COPPER AND MERCURY 513 



of copper hydroxide in water is very small; in other words, we 

 cannot have a high concentration of Cu 4 ^" and OH~ simultane- 

 ously in solution. But when ammonia is added to a solution 

 containing the very small amount of Cu" 1 " 1 " and OH~ requisite to 

 be in equilibrium with precipitated Cu(OH) 2 , it combines with 

 the Cu" 1 " 1 " to form complex ions of the formula [Cu(NH 3 )4J ++ . To 

 replace Cu ++ , more Cu(OH) 2 goes into solution, and the process 

 continues, if enough ammonia is added, until all Cu(OH) 2 has 

 been dissolved. We shall meet with other cases of a similar char- 

 acter later; in fact the principle involved the formation of a 

 complex ion is used very extensively in analysis (see p. 538) 

 and in industrial processes (see p. 523). 



An ammoniacal solution of cupric hydroxide is employed as a 

 solvent for cellulose. It dissolves also in a solution of sodium- 

 potassium tartrate NaKC 4 H 4 6 (p. 354), giving Fehling's solu- 

 tion, a reagent used in testing for glucose and similar reducing 

 agents. When this reagent is added to a liquid containing glu- 

 cose (p. 401), red cuprous oxide Cu 2 O is precipitated. 



Cuprous Oxide Cu 2 O. This oxide, mixed with CuO, is 

 formed by gentle heating of copper in air, and is best prepared by 

 use of Fehling's solution. It is employed in making ruby glass 

 and in coloring porcelain. 



Cupric Oxide CuO. When the liquid containing the blue 

 precipitate of cupric hydroxide is boiled, the blue color changes 

 to black and cupric oxide is thrown down: 



Cu(OH) 2 -* CuO + H 2 0. 



This oxide is used in the laboratory to ascertain the composi- 

 tions (and formulae) of organic compounds (determination of car- 

 bon and hydrogen). A weighed amount of the organic compound 

 is placed in a horizontal tube, between heated masses of the oxide. 

 A stream of oxygen or air carries the vapor of the organic 



