Common Ion Effect Applications, problems and examples

Common Ion Effect can be described as“The lowering of the degree of discussion of weak electrolytes by adding a

strong electrolyte having a common ion ”.

Or

“The decrease in the solubility of the salt in a solution that already contains an ion common to that salt is called common ion effect”.

For Example;

Consider the equilibrium state of sparingly soluble electrolyte AgCl in a saturated solution.

AgCl  →   Ag⁺  + Cl^–

Now add NaCl to this solution, which is completely ionized.

NaCl  →   Na⁺  + Cl^–

Then there will be common ion i.e. chlorine ion (Cl).

AgCl →    Ag⁺  + Cl^–

NaCl  →   Na⁺  + Cl^–

Consequently, equilibrium will be shifted to the left in accordance with Le-Chatlier’s principle AgCl will be precipitated. In other words, the ionization of AgCl is suppressed due to common ion, chlorine (Cl) and it forms the precipitate.

Applications of Common Ion Effect

The common ion effect finds a useful application in a qualitative salt analysis.

Precipitation of Sulphides of Group II

In order to precipitate the Sulphides of group II, H₂S is passed through the original solution (O.S) in the presence of HCl.

H₂S  →   2H⁺  + S^2-

HCl furnishes H⁺ as common ions, which shift the above equilibrium to left according to Le-Chatlier’s principle. In other words, the addition of HCl suppresses the ionization of H₂S thereby lowering the concentration of sulfide ions (S^2-), which is however just enough to exceed the solubility product of group II Sulphides. In this way group, II cations are precipitated such as CuS, PbS, CdS, etc.

Precipitation of Sulphides in group III

Group III cations are precipitated as hydroxides’ by the NH₄OH in the presence of NH₄Cl. The common ion, in this case, is NH₄⁺, which suppresses the ionization of NH₄OH.

NH₄OH      →    NH₄⁺   + OH^–

Thus with the addition of the common ion (NH₄⁺) The equilibrium is shifted towards left and the concentration of OH^– decreases. Under these conditions, the solubility products of hydroxides of Al, Fe, and Cr is only exceeded due to which they are precipitated.

The hydroxides of the other cations such as Zn, Ni, Co, etc. are not precipitated due to their higher solubility product.

Precipitation of Sulphides of Group IV

Group IV Sulphides having higher solubility products are precipitated by H₂S in the presence of NH₄OH. The OH^– of NH₄OH combines with the H⁺ of H₂S to form H₂O.

H⁺   + OH^–     →   H₂O

The removal of H⁺ from the product side shifts the equilibrium to right.

H₂S       →      2H⁺  + S^2-

In this way, the concentration of the sulfide ion (S^2-) increases which the enough to exceed the solubility product for the precipitation of Sulphides, e.g. CoS, NiS, ZnS.

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