Buffer action

The property of a buffer solution to resist change in its pH value even when small amounts of the acid or the base are added to it is called buffer action.
Consider the acidic buffer solution containing acetic acid and sodium acetate. They dissociate as
CH3COONa <=======> CH3COO_- + H₊
CH3COONa <=======> CH3COO_- + Na₊
When a few drops of an acid, HCl is added to this buffer solution, the H₊ ions combine with CH3COO_- ions to form weakly ionized molecules of CH3COOH.
CH3COO_- + H₊ <=======> CH3COOH
Thus H₊ ion concentration does not change and hence the pH of the solution remains constant.
When a few drops of base, NaOH is added to the buffer solution, hydroxyl ions of the base neutralize the acid, forming salt and water.
Similarly, in a basic buffer solution of NH4OH and NH4Cl, they dissociates as
NH4OH <======> NH4₊ + OH_-
NH4Cl ----------> NH4₊ + Cl_-
When a few drops of a base added, the OH_- ions given by it combine with NH4₊ ions to form the weakly ionized NH4OH.
NH4₊ + OH_- ---------> NH4OH
Thus the OH_- ion concentration or the pH of the solution remains unaffected.
When a small amount of an acid is added, the H₊ ions given by it combines with the OH_- ions already produced by NH4OH.
H₊ + OH_- --------> H2O
Therefore the H₊ ions concentration or the pH of the solution remains unaffected.
The buffer capacity of a buffer solution is defined as the number of moles of acid or base added per liter of the solution to change the pH by one unit.