In aqueous formulations there is often a requirement for a fixed pH that will remain stable and resist the effect of small amounts of acid or alkali getting into the preparation or being formed as the ingredients age. In other words, they need some buffer capacity.

Many salts, usually in combination with the weak acids or weak alkalis of that particular salt, will provide buffer capacity. For example, a dilute solution of sodium acetate and acetic acid behaves as an acid buffer (pH 4.5) due to the following reaction.

This dissociation of acetic acid occurs only to a small extent because of the small dissociation constant. Thus, most of the acid remains as molecules. However, if some of the hydrogen ions were removed by reaction with alkali this disturbs the equilibrium which then re-establishes itself by more of the acid molecules dissociating. As such it has the capacity to resist pH change due to small additions of alkali.

If, instead of alkali being added, there is a small amount of acid put in this increases the concentration of hydrogen ions but equilibrium is re-established by the dissociation reaction going into reverse and consuming those extra hydrogen ions to form more acetic acid molecules.

Again, the acetic acid dissociation reaction has compensated to keep the hydrogen ion concentration and hence the pH constant. The dissociation swings either way to add or remove hydrogen ions and buffer the effect of small disturbances. To ensure that there is always a sufficiency of...