The buffering capacity of a buffer is maximum when the pH is equal to:
Buffering capacity of a buffer is most effective when the concentrations of salt and acid are equal or when pH = pKa. The effective range of a buffer is 1 pH unit higher or lower than pKa. pKa refers to the pH at which the concentration of the acid equals that of the base. Phosphate buffer is effective at a wide range because it has three pKa values. Ref: Vasudevan, Biochemistry, 3rd Edition, Page 320
0.5 pKa
pKa
pKa+1
2 pKa