Acid-base indicators are substances which change colour according to hydrogen ion concentration (or pH) of the solution to which they are added. They are usually weak acids or weak bases. A typical indicator is a weak organic acid that has different colour than its conjugate base. Let us consider an indicator which is a weak acid (H/n). The equilibrium between indicator and its conjugate base (In-) may be represented as:
At low pH, the concentration ofH3O+ is high so the equilibrium is pushed to the left and the equilibrium solution has colour A. At high pH values the concentration of H3O+ is low and the equilibrium is pushed to the right and the equilibrium solution has colour B. We can write equilibrium constant expressions for indicator equilibria. For a weak acid indicator at equilibrium we can write
K In = [H3O+] [In-] / H In]
K in is known as indicator dissociation constant. The colour of the indicator changes from colour A to colour Bat a particular point, known as end point of the indicator. At this point,[H In] = [In -]
K In = [H3O +]
-log K In = – log [H3O +]
Pk In = pH
The pH of the solution at the end point is equal to pK in. When pH of the solution is equal to pK in half the indicator is in the acid form and half in the form of its conjugate base. The colour of the solution at the end point is intermediate between the colours A and B. This is known as the midpoint colour. For example, the indicator methyl orange is red in acidic medium and yellow in alkaline medium. Its midpoint colour is orange.