Common ion effect is the phenomenon that causes the suppression of electrolysis of weak electrolytes upon the addition of strong electrolytes having a common ion. This effect is due to the fact that the common ion (from the strong electrolyte) will compete with the other solute, with less solubility product (Ksp), leading to a decrease in the solubility of the solute with a lesser Ksp value.

For example, when strong electrolytes such as salts of alkali metals, are added to the solution of weak electrolytes, having common ions, they dissociate strongly and increase the concentration of the common ion. This will decrease the solubility of weak electrolytes by shifting the equilibrium backward.

The common ion effect is often used to control the concentration of ions in solutions. It can also be used in the separation of mixtures, by adding a common ion to one of the components of the mixture to decrease its solubility and allow it to be precipitated out of the solution.

The common ion effect is applicable to reversible reactions.

Principle of common ion effect

Common ion effects work on Le Chatelier’s principle. According to this principle, the system adjusts itself to nullify the effect of changes in physical parameters like pressure, concentration, temperature, etc.

In a reversible reaction, when the concentration of ions increases on the product side it will shift the equilibrium toward reactants.

Hydrofluoric acid (HF) is a weak acid. It slightly dissociates in water. At equilibrium, we have H+ and F ions.

Hydrogen fluoride dissociation: example of common ion effect

When sodium fluoride (NaF) is added to the aqueous solution of HF, it further decreases the solubility of HF. Because it dissociates to increase the concentration of F ion. It shifts the equilibrium toward the reactant side.

sodium fluoride dissociation example of common ion effect

Examples of common ion effect

Dissociation of NH4OH

Ammonium hydroxide (NH4OH) is a weak electrolyte. It dissociates in water and equilibrium is established between ions and undissociated molecules.

NH4OH dissociation example of common ion effect

When sodium chloride, a strong electrolyte, NH4Cl containing a common ion NH4+ is added, it strongly dissociates in water. 

As the concentration of NH4+ ion increases. It will shift the equilibrium toward the left. It suppressed the dissociation of NH4OH.

Dissociation of silver chloride (AgCl)

Silver chloride is merely soluble in the water, such that only one formula unit of AgCl dissociates into Ag+ and Clions from one million of them. dissociates as

At equilibrium is

Silight Dissociation of silver chloride

When we add NaCl into the aqueous solution of AgCl. It decreases the solubility of AgCl2 because it has the common ion Cl. NaCl dissociates into Na+ and Cl ions.

As the concentration of Cl ion increases AgCl2 gets precipitated and equilibrium is shifted toward the left.

Dissociation of acetic acid (CH3COOH)

CH3COOH is a weak acid. It weakly dissociates in water and establishes an equilibrium between ions and undissociated molecules.

Disscoiation of acetic acid

When sodium acetate CH3COONa containing a common ion CH3COO, is added, it strongly dissociates in water.

Dissociation of sodium acetate

As a result, the concentration of CH3COO ion increases, and the equilibrium shifts toward the left, This way, the dissociation of CH3COOH is suppressed.

Effect of common ion on solubility

Common ion has an effect on the solubility of solutes. When we add a compound having a common ion it decreases the solubility of dissolved compounds. Ionic compounds are less soluble in an aqueous solution having a common ion rather they are more soluble in water having no common ion.

This is due to an increase in the solubility product of that ion. For example, 

Barium sulfate dissociates in water as Ba+2 and SO4-2 ions. At equilibrium we have:

Ionization of Barium sulphate BaSO4

When we add sodium salt of sulfate it decreases the solubility of BaSO4. This is because Na2SO4 has a common ion(SO4-2).

Dissolution of Sodium sulphate

As the concentration of SO4-2 ions increases equilibrium is shifted toward the left. Solubilities vary according to the concentration of a common ion in the solution. The solubility of solid decreases if a solution already contains a common ion.

Importance of common ion effects

Gravimetric analysis

The common ion effect is used in gravimetric analysis to decrease the solubility of precipitate in a medium. This is done by adding an excess precipitating agent. This help to estimate the accurate quantity of analyte.

Purification of salt

The common ion effect is used for the purification of crude common salt. Crude salt has different impurities like CaCl2, MgCl2, KBr, etc. These impurities are removed by passing HCl gas through a concentrated solution of salt. NaCl precipitated and crystallized out of the solution.

Salting out of soap

Soap is the sodium salt of higher fatty acids. By using the common ion effect we can remove dissolved salts from soap. This is done by adding NaCl to the boiling soap solution.

Quantitative analysis

By using the common ion effect we can analyze substances to the desired extent. This is done by decreasing the solubility of substances by adding other substances having common ions.

Common ion effect and pH

As the concentration of ions changes pH of the solution also changes. When H+ ions increases in the solution the pH of the solution decreases whereas when the concentration of OH ion increase pH of the solution also increases. The solubilities of many substances depend upon the pH of the solution. For example,

As the concentration of OH ion increases pH of the solution also increases. This will shift the equilibrium toward the left.

water dissociation

But if we add H+ ions then the equilibrium will shift toward the right and the pH of the solution decreases.

Related Resources

Key Takeaways

  • The common ion effect is a decrease in the solubility of a weak electrolyte by adding a common ion.
  • The common ion effect works on the basis of Le Chatelier principle.
  • The shift the equilibrium is toward the reactant side.

Concepts Berg

What is Le Chatelier principle?

According to the Le Chatelier principle, the system adjusts itself to nullify the effect of change in physical parameters i.e, pressure, temperature, concentration, etc.

How does the common ion effect works?

Common ion effect by suppressing the ionization of weak electrolytes or by reducing the solubility of dissolved salt and shifting the equilibrium toward reactants.

Why dissociation of weak electrolytes is suppressed?

Dissociation of weak electrolytes is suppressed because the strong electrolyte can more easily dissociate and increase the concentration of the common ion. This results in the suppression of the dissociation of weak electrolytes.

What is the effect of a common ion on the degree of dissociation of weak electrolytes?

The degree of dissociation of weak electrolytes is reduced due to the common ion effect.

What is meant by the term common ion?

The term common ion means the two substances having the same ion. For example, sodium chloride NaCl and HCl have common Cl ions.

Why does the common ion effect decrease solubility?

As the concentration of a particular ion increases system shifts the equilibrium toward the left to nullify the effect of change.

References

  • Vogel’s Textbook of Quantitative Chemical Analysis sixth edition by J Mendham, RC Denney, JD Barnes, M Thomas.
  • Chemistry by Zumdahl and DeCoste.
  • Chemistry by Raymond Chang
  • Common Ion Effect Study Guide (inspiritvr.com)
  • Common ion Effect (ecampusontario.pressbooks.pub)