A buffer is able to resist changes in pH because the solution contains a high amount of both a conjugate acid and a conjugate base in equilibrium, allowing it to neutralize small amounts of acids or bases that are added. Once the buffering capacity is reached, the pH quickly changes because the conjugate acid or base has been depleted through neutralization and the solution is no longer in equilibrium.
A buffer consists of a weak conjugate acid-base pair in which the solution has a weak acid. It also has a salt containing its conjugate base or a weak base and a salt containing its conjugate acid. The conjugate acid of a base is what is formed when the base accepts a hydrogen atom, while the conjugate base of an acid is what is formed when the acid loses a hydrogen atom.
If an acid is added to a buffer solution, it takes the form of an extra hydrogen atom. The conjugate base accepts this atom and the reaction creates water and a salt. If a base is added to the buffer solution, the buffer removes a hydrogen atom from the water molecules. The conjugate acid releases a hydrogen atom and the reaction creates water and a salt. The ratio of the amount of conjugate acid to base depends on the desired pH level of the solution.