DETECTION OF IMPURITIES. 231 



ammonium hydrate and filtered, should not impart a blue color 

 to the filtrate (absence of copper), and this filtrate should not 

 be precipitated by ammonium carbonate or phosphate (absence 

 of alkaline earths and magnesium) and should not leave a 

 residue on evaporation and gentle ignition in order to expel the 

 ammonium compounds (absence of zinc, manganese, alkalies, 

 etc.). 



Solution of potassium hydrate added in excess to ferric solu- 

 tions and filtered, should not be precipitated by ammonium 

 carbonate after the alkaline solution has been neutralized by 

 hydrochloric acid (absence of aluminium). 



Compounds of iron may contain arsenic, and should therefore 

 be examined by Marsh's test. 



Ferrous salts (sulphate) should give no immediate blue pre- 

 cipitate with potassium ferrocyanide (absence of ferric com- 

 pounds). Ferric salts should give no blue color with potassium 

 ferridcyanide (absence of ferrous compounds). 



Ferric chloride should also be tested for nitric acid by placing 

 a crystal of ferrous sulphate into the solution to which sulphuric 

 acid has been added (a brown color indicating nitric acid), and 

 for sulphuric acid by barium chloride. 



Examination of compounds of zinc. Chloride or sulphate of zinc 

 when dissolved in water, oxide or carbonate of zinc when dissolved 

 in hydrochloric acid, should give no precipitate in the acidulated 

 solution with hydrosulphuric acid (absence of the metals of the 

 arsenic and lead groups). 



Ammonium hydrate, added to zinc solutions, should cause a 

 white precipitate, which is completely dissolved by an excess of 

 the reagent (absence of iron, aluminium, etc.). 



The filtrate of zinc solutions from which the metal has been 

 precipitated by an excess of ammonium sulphide, should not 

 be precipitated by ammonium carbonate or phosphate (absence 

 of alkaline earths apd magnesium), nor should it leave a residue 

 on evaporation and gentle ignition (absence of alkalies). 



"Water, digested with zinc oxide or carbonate, should leave no 

 residue on evaporation. 



Zinc sulphate should be tested for chlorides by silver nitrate, 

 and zinc chloride for sulphates by barium chloride. 



