A buffer is a mixture of a weak acid and its salt, or a weak base and its salt, such as:

1. acetic acid (HOAc) and sodium acetate (NaOAc);

2. carbonic acid (H₂CO₃) and sodium bicarbonate (NaHCO₃);

3. potassium dihydrogen phosphate (KH₂PO₄) and dipotassium hydrogen phosphate (K₂HPO₄);

4. ammonia (NH₃) and ammonium chloride (NH₄Cl)

The secret of pH control with acid buffers is that, if a small amount of strong acid is added to the buffer solution, some of the anions from the buffer salt will combine with the added protons to form more undissociated buffer acid. The change in pH is less than if the protons from the added acid had remained uncombined. Similarly, if a small amount of extra base is added, some of the buffer acid reacts with it to form more buffer salt. The reactions are).

In the example on the opposite page, the buffer mixture is symbolized by five undissociated HA molecules and five A^- ions from NaA salt. As soon as three protons are added to the buffer, they are neutralized by three of the five A^- ions, and the pH of the solution is affected very little:

Conversely, if three OH^- ions are added to this buffer mixture, as shown at the far right, they steal protons from three of the five undissociated HA molecules and are neutralized to form three more water molecules:

Without the buffer, the added H⁺ and OH^- ions would have caused a large alteration in pH. Of course, if six H⁺ ions had been added instead of three, then pH control would have broken down. This is called exceeding the buffer capacity of the solution.

Buffers are designed to damp down minor perturbations in acidicity, and the amount of buffer acid and salt must be greater than the amount of perturbing acid or base encountered.