Exercise 91. Potassium Cupric Chloride

EXERCISE 91. POTASSIUM CUPRIC CHLORIDE, 2KC1CuC12-2H20 

This double salt is made easily by dissolving potassium chloride, KC1, and cupric chloride, CuCl2-2H20, in water in the proportion of two molecules of the former to one of the latter and evaporating the solution to crystallization. This salt, as well as the corresponding ammonium salt, is used in the determination of carbon in iron. Iron dissolves in a concentrated solution of the salt, leaving the carbon undissolved, and the latter can be filtered off and estimated by combustion. 

At a temperature of about 92°, the blue, plate-shaped crystals change into a brownish-colored mass in which can be distinguished the white octahedra of potassium chloride. The decomposition may be expressed in the equation 

The change is enantiotropic, but the reverse transformation is much slower than the direct decomposition. 

Obtain the salt in small crystals and carefully dry them. Seal a small sample in a thin-walled glass tube and determine the transition point by using the tube as a stirring rod in a beaker of water that is being heated. When approaching the transition point, regulate the flame so as to raise the temperature of the water very slowly. 

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