Fundamental concepts in chemistry: What is common ion effect?

When a common ion is added from outside in an ionic reaction to suppress the ionization, the phenomenon is called common ion effect. To illustrate it further common ion effect can be explained as the addition of a common ion in a solution of less soluble ion or electrolyte, it considerably suppresses the ionization of this electrolyte resulting in increased concentration of this electrolyte which may further precipitate out from the solution.

Common example of common ion effect:

Purification of saturated brine is done through the phenomenon of common ion effect. In this reaction hydrogen chloride gas serves as common ion and it is passed through saturated solution of sodium chloride. This reaction can be explained in the form of equilibrium constant expression given as follows

Equilibrium constant for equation can be written as follow

When HCl gas is passed through saturated solution of NaCl it also ionizes in this solution as follows

On passing hydrochloric gas from saturated solution of NaCl, the concentration of Cl^- ion increases in the solution and as a result sodium chloride crystallizes out. Thereby, the value of equilibrium constant maintains at constant value.