A buffer solution is one that resists changes in pH upon dilution or the addition of acids and bases. The pH of a buffer solution does change when diluted or when acids and bases are added but the change is much less than that compared to an unbuffered solution.
A buffer solution is usually composed of a weak conjugate acid-base pair, e.g. the ethanoic acid/ethanoate pair (acidic buffer) or the ammonium chloride/ammonia pair (basic buffer). It is most effective (or has a maximum buffer capacity) when it contains equal equilibrium concentrations of the weak acid and its conjugate base.
For an acidic buffer, we have the following equilibrium:
If a small amount of acid H3O+ is added to the solution, it reacts with the conjugate base A– to form the weak conjugate acid HA, thereby removing the added acid and resisting the change in pH. Similarly, when a small amount of base OH– is added, it combines with H3O+ to form water; again, resisting the change in pH. The degree to which a buffer solution resists the change in pH is dependent on a few factors, which shall be discussed in the next article.