what is buffering capacity

Buffer capacity

Buffer capacity for pK_a=7 as percentage of maximum

Buffer capacity is a quantitative measure of the resistance of a buffer solution to pH change on addition of hydroxide ions. It can be defined as follows.

buffer capacity =

where dn is an infinitesimal amount of added base and d(pH) is the resulting infinitesimal change in pH. With this definition the buffer capacity can be expressed as^[1]

where K_w is the self-ionization constant of water and C_A is the analytical concentration of the acid, equal to [HA]+[A^-]. The term K_w/[H⁺] becomes significant at pH greater than about 11.5 and the second term becomes significant at pH less than about 2. Both these terms are properities of water and are independent of the weak acid. Considering the third term, it follows that

1. Buffer capacity of a weak acid reaches its maximum value when pH = pK_a
2. At pH = pK_a ± 1 the buffer capacity falls to 33% of the maximum value. This is the approximate range within which buffering by a weak acid is effective. Note: at pH = pK_a - 1, The Henderson-Hasselbach equation shows that the ratio [HA]:[A^-] is 10:1.
3. Buffer capacity is directly proportional to the analytical concentration of the acid.