The phenomenon in which the degree of ionization of a less soluble electrolyte is suppressed by the addition of a highly soluble electrolyte containing a common ion and the concentration of unionized species increases, which may come out as a precipitate is called common ion effect.
Suppression of ionization resulted by common ion causes to decrease the solubility of the electrolyte.
Precipitation of potassium perchlorate
Highly soluble KCl is added to the saturated solution of less soluble KCIO4 in which ionized and un-ionized KCIO4 is at equilibrium. Ionization takes place as given below:
KCIO4 (s) ⇆ K+ (aq) + C1O4- (aq)
KCI (s) ⇆ K+ (aq) + CI- (aq)
KCl causes to increase the concentration of K+ ion (common ion). According to Le Chatlier’s principle, K+ ions will react with C104- ions to form KCIO4 (ppt) to restore equilibrium. This suppresses the ionization of KCIO4, which causes to decrease the solubility of KC104. It means the solubility of KCIO4 is less in the solution of KCl than in pure water.
Purification of sodium chloride
HCI gas is passed through saturated solution of NaCl in which ionized and unionized NaCl is at equilibrium. Ionization takes place as given below:
NaCl (s) ⇆ Na+ (aq) + Cl- (aq)
HCl (aq) ⇆ H+ (aq) + Cl- (aq)
HCl causes to increase the concentration of Cr ions (common ion).
According to Le Chatleier’s principle, Cl- ions will react with Nations to form ppt of pure NaCl.