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.
CnH2n+1 + COONa ⇌ CnH2n+1 COO– + Na+
NaCl ⇌ Na+ + Cl–
Hence, the ionic product [CnH2n+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.