98 NON-METALS AND THEIR COMBINATIONS. 



and ammonia water; a crystalline precipitate falls, which is 

 dimagnesium ammonium phosphate : 



H 3 P0 4 + MgSO 4 + 3NH 4 HO = MgNH 4 P0 4 + (NH 4 ) 2 SO 4 + 3H 2 ; 

 Na 2 HPO 4 -f MgS0 4 + NH 4 HO = MgXH 4 PO 4 + Na,SO 4 + H 2 O. 



2. Add to a neutral solution of a phosphate, silver nitrate: 

 a yellow precipitate of phosphate of silver is produced, which 

 is soluble both in ammonia and nitric acid : 



Na 3 P0 4 + 3AgN0 3 = Ag 3 PO 4 + 3NaNO 3 . 



3. Add to phosphoric acid, or to a phosphate dissolved in 

 nitric acid, an excess of a solution of ammonium molybdate in 

 dilute nitric acid and apply heat : a yellow precipitate of phos- 

 pho-molybdate of ammonium is produced. This test is by 

 far the most delicate, and even traces of phosphoric acid may 

 be recognized by it ; moreover, it can be used in an acid solu- 

 tion, while the first two. tests cannot. 



4. Add to a neutral solution of a phosphate, calcium or 

 barium chloride: a white precipitate of calcium or barium 

 phosphate is produced, which is soluble in acids. 



Hypophosphorous acid, H 3 P0 2 or HPH 2 2 . This acid is of little 

 interest in the free state, but many of its salts, named hypo- 

 phosphites, are frequently used in medicine. 



Although containing three atoms of hydrogen, hypophos- 

 phorous acid is a monobasic acid, only one of the hydrogen 

 atoms being replaceable by metals. 



Tests for hypophosphites. 



1. Heated over a flame they burn with a phosphorescent 

 light. 



2. From solutions of mercuric chloride and silver nitrate 

 they precipitate the metals in consequence of the deoxidizing 

 action of hypophosphorous acid. 



3. With zinc and diluted sulphuric acid they evolve hydrogen 

 and phosphoretted hydrogen. 



4. An acid solution of potassium permanganate is readily 

 decolorized. 



