PHOSPHORUS. 113 



Phosphorous acid, H 3 PO 3 = 82. This acid is obtained by dis- 

 solving phosphorous oxide in water : 



P 2 3 + 3H 2 = 2H 3 P0 3 . 



It is a colorless, acid liquid, which forms salts known as phos- 

 phites; it is a strong deoxidizing agent, easily absorbing oxygen, 

 forming phosphoric acid. 



Tests for phosphorous acid. 



1. Added to mercuric chloride, a white precipitate of mercurous 

 chloride is formed. 



2. Added to silver nitrate, a black precipitate of metallic silver is 

 produced. 



3. After being heated with nitric acid, it shows reactions of phos- 

 phoric acid. 



Phosphoric acids. Phosphoric oxide is capable of combining 

 chemically with one, two, or three molecules of water, forming 

 thereby three diiferent acids. 



P 2 O 5 + H 2 O = H 2 P 2 O 6 = 2HPO 3 Metaphosphoric acid. 

 P 2 O 5 + 2H 2 O = H 4 P 2 O 7 Pyrophosphoric acid. 



.P 2 O 5 + 3H 2 O = H 6 P 2 O 8 = 2H 3 PO 4 Ortho-phosphoric acid. 



These three acids show different reactions, act differently upon the 

 animal system, and form different salts. 



Metaphosphoric acid, HPO 3 = 80 (Glacial phosphoric acid). 

 This acid is always formed when phosphoric oxide is dissolved in 

 water ; gradually, and more rapidly on heating with water, it absorbs 

 the latter, forming orthophosphoric acid ; by evaporating the latter 

 sufficiently, metaphosphoric acid is re-formed. 



Metaphosphoric acid is a monobasic acid which coagulates albumin 

 (pyro- and orthophosphoric acids do not) and gives a white precipi- 

 tate with ammonio silver nitrate ; it is not precipitated by magnesium 

 sulphate in the presence of ammonia and ammonium chloride. It 

 acts as a poison, whilst common phosphoric acid is comparatively 

 harmless. 



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

 which gives a white precipitate with arnmonio-silver nitrate, whilst 

 orthophosphoric acid gives a yellowish precipitate ; it is not precipi- 

 tated by ammonium molybdate, and does not coagulate albumin. 



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