilst common phosphoric acid is comparatively harm- 

 less. 



Pyrophosphoric acid, H 4 P 2 7 = 178. This is a tetra-basic acid 

 which gives a white precipitate with ammonio-silver nitrate, 

 whilst orthophosphoric acid gives a yellow precipitate. 



Phosphoric acid, Orthophosphoric acid, Aciduni phosphoricum, 

 H 3 P0 4 = 98 (Trihydric phosphate, Common or tribasic phos- 

 phoric add}. Nearly all phosphates found in nature are ortho- 

 phosphates. 



Phosphoric acid may be made by burning phosphorus, dis- 

 solving the phosphoric oxide in water and boiling for a sufficient 

 length of time to convert the meta- into ortho-phosphoric acid. 



For medicinal purposes, phosphoric acid is made by gently 

 heating pieces of phosphorus with diluted nitric acid, when the 

 phosphorus is oxidized, red fumes of nitrogen tetroxide escaping. 

 The liquid is evaporated until the excess of nitric acid has been 

 expelled and enough of water added to obtain an acid which 

 contains 50 per cent, of the pure acid, H 3 P0 4 . 



Phosphoric acid is a colorless, odorless liquid, which, on heat- 

 ing, loses water, and is finally volatilized at a low red heat. It 

 is a tribasic acid forming three series of salts, namely : 



Na 3 PO 4 Trisodium phosphate. 

 Na 2 HPO 4 = Disodium hydrogen phosphate. 

 NaH 2 PO 4 = Dihydric sodium phosphate. 

 H 3 PO 4 = Trihydric phosphate. 



If the metal is bivalent, the formulas are thus : 



Ca 3 2PO 4 = Tricalcium phosphate. 

 Ca 2 H 2 2PO 4 = Dicalcium orthophosphate. 

 CaH 4 2PO 4 Monocalcium orthophosphate. 



Tests for phosphoric acids and phosphates. 



1. Add to phosphoric acid, or to an aqueous solution of a phos- 

 phate, a mixture of magnesium sulphate, ammonium chloride, 



7 



