A buffer is a solution containing an equilibrium between a weak acid and its conjugate base, or between a weak base and its conjugate acid. The buffer is most effective when these two substances are present in equal concentrations. Buffers are special because they can maintain a relatively constant pH when they are diluted or when moderate amounts of acid or base are added to them.
This solution can be prepared directly, but it also occurs during a titration of a weak acid or weak base. Specifically, it occurs when the volume of titrant is half the volume that causes the equivalence point.
For example, a buffer solution containing H2CO3(aq) and HCO3−(aq) can be represented by the following equilibrium equation:
H2CO3(aq) ⇌ H+(aq) + HCO3−(aq).
Suppose we add HCl(aq) to this solution. Normally, this would decrease the pH of the solution significantly. With a buffer, though, it will not change very much because the added hydrogen ions cause the equilibrium to shift left according to LCP.
If we add NaOH(aq), the pH only increases slightly because the hydroxide ions combine with the hydrogen ions, causing the equilibrium to shift right to compensate for the decrease in hydrogen ions.