The buffering capacity of a buffer is maximum at pH equal to -

Buffering is the tendency of a solution to resist a change in pH more effectively after addition of a strong acid or base than does an equal volume of water. Solutions of weak acids & their conjugate bases (& vice versa) exhibit buffering. Strong acid/bases do not have buffering quality as great as the weak acid/bases. pKa refers to the pH at which the concentration of the acid equals that of the base. At pH = pKa the solution would contain acid and base in equal amounts, and therefore would resist a change most effectively in either direction. At pH < pK i.e. more acidic environment, much of the acid buffering capacity of the solution has already been neutralized so adding even minute quantities of acid would decrease the pH, i.e. it no more has buffering capacity to acids, but it would still have buffering capacity towards bases. At pH > pK, i.e. more alkaline environment, much of the alkaline buffering capacity has been neutralized after all that's the reason why the pH rose above pK in the first place so adding even minute quantities of alkali would increase the pH. "At values close to pKa, the buffer solution resists changes in pH most effectively"