How an Acid-Base Indicator Works

How an Acid-Base Indicator Works?

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,
Kin = [H+] [In] / [HIn] --- Eq-1
where Kin 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 Kin 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