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 .
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.