Buffers

A buffer is a system that is able to resist changes to pH when small amounts of a strong acid or strong base are added.

Buffers are formed by a combination of a weak acid and its conjugate weak base or a weak base and its conjugate weak acid. These combinations establish equilibrium systems with similar amounts of both reactants and products present...

An example of a buffer would be the dynamic equilibrium between ethanoic acid (a weak acid) and the ethanoate ion (its weak conjugate base)...

As a buffer, the above system is able to resist small changes to pH upon the addition of a strong acid or strong base. This is because the system is in dynamic equilibrium.

But why are dynamic equilibria able to resist small changes to pH?

When a strong acid/base is added to a buffer, there will be an initial increase in the concentration of hydronium/hydroxide ions. As they are in dynamic equilibrium, buffers will partially oppose the initial increase in the concentration of hydronium/hydroxide ions by favouring either the forward or reverse reaction. This will return the hydronium/hydroxide ion concentration towards its original concentration, therefore causing pH to remain largely unchanged.

Thus, the addition of the strong acid has been resisted.

Buffer Capacity

A buffer will be able to resist a pH change up until the point that either its weak acid or weak base is completely consumed. This point is known as the buffer capacity.

By definition, buffer capacity is a measure of the number of moles of strong acid or strong base that can be added to a buffer system without causing a significant change in pH.

Maximising the buffer capacity is all about increasing the amount of the weak acid and weak base. Therefore, to achieve a high buffer capacity, we can...

1. Add more of the weak acid and weak base so that more acid or base needs to be consumed before the buffer can no longer operate.

2. Ensure the concentrations of the weak acid and weak base are equal so they can equally accomodate the addition of a strong acid or strong base.