A buffer is a solution that resists changes in pH upon the addition of acid or base. Buffers typically contain a species in solution that reacts with added acid and another that reacts with added base. Although acids or bases themselves could accomplish this task in theory, in practice if they are in solution together they interact and neutralize each other. Buffers are most often prepared one of two ways from conjugate acid-base pairs: from a weak acid and its conjugate base, or from a weak base and its conjugate acid.

If 10 mL of 0.10 M HCl is added to a liter of water at pH 7, the pH drops four units to pH 3. If 10 mL of 0.10 M NaOH is added to a liter of water at pH 7, the pH rises about four units to pH 11. But if the 10 mL of 0.10 M strong acid or strong base is added to...