Question

Derive Henderson equation for the determination of pH of an acid buffer.

Answer 1

An acid buffer consists of a mixture of a weak acid and salt with strong acid. The ionization of weak acid, HA is given by

HA ⇌ H⁺ + A^–

The dissociation constant of the weak acid is given by,

It can be assumed that the concentration of [A^–] ions from the complete ionization of the salt compared to the ionization of the weak acid. Hence, [A^–] concentration may be considered as the concentration of the salt.

So,

Taking logarithms on both sides and reversing the sign,

This is known as Henderson’s equation.