Application of solubility product principle in qualitative analysis

The concepts of solubility product and common ion effect play an important role in qualitative analysis for the separation of basic radicals (cations) into different groups.

Weak acids and weak bases ionise in water slightly and an equilibrium is established in their solutions. For example, in the ionization of a weak base NH₄OH as:

The ionization constant for the base,

If solid NH₄Cl is added to the solution, the concentration of NH₄⁺ ions increases. According to Le Chatelier's principle, the equilibrium shifts to the left. As a result, the concentration of OH^- is considerably decreased and the weak base NH₄OH becomes even weaker in the presence of its salt.

This is common ion effect and may be defined as the suppression of the degree of dissociation of a weak acid or a weak base by the addition of a strong electrolyte containing a common ion.

Qualitative analysis

The common ion effect is generally employed in qualitative analysis.

The cations of group II (Hg²⁺, Pb²⁺, Bi³⁺, Cu²⁺, As³⁺, Sb³⁺, Sn²⁺) are precipitated as their sulphides (such as CuS, PbS) by passing H₂S gas in the presence hydrochloric acid (Common H⁺ ions).

The cations of group III are precipitated as their hydroxides by NH₄OH in the presence of NH₄Cl.

The cations of group V are precipitated as their carbonates by the addition (NH₄)₂CO₃, in the presence of HCl.

Purification of sodium chloride

Sodium chloride obtained from sea-water or lakes is always impure. It can be purified on the basis of common ion effect as described below:

The saturated solution of impure sodium chloride is prepared by dissolving in minimum quantity of water. HCl gas is then passed through this solution. The following equilibria are set up:

Due to the presence of common chloride ions, the dissociation of sodium chloride is suppressed. This is known as common ion effect. The dissociation of sodium chloride is decreased to such an extent that the ionic product of NaCl exceeds its solubility product and it is thrown down as a precipitate.

Salting out of soap

Soap is a sodium salt of higher fatty acids e.g. sodium stearate, sodium oleate etc. When soap is prepared it floats over spent lye (the residual aqueous solution containing unused alkali, glycerol etc.). A significant amount of soap remains dissolved in this solution. To recover this soap, sodium chloride is added to the boiling soap solution.

The recovered soap separates out due to the common ion effect of Na⁺, in accordance with the reactions.

The increased concentration of Na⁺ in the solution due to the dissociation of NaCl, shifts the equilibrium towards left and thus soap is precipitated. The recovery of a dissolved salt by adding another salt to the solution is termed salting out.