Common Ion Effect

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  • The phenomenon in which the degree of dissociation of any weak electrolyte is suppressed by adding a small amount of strong electrolyte containing a common ion is called as common ion effect.

Example – 1: (Dissociation of a Weak Acid)

  • Ionization of weak electrolyte acetic acid (CH3COOH) is suppressed by adding strong electrolyte sodium acetate (CH3COONa) containing common acetate ion (CH3COO)
  • Explanation: 
  • Suppose, an electrolyte acetic acid (CH3COOH) is treated with water.  It dissociates and an equilibrium exists as follows,

CH3COOH(aq)  ⇌ CH3COO(aq) + H+(aq)

By applying law of mass action,

Where ‘Ka’ is dissociation constant of acid.



  • If small amount of a strong electrolyte like sodium acetate (CH3COONa) is added to the aqueous solution of CH3COOH, it gets dissociated and equilibrium exists, as

CH3COONa(aq)  → CH3COO(aq) + Na+(aq)

  • Here CH3COO ions are common hence their concentration increases. According to Le-Chatelier’s principle, equilibrium shifts towards left.  To keep the value of Ka constant, the concentration of CH3COOH molecules is increased. In this way ionisation of CH3COOH is suppressed by adding CH3COONa. Thus the pH of the solution increases.

Example – 2: (Dissociation of a Weak Base):

  • Ionisation of weak electrolyte Ammonium hydroxide (NH4OH) is suppressed by adding strong electrolyte Ammonium chloride (NH4Cl) containing common ammonium ion (NH4+)
  • Explanation:
  • Suppose, an electrolyte Ammonium hydroxide (NH4OH) is treated with water.  It dissociates and an equilibrium exists as follows,

NH4OH(aq)   ⇌     NH4+(aq)  +    OH(aq)

By applying law of mass action,

Common Ion Effect



Where ‘Kb’ is dissociation constant of the base.

  • If small amount of a strong electrolyte like Ammonium chloride (NH4Cl) is added to the aqueous solution of NH4OH, it gets dissociated and equilibrium exists, as

NH4Cl(aq)    →   NH4+(aq) +  Cl(aq)

  • Here NH4+ ions are common hence their concentration increases. According to Le-Chatelier’s principle, equilibrium shifts towards left.  To keep the value of Kb constant, the concentration of NH4OH molecules is increased. In this way ionisation of NH4OH is suppressed by adding NH4Cl. Thus the pH of the solution decreases.

Applications of Common Ion Effect:

Purification of Common Salt:

  • Principle: Addition of common ion to a saturated solution of a salt causes the precipitation of salt. When ionic product exceeds the solubility product, precipitation takes place.
  • Process and Explanation: 
  • A saturated solution of common salt, free from suspended impurities is taken and HCl gas is passed through it. In a saturated solution of impure NaCI, equilibrium exists as follows,

NaCl(aq)    →   Na+(aq) +  Cl(aq)

  • If pure HCI gas is passed through this solution, being strong elec­trolyte, it dissociates almost completely and equilibrium exists,

HCl(aq)    →   H+(aq) +  Cl(aq)

  • Now Cl is a common ion. The concentration of CI ions (common ions) is increased.  According to Le-Chatelier’s principle equilibrium shifts towards the left.  Dissociation of NaCI is suppressed and pure  NaCI is precipitated as solid salt.  Thus pure NaCI can be precipitated by passing HCI gas in the saturated solution of impure NaCI.

Salting Out of Soap:

  • Principle: Addition of common ion to a saturated solution of a salt causes the precipitation of salt. When ionic product exceeds the solubility product, precipitation takes place.
  • Process and Explanation:
  • Soap is sodium salt of higher fatty acids (RCOONa).  It is prepared by hydrolysis of oils with NaOH. In saturated soap solution there exist an equilibrium,

ROONa(aq)  → RCOO(aq) + Na+(aq)



  • If small amount of NaCl is added to the saturated soap solution, it dissociated as

NaCl(aq)    →   Na+(aq) +  Cl(aq)

  • Concentration of Na+ ions (common ion) increases.  Due to common ion effect, dissociation of soap is decreased and soap gets precipitated and then can be easily removed from soap solution. This process of getting solid soap from soap solution, by adding salt like NaCI is called as salting out of a soap.
Science > Chemistry > Ionic EquilibriaYou are Here
Physics Chemistry  Biology  Mathematics

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