Occlusion by solid-solution formation

Kolthoff proposed the concept that entrapment of foreign ions, involving growth of precipitate around adsorbed ions, is an important source of contamination, particularly of crystalline precipitates such as barium sulfate and calcium oxalate. Essential to this concept is that occlusion is not an equilibrium process and that recrystaUization during aging can effect purification. The foreign ions represent lattice imperfections unless they are actually held in solid solution. 

One of the ions of a precipitate can be replaced by a foreign ion of the same charge, provided the ions do not differ in size by more than about 5% and provided the two salts crystallize in the same system. Thus silver chloride and silver bromide form a complete series of solid solutions by isomorphous replacement of bromide by chloride. 

If pure silver bromide is shaken with a solution containing chloride ions, an equilibrium is set up  

This may be regarded as a distribution equilibrium of the two halide ions between solid and solution  

To express theoretically the relations involved, we follow the treatment given by Flood and by Flood and Bruun. TheworkofVaslowand Boyd may be consulted for a more complete thermodynamic consideration of silver chloride-silver bromide equilibria. An equation for the distribution ratio D corresponding to Equilibrium (9-6) can be obtained in terms of solubility products. First, consider the solid to be an impure silver bromide, with chloride as a foreign ion. Thus, 

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