228 NON-METALS AND THEIR COMBINATIONS. 



evidenced by the slightly alkaline reaction of disodium phosphate, which dis- 

 sociates thus, 



Na 2 HP0 4 i 2Na- + HPO 4 ". 



The alkaline reaction is due to the formation of a slight quantity of free alkali 

 by the ions of water (hydrolysis) thus, 



2Na + HPO/'l^ Hjpo, 

 (OH)' + H- / 



Na 2 HPO 4 + H 2 = NaOH + NaH 2 PO 4 , 



Trisodium phosphate is completely hydrolyzed in solution into disodium phos- 

 phate and sodium hydroxide, Na 3 PO 4 + H 2 O = Na 2 HPO 4 + NaOH. 



Tests for phosphoric acid and phosphates. 

 (Sodium phosphate, Na^HPO^ may be used.) 



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

 phate, a mixture of magnesium sulphate, ammonium chloride, and 

 ammonia water ; a white crystalline precipitate falls, which is mag- 

 nesium ammonium phosphate : 



H 3 P0 4 + MgS0 4 + 3NH 4 OH = MgNH 4 PO 4 + (NH 4 ) 2 SO 4 + 3H 2 O; 

 Na-jHPO, -f MgS0 4 + NH 4 OH = MgNH 4 PO 4 + NaO 4 + H 2 O. 



2. Add to a solution of disodium phosphate, silver nitrate ; a yellow 

 precipitate of silver phosphate is produced, which is soluble both in 

 ammonia and nitric acid : 



Na,HP0 4 + 3AgN0 3 = Ag 3 PO 4 + 2NaNO 3 + HN0 3 . 



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

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

 dilute nitric acid, and heat gently ; a yellow precipitate of phospho- 

 molybdate of ammonium, (NH 4 ) 3 PO 4 .10MoO 3 .2H 2 O, is produced ; 

 the precipitate is readily soluble in ammonia water. 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 solution, while 

 the first two tests cannot. Only a few drops of the solution to be 

 tested should be used. 



4. Add to a solution of a phosphate, calcium or barium chloride ; 

 a white precipitate of calcium or barium phosphate is produced, which 

 is soluble in acids. 



5. Ferric chloride produces a yellowish-white precipitate of ferric 

 phosphate, Fe 2 (PO 4 ) 2 , thus : 



2Na,HP0 4 + Fe.Cle = Fe 2 (PO 4 ) 2 + 4NaCl + 2HC1. 

 The liberated hydrochloric acid dissolves some of the precipitate, 

 which may be avoided by adding previously some sodium acetate ; 



