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.