Common Ion Effect

If to an ionic equilibrium, AB A⁺ + B‾ , a salt containing a common ion is added, the equilibrium shifts in the backward direction. This is called common Ion effect.

Acetic acid being a weak acid, ionizes to a small extent as:

CH₃COOH  CH₃COO‾ + H⁺        

To this solution , suppose the salt of this weak acid with a strong base is added. It ionizes almost completely in the solution as follows and provides the common acetate ions:

CH₃COONa ————> CH₃COO‾ + Na⁺

the concentration of CH₃COO‾  ions increases and by Le Chatelier’s principle, the dissociation equilibrium shift backwards i.e. dissociation of acetic acid is further suppressed.

2) To the solution of the weak base , NH₄OH , NH₄Cl is added which provides the common NH₄⁺ ions ,dissociation of NH₄OH is suppressed:

NH₄OH  NH₄⁺ +OH‾

NH₄Cl ———->NH₄⁺ +Cl‾

3) To the solution of silver chloride in water, if NaCl is added which provide the common Cl‾ ions, the solubility AgCl decreases :

AgCl Ag⁺ + Cl‾

NaCl ——–> Na⁺ + Cl‾

Increase in the concentration of Cl‾ ions shifts the equilibrium in the backward direction i.e. some solid AgCl separates out.

If to the solution of a weak electrolyte, which ionises to a small extent ,a strong electrolyte having a common ion is added which ionizes almost completely, the ionization of the weak electrolyte is further suppressed .

If to the solution of a sparingly soluble salt, if a soluble salt having a common ion is added ,the solubility of the sparingly soluble salt further decreases.