The common ion effect is an application of the Le-Chateliers principle (1884) which states as: “If a change in concentration is caused to a chemical reaction in equilibrium, the equilibrium will shift to the right or left so as to minimize or reduce the change that has occurred”.
The common-ion effect is also defined in the following ways:
1. The reduction or repression of the degree of ionization or dissociation of an acid or salt by the addition of a common ion.
2. The effect on a solution of two dissociated solutes that contain the same ion.
3. When a solution has an ion introduced from another source that is the same as one of the ions in the original solution. The presence of a common ion suppresses the ionization of a weak acid or a weak base.
When a soluble salt (AC) is added to a solution of another salt (AB) containing a common ion A+, the dissociation of AB is suppressed according to the Le-Chateliers principle as equilibrium will shift to the left thereby decreasing the concentration of A+.
The dissociation is shown as;
[AB] ⇌ [A+] [B+]
[AC] ⇌ [A+] [C+]
Therefore, due to the common ion effect, the degree of dissociation of the AB will be reduced.
Example: If sodium acetate and acetic acid are dissolved in the same solution, they will dissociate and ionize to produce acetate ions. Acetic acid is a weak acid so it ionizes slightly in the solution whereas sodium acetate is a strong electrolyte so it dissociates completely in solution as;
CH3COOH(l) ⇌ H+(aq) + CH3COO−(aq)
CH3COONa(s) → Na+(aq) + CH3COO−(aq)
According to the Le-Chateliers principle, the addition of acetate ions as common ions from sodium acetate will suppress the ionization of acetic acid and shift its equilibrium to the left. Thus, the dissociation of the acetic acid will decrease and the pH of the solution will increase. This will decrease the hydrogen ion concentration as, pH α 1/[H+], and thus the common ion solution will be less acidic than a solution containing acetic acid. Therefore, 0.1M acetic acid solution has a pH of 2.87 but a solution of 0.1M acetic acid and 0.1M sodium acetate has a pH of 4.74. The pH of a buffer is 4.74 which is higher than the acid itself due to the common ion effect.
Make sure you also check our other amazing Article on : The Arrhenius Theory of Acids and Bases