PHOSPHORUS. 115 



Tests for phosphoric acid and phosphates. 



(Sodium phosphate, Na 2 HPO 4 , 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 dimag- 

 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 2 HP0 4 + MgS0 4 + NH 4 OH = MgNH 4 PO 4 + Na 2 SO 4 + H 2 O. 



2. Add to a neutral solution of a phosphate, silver nitrate ; a yel- 

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

 in ammonia and nitric acid : 



Na 3 P0 4 + 3AgN0 8 = Ag 3 PO 4 + SNaNO,. 



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 apply heat ; 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. 



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

 chloride ; a white precipitate of calcium or barium phosphate is pro- 

 duced, which is soluble in acids. 



5. Ferric chloride produces in neutral solution a yellowish- white 

 precipitate of ferric phosphate, Fe 2 (PO 4 ) 2 , thus : 



2Na 2 HPO 4 + Fe 2 Cl 6 = 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 ; 

 the hydrochloric acid combines with the sodium of the acetate, and 

 the acetic acid w^hich is set free has no dissolving action upon the 

 ferric phosphate. 



Hypophosphorous acid, H 3 PO 2 , or HPH 2 O 2 . When phosphorus 

 is heated with solution of potassium, sodium, or calcium hydroxide, 

 the hypophosphite of these metals is formed, while gaseous hydrogen 

 phosphide, PH 3 , is liberated and ignites spontaneously. The action 

 may be represented thus : 



3KOH + 4P + 3H 2 O = 3KPH 2 O 2 -f PH 3 . 

 or 



3Ca(OH) 2 + 8P + 6H 2 O = 3Ca(PH 2 O 2 ) 2 + 2PH 3 . 



