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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