A buffer solution moderates changes in pH because it contains both a weak acid that can react with added strong base and a weak base that can react with added strong acid. This leads to several criteria for selecting a suitable buffer solution for a given purpose.
- The pKa of the weak acid in the buffer should be close to the desired pH of the buffer solution. According to the Henderson-Hasselbalch equation, if the concentrations of weak acid and weak base are equal, the pH of the buffer solution equals the pKa of the weak acid involved.
- A buffer solution should have approximately equal concentrations of the weak acid and weak base. A ratio of >10 or <0.1 makes for a poor buffer solution. Figure below shows how the pH of an acetic acid-acetate ion buffer increases as strong base is added. The initial pH is pKa = 4.74. When pH reaches 5.74, a change of 1 pH unit, the ratio . After that the pH increases more rapidly and the solution no longer provides significant buffering.
- Larger amount (mol) of weak acid and weak base give a buffer with greater capacity.
When designing a buffer system, look for weak conjugate acid-base pairs that have pKa, weak acid near the desired pH. Then adjust the ratio of the weak base to weak acid concentrations to achieve the exact pH desired. Make certain that the concentrations of weak base and weak acid are large enough to react with the quantities of acid or base that might be added to the buffer solution.