PHOSPHORUS. 225 



2. Acidify about 5 c.c. of the solution with dilute hydrochloric 

 acid, and add some mercuric chloride solution. A white precipitate 

 of mercurous chloride is formed. Above 60 C. and with excess of the 

 hypophosphite, further reduction to dark metallic mercury takes place. 



4HgCl 2 + Na.PHA + 2H 2 O = 4HgCl + H,PO 4 + NaCl + 3HC1. 

 4HgCl + NaPHA + 2H 2 O == 4Hg + H S PO 4 + NaCl + 3HC1. 



3. Addition of silver nitrate solution causes a dark precipitate of 

 metallic silver. In the first instant, a white precipitate of silver hypo- 

 phosphite is seen, but this is very unstable. 



NaPH 2 Q 2 + 4AgN0 3 + 2H 2 O == 4Ag + NaH 2 PO 4 + 4HN0 3 . 



4. When the solution is added to acidified potassium permanganate 

 solution, the latter is decolorized. The essential reaction is this, 



NaPH 2 2 + 2 = NaH 2 P0 4 . 



Tests 1 and 2 are very distinctive and usually sufficient to recognize 

 the acid or its salts. 



Phosphorous acid, H 3 PO 3 , PO.H.(OH) 2 . This is a dibasic acid 

 obtained by dissolving phosphorous oxide in water: 



PA + 3H 2 = 2H 3 P0 3 . 



or still better by the action of water on phosphorus trichloride : 

 PC1 3 + 3H 2 O == H 3 P0 3 + 3HC1. 



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. Phosphorous acid and its salts give practically the same 

 reactions as the hypophosphites. The following are the chief dis- 

 tinctions : Phosphites when added to solutions of calcium, barium, and 

 strontium salts give precipitates of phosphites of these metals, whereas 

 hypophosphites do not give a precipitate. Acidified permanganate 

 solution is decolorized only after some time by phosphites, but imme- 

 diately by hypophosphites. 



Phosphites are of very little importance. 



Phosphoric acids. Phosphoric oxide is capable of combining 

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

 thereby three different acids. 



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

 P 2 O 5 -f 2H/) = 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 Orthophosphoric acid. 

 15 



