Page 27 - Pharmaceutical analytical chemistry |

An example of this salt is ammonium chloride which is a strong
electrolyte and is completely ionized. Its hydrolytic reaction is:

NH4+ + H2O ↔ NH4OH + H+
The pH of these salts can be calculated in the same way as before so that

pH = ½ (pKa + pCs)

 Buffer Solutions

A buffer is defined as a solution that resists the change in pH when a
small amount of an acid or a base is added or when the solution is diluted.
A buffer solution consists of a mixture of a weak acid and its conjugate
base (its salt) or a weak base and its conjugate acid (its salt). Therefore,
we have two types of buffer.
a) Acidic buffer: a solution that contains a weak acid and its salt, e.g.
acetic acid and acetate ion. The acid equilibrium that governs the system
is

HA ↔ H+ + A-
The buffering mechanism for this mixture can be explained as follows: if
a small amount of strong acid is added, it will combine with an equal
amount of the A- to form HA. The change in the ratio [A-] / [HA] is small
and hence the change in pH is small. On the other hand, if a small amount
of a strong base is added, it will combine with part of the HA to form an
equivalent amount of A-.
Henderson – Hasselbalch equation
It is used to calculate the pH of buffer solutions.

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