The common ion effect

The common-ion effect is the shift in an ionic equilibrium caused by the addition of a solute that provides an ion that takes part in the equilibrium. The presence of the common ion suppresses the ionization of a weak acid or a weak base. For example, if sodium acetate and acetic acid are dissolved in the same solution, they both dissociate and ionize to produce CH₃COO^– ions.

Sodium acetate, CH₃COO^–Na⁺, is a strong electrolyte, so it dissociates completely in solution, but acetic acid, CH₃COOH, is a weak acid and ionizes partially. According to Le Chatelier’s principle, the addition of CH₃COO^– ions from CH₃COO^–Na⁺ to a solution of CH₃COOH will suppress the ionization of CH₃COOH and decrease the concentration of hydrogen ions.

Buffer Solutions

A buffer solution is a solution that resists changes in pH from the addition of a limited amount of an acid or a base.

Buffers contain either a weak acid and its conjugate base or a weak base and its conjugate acid. A buffer solution must contain a relatively large concentration of acid to react with any OH^– ions that are added to it. Similarly, it must contain a relatively large concentration of base to react with any H⁺ ions.

To understand the action of buffer, consider a buffer that contains approximately equal molar amounts of a weak acid, HA, and its conjugate base, A^–. When a strong acid is added to the buffer, it supplies hydrogen ions that react with the base A^–.

On the other hand, when a strong base is added to the buffer, it supplies hydroxide ions that react with the acid, HA.

Thus, a buffer solution resists changes in pH through its ability to combine with the H⁺ and OH^– ions.

Buffers are very important to chemical and biological systems.