DETECTION" OF IMPURITIES. 233 



added to a cupric solution should form a dark blue solution 

 without leaving a residue. 



Examination of compounds of bismuth. Subnitrate and sub- 

 carbonate of bismuth should be completely soluble in about 8 parts 

 of a mixture of equal parts of nitric acid and water. When 

 this solution is poured into 50 parts of water a white precipi- 

 tate falls; the filtrate from this precipitate may be used to 

 test for lead by sulphuric acid, for silver by hydrochloric acid, 

 for sulphates by barium chloride, for chlorides by silver nitrate 

 (all of which reagents give white precipitates if the impu- 

 rities named are present), and for copper by an excess of 

 ammonium hydrate, which precipitates the bismuth yet in solu- 

 tion, while copper would be indicated by the blue color of the 

 filtrate. 



Another portion of the bismuth salt is dissolved in hydro- 

 chloric acid and all bismuth precipitated by hydrosulphuric acid ; 

 the filtrate should leave no residue (absence of metals of the 

 iron group and of the light metals). 



Examination of compounds of silver. A solution of silver 

 nitrate in water, or of silver oxide in nitric acid, should give a 

 white precipitate with hydrochloric acid, which precipitate 

 should be completely dissolved by ammonium hydrate. The 

 filtrate from a solution from which all silver has been precipi- 

 tated by hydrochloric acid, should leave no residue on evapora- 

 tion. Silver oxide is readily dissolved by ammonia water. 



Examination of compounds of mercury. All compounds of 

 mercury are completely volatilized on heating. 



The oxides of mercury are dissolved by heating with about 10 

 parts of diluted nitric acid; mercuric oxide, when heated in a 

 tube, should evolve no red fumes (absence of nitric acid). 



Mercuric chloride should be soluble in water and in alcohol; it 

 should be tested for arsenic by Marsh's test. 



Mercurous chloride, when digested with water, should yield a 

 filtrate which, on evaporation, leaves no residue, and which 

 is not changed by either hydrosulphuric acid, silver nitrate, 

 or potassium iodide (absence of mercuric chloride). 



