Buffer solutions

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 H₂CO_3(aq) and HCO₃⁻_(aq) can be represented by the following equilibrium equation:

H₂CO_3(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.