Define Buffer Action?

Buffer action: the ability of the buffer solution to resist the changes in pH value on the addition of small amount of an acid or a base is known as buffer action.

For example: consider an acidic buffer such as a solution containing an equimolar amounts of acetic acid and sodium acetate, the solution contains a large number of sodium ions (Na⁺), acetate ions (CH₃COO^–) and also a large number of undissociated acetic acid molecules.

CH₃COONa (aq) ————-> CH₃COO^– (aq) + Na⁺ (aq)
Suppose, a few drops of HCl are added to this buffer solution. This would provide hydrogen (H⁺) ions. These additional H⁺ ions would combine with the large reserve of CH₃COO^– ions to form undissociated acetic acid molecules.
CH₃COO^– (aq) + H⁺ (aq) ⇌ CH₃COOH

The additional H⁺ ions are neutralized by CH₃COO^– ions in the solution, hence there will be no change in its pH value. The reserve basicity of the solution is due to acetate ions.