190 METALS AND THEIR COMBINATIONS. 



metallic copper is not affected by any of the substances used in the preparation 

 of food, but as the metal is very apt to become covered with a film of oxide 

 when exposed to the air, and as the oxide is easily dissolved by the combined 

 action of water, carbonic or other acids, such as are found in vinegar, the juice 

 of fruits, or rancid fats, the use of copper for culinary vessels is always 

 dangerous. Actual adulterations of food with compounds of copper have been 

 detected. 



In cases of poisoning by copper the stomach-pump should be used, vomiting 

 induced, and albumen (white of egg) administered, which forms an insoluble 

 compound with copper. Reduced iron, or a very dilute solution of potassium 

 ferrocyanide, may be of use as antidotes. 



Analytical reactions. 

 (Cupric sulphate, CuSO 4 , may be used.) 



1. Add to solution of copper, hydrogen sulphide or ammonium sul- 

 phide : a black precipitate of cupric sulphide is formed. (Plate III., 1): 



CuSO 4 -f H 2 S = H 2 SO 4 + CuS. 



2. Add sodium or potassium hydroxide : a bluish precipitate of 

 cupric hydroxide, Cu(OH) 2 , is formed which is converted into dark- 

 brown cupric oxide, CuO, by boiling. (See equation above.) (Plate 

 III., 2.) 



3. Add ammonium hydroxide : a bluish precipitate of cupric 

 hydroxide is formed which readily dissolves in an excess of the 

 reagent, forming a deep azure-blue solution containing an ammonio- 

 copper compound. (See explanation above.) (Plate III., 3.) 



4. Add potassium ferrocyanide : a reddish-brown precipitate of 

 cupric ferrocyanide, Cu 2 Fe(CN) 6 , is obtained. (Plate III., 5.) 



5. Add solution of arsenous acid and carefully neutralize with 

 sodium hydroxide : green cupric arsenite is precipitated. (Plate V., 2.) 



6. Add sodium or potassium carbonate : green cupric carbonate 

 with hydroxide is precipitated. (Plate III., 4.) 



7. Immerse a piece of iron, or steel, showing a bright surface, in 

 an acidified solution of copper : the latter is precipitated upon the 

 iron, an equivalent amount of iron passing into solution : 



CuS0 4 + Fe = FeS0 4 + Cu. 



8. Most compounds of copper color the flame green, cupric chloride 

 blue. 



9. Cupric compounds give a blue, cuprous compounds a red borax 

 bead. 



10. Cupric salts (when not anhydrous) have mostly a blue or green 



