Applications of Solubility Product

(i)  Purification of common salt

Natural common salt consists of many insoluble and soluble impurities. Saturated solution of common salt is prepared and insoluble impurities are filtered off. Hydrogen chloride gas (HCl) is circulated through the saturated solution. HCl and NaCl dissociate into their respective ions as:

            NaCl ↔  Na⁺ + Cl^-

            HCl  ↔  H⁺ + Cl^-

The concentration of Cl^- ions increases considerably in solution due to ionization HCl. Hence, the ionic product [Na⁺][Cl^-] exceeds the solubility product of sodium chloride and, therefore, pure sodium chloride precipitates out from solution.

(ii)   Salting out of soap

Soap is a sodium salt of higher acids. From the solution, soap is precipitated by the addition of concentrated solution of sodium chloride. Soap and sodium chloride are present in the form of ions.

C_nH_2n+1 COONa ↔  C_nH_2n+1 COO^- + Na⁺

             Soap

          NaCl ↔  Na⁺ + Cl^-

Thus, the concentration of Na+ ions increases considerably on addition of NaCl solution. Hence, the ionic product [C_nH_2n+1COO^-] [Na⁺] exceeds the solubility product of soap and, therefore, soap precipitates out from the solution.

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

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

                NH₄OH + CO₂ → NH₄HCO₃

            NH₄HCO₃ + NaCl → NaHCO₃ + NH₄Cl

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

    Thus, baking soda (NaHCO₃) can be quantifiably estimated.

(iv) Application of solubility product in quantitative analysis

1. Estimation of barium as barium sulphate:

H₂SO₄ as precipitating agent is added to the aqueous solution of BaCl₂.

            BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

Precipitation of BaSO₄ takes place when its ionic product exceeds solubility product. H₂SO₄ is added in slight excess to ensure complete precipitation. Large excess of H₂SO₄ is harmful for complex formation.