Common ion effect

Common ion effect

The degree of dissociation of an electrolyte (weak) is suppressed by the addition of another electrolyte (strong) containing a common ion, this is termed as common ion effect. Acetic acid is a weak electrolyte and its ionisation is suppressed in presence of a strong acid  ( ion as common ion) or a strong salt like sodium acetate (acetate ion is a common ion). Similarly, the addition of  or  to  solution will suppress the dissociation of  due to common ion either  or .




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As a result of common ion effect, the concentration of the ion of weak electrolyte which is not common in two electrolytes, is decreased. The use of this phenomenon is made in qualitative analysis to adjust concentration of  ions in second group and  ion concentration in third group.

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