It is observed that a buffer solution resists the change in pH to a certain limit only. The addition of excess of strong acid or strong base to a buffer solution leads to a considerable change in pH. The capacity of a buffer solution in maintaining the constant pH upon the addition of acid or base is known as buffer capacity. The buffer capacity of a buffer solution is defined as the ratio of the number of moles of an acid or a base added to the buffer solution and change in pH, i.e. ,
Buffer capacity = No of moles of acid or base added / ΔpH
A buffer solution has maximum buffer capacity when the ratio [Salt] / Acid or [Salt] / base = 1
In such a solution pH = pKa for acidic buffers and pOH = pKa for basic buffers. A solution can act as buffer only if the ratio of cone of salt to acid or base is between 0.1 and 10. Thus, pH of an acidic buffer can have range from pKa – 1 to pK4 + 1. Similarly, a basic buffer can act as buffer in the pOH range from pKa – 1 to pKb + 1.
For example, CH3COOHICH3COONa buffer has pH range 3.76 to 5.76 because pKa of CH3COOH is 4.76.