Page 28 - Pharmaceutical analytical chemistry |

It is clear from the equation that the pH of a buffer is determined by the
ratio of the conjugate acid-base pair concentration.
The mixture of a weak acid and its salt may also be obtained by mixing
an excess of a weak acid with a strong base to produce the salt by
neutralization or by mixing an excess of salt with a strong acid to produce
the weak acid component of the buffer.
b) Basic buffer: a solution that contains a weak base and its salt, e.g.
ammonium hydroxide and ammonium ion. The equilibrium between the
base B and its conjugate acid BH+ is considered as follows:

BOH ↔ B+ + OH-
This mixture acts as a buffer in the same manner as a weak acid and its
salt. When a strong acid is added, it combines with some of the base B to
form the salt BH+. Conversely, a base combines with BH+ to form B,
since the change in the ratio will be small, the change in pH will be small.
The pH of the basic buffer can be derived as follows

Buffer capacity

It is defined as the number of moles of strong acid or strong base that

causes one liter of the buffer to undergo a one unit change in pH. The

buffer capacity () is governed by the ratio of salt to acid or base. It

increases as the concentration of buffer components increases and reaches

a maximum when the ratio is unity, i.e. when the pH = pKa. The buffer

range is the pH range over which a buffer effectively neutralizes added

acids and bases and maintains a fairly constant pH. It is equal to 2 pH

units (pKa ± 1).

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