Common Ion Effect

The degree of ionisation of a weak electrolyte is suppressed by the addition of strong electrolyte containing a common ion. It is known as common ion effect.

For example, ammonium hydroxide dissociates in solution as:

NH_4OH \leftrightharpoons NH^+_4 + OH^-

Applying the law of mass action, we get


\dfrac{[NH^+_4][OH^-]}{[NH_4OH]} = K


In the presence of ammonium chloride i.e., addition of ammonium ions (NH_4^+) to the solution and NH^+_4 ions are also obtained from the compound NH_4 OH in the solution; hence it is named as common ion. Thus the concentration of NH^+_4 increases, but K is constant at a particular temperature, there must be an increase in the value of [NH_4OH] or decrease in the value of [OH^-]

NH_4Cl \leftrightharpoons NH^+_4 + Cl^-


Thus the ionisation of NH_4OH is diminished by the addition of NH_ 4Cl which furnishes the common ion, NH^+-4.

In the same way the ionisation of CH_3COOH is diminished in the presence of CH_3COONa which furnishes the common ion CH_3 COO^-.

The principle of common ion effect has a great importance in qualitative analysis.

Dissociation of hydrogen sulphide in the presence of hydrochloric acid

HCl \leftrightharpoons H^+ + Cl^- \hspace{20mm}H^+ ion being common

H_2S \leftrightharpoons 2H^+ + S^{2-} \hspace{20mm} H^+ ion being common

Dissociation of ammonium hydroxide in the presence of ammonium chloride:

NH_Cl \leftrightharpoons NH^+_4 + Cl^- \hspace{20mm} NH^+_4 ion being common

NH_4OH \leftrightharpoons NH^+_4 + OH^- \hspace{20mm} NH^+_4 ion being common

Thus, in above system [S^{2-}] and [OH^-] decreases in comparison the usual [S^{2-}]  and [OH^-] obtained from [OH^-]H_2S \space{2mm} \text{and} \hspace{2mm} NH_4OH, respectively in water.

The common ion effect provides a valuable method for controlling the concentration of the ions furnished by a weak electrolyte.

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