What is buffer capacity?

A buffer solution resists change in pH upon addition of any compound that tend to alter hydrogen ion concentration. Buffer capacity is defined as the magnitude of of the resistance of a buffer to pH changes. Other names for it include buffer index, buffer value, and buffer efficiency. To have a better understanding, lets assume that we have a 1 liter acetate buffer solution containing 0.2 moles acetic acid and 0.2 moles sodium acetate. According to the buffer equation, the pH of this solution is equal to pKa value of acetic acid which is 4.76.

Furthermore, lets assume that 0.02 moles of KOH are introduced into this solution without significantly changing the volume. Accordingly, 0.02 moles of acetic acid will change into acetate. Thus, acetic acid concentration becomes equal to 0.2-0.02 = 0.18 M and acetate concentration becomes equal to 0.02+0.02 = 0.22. The final pH is calculated according to the following:

Thus, for an addition of 0.02 moles of a strong base the pH changed by 4.85-4.76 = 0.09 units. If another portion (0.02 moles) of the base is added this will lead to a solution pH = 4.94 and a pH change = 0.09 units (try the calculation). A third portion (0.02 moles) of the base will lead to a solution pH = 5.03 and a pH change = 0.09 units. The buffer capacity,β, can now be approximately calculated  using the following equation:

Where, ΔB is the molar amount of the base added to the buffer to introduce a pH change of ΔpH. Accordingly, the buffer capacity of the above mentioned acetate buffer is 0.22. 

Note: buffer capacity for a given acid-conjugate base (e.g. acetic acid-acetate) buffer decreases as the pH moves further away from pKa value. Try it..

Related post: How to calculate exact buffer capacity?