Common Ion Effect on Degree of Dissociation

Weak electrolytes are poorly ionized in aqueous solution. Their ionization may further be reduced if one of the ions are present from another source. This called a common ion effect.

i) Ammonium hydroxide is a weak base. On addition of ammonium chloride (a salt) ammonium ion from it, will make the ammonium ions combine with the hydroxide to form unionized ammonium hydroxide.

NH₄Cl → NH₄⁺ + Cl^–

NH₄OH ⇌ NH₄⁺ + OH^–

ii) In the base hydrolysis of oil, the sodium salt of the fatty acid (soap) is in a dissolved state.

When sodium chloride salt is added, the concentration of Na+ ions increases considerably.

C_nH_2n+1 + COONa ⇌ C_nH_2n+1 COO^– + Na⁺

NaCl ⇌ Na⁺ + Cl^–

Hence, the ionic product [C_nH_2n+1COO^–] [Na⁺] exceeds the solubility product of soap and, therefore, soap precipitates out from the solution. This is called salting out of soap.

(iii)  Manufacture of sodium bicarbonate (baking soda):

In Solvay’s soda process. The CO₂ gas is passed through ammonical brine to precipitate out NaHCO₃.

NH4OH + CO2 → NH4HCO3

NH4HCO3 + NaCl → NH4HCO3 + NH4Cl

NaHCO₃ is precipitated first because of its lower solubility product as compared to those of NH₄Cl, NH₃HCO₃ and NaCl.