Is buffer capacity constant?

At what pH does a buffer solution exhibit maximum buffer capacity?

• Answer:

  Maximum buffer capacity occurs when the pH of the solution is equal to the pKa of the buffer. For example, a H2PO4-/HPO4-2 buffer has a maximum buffer capacity at a pH of 7.21 because the corresponding pKa (pKa2 of phosphoric acid) is 7.21. Another example is the acetate buffer; the pKa of acetic acid is 4.76, therefore, it has a maximum buffer capacity when the pH is 4.76