How an Acid-Base Indicator Works

How an Acid-Base Indicator Works ?

Let us explain the indicator action by taking an example of methyl orange. Methyl orange is a weak acid and gives the following ionisation equilibrium in solution-
HIn ⇌ H⁺ + In⁻
red yellow
In accordance with the law of mass action,
K_in = [H⁺] [In⁻] / [HIn] --- Eq-1
where K_in is the dissociation constant of the indicator and is called the Indicator constant.

The anion In^– is yellow and the nonionised form HIn is red. If an acid is added to the solution, the hydrogen ion concentration, [H⁺], in the equilibrium expression (Eq-1) increases. To maintain K_in constant, the equilibrium shifts to the left. Thereby the concentration of [In^–] is reduced and the concentration of [HIn] increases so that the solution is red. On the other hand, upon addition of a base to the solution, H⁺ ions are removed as H₂O by reacting with OH^– ions of the base. This shifts the equilibrium to the right, resulting in the increase of In^– ions that are yellow. Thus in acid solution the unionised HIn molecules predominate and the solution is pink, while in basic solution In^– ions are in excess and the solution is yellow.

Source: Essentials of Physical Chemistry: B.S.Bahl