Coupon Accepted Successfully!


Common Ion Effect

Weak acids and weak bases are ionized only to small extent in their aqueous solutions. In their solutions, unionized molecules are in dynamic equilibrium with ions. The degree of ionization of a weak electrolyte (weak acid or weak base) is further suppressed if some strong electrolyte which can furnish some ion common with the ions furnished by weak electrolyte, is added to its solution. This effect is called common ion effect. For example, degree of ionization of NH4OH (a weak base) is suppressed by the addition of NH4Cl. 

The ionization of NH4OH and NH4Cl in solution is represented as follows:

NH4OH(aq) NH4+(aq) OH -(aq) .... Weakly ionized ........ (1)
NH4Cl NH4+(aq) + Cl-(aq) ... strongly ionized ....... (2)

Due to the addition of NH4Cl, which is strongly ionized in the solution, concentration of NH4+ ions increases in the solution. Therefore, according to Le-Chatelier's principle, equilibrium in equation (1) shifts in the backward direction in the favour of unionized NH4OH. Thus, the concentration of OH- ions in the solution is considerably reduced and the weak base NH4OH becomes still weaker base.

The suppression of the degree of ionisation of a weak electrolyte (weak acid or weak base) by the addition of some strong electrolyte having a common ion is called common ion effect.
The solubility of salts of weak acids like phosphates increases at lower pH. This is because at lower pH the concentration of the anion decreases due to its protonation. This in turn increase the solubility of the salt so that Ksp = Qsp.

Test Your Skills Now!
Take a Quiz now
Reviewer Name