Buffer capacity is a quantitative measure of the ability of a buffer to resist changes in pH. It is defined as the moles of an acid or base that can be added to a buffer solution without causing a significant change in its pH.

Buffer capacity depends on the concentration of the buffer components (the weak acid and its conjugate base), as well as the pH of the solution. Buffer solutions have the greatest buffer capacity when the pH of the solution equal to the pKa value of the weak acid (pH = pKa). Outside of this range, the buffer may become less effective at resisting changes in pH, leading to greater fluctuations in the pH of the solution.