If the solution of a strong electrolyte is mixed with that of a weak electrolyte having a common ion, the degree of dissociation of the weak electrolyte is suppressed. This is known as common in effect. For example, on adding NH₄CI (a strong electrolyte) to NH₄OH (a weak electrolyte) both having common NH₄ ion, the degree of dissociation of NH₄OH is suppressed due to common ion effect as:

NH₄CI                  →        NH₄⁺  +   CI ^-
(Strong electrolyte)
NH₄OH            <==>       NH₄⁺  +   OH ^-
(Weak electrolyte)

The common ions (NH₄⁺ ions) form NH₄CI as well as NH₄OH lead to suppression of degree of degree of dissociation of NH₄ OH and hence decrease in the concentration of hydroxide ions in the medium.

Importance of common ion effect in qualitative analysis

In second group of qualitative analysis we suppress the ionization of H₂S (a weak electrolytes) by adding HCI (a strong electrolyte) having common ion (H⁺ ion). As a result, the concentration of sulphide ion is reduced as :

HCI ( a strong electrolyte)       →      H⁺  +   CI ^-
H₂S ( a weak electrolyte)      <==>      2H⁺  +  S^{2 -}

Similarly, in third group of qualitative analysis, by adding NH₄CI (a strong electrolyte) to NH₄OH (a weak electrolyte having common NH₄⁺ ion, the degree of dissociation of NH₄OH is suppressed leading to decrease in the concentration of hydroxide ions.

For more help in Common Ion Effect click the button below to submit your homework assignment