Acids and Bases

Acid-Base Indicators

Acid-Base Indicators

Theory of indicators

An acid-base indicator is a weak acid. It works by forming an anion of a different colour when it dissociates.

Take the weak acid HIn:

HIn(aq) + H2O(l) ? H3O+(aq) + In(aq)

HIn and In(its conjugate base) are different colours. The relative concentrations of these species dictates the solution’s colour:

  • In a strongly acid solution: the system will shift to the left meaning HIn (or Red) dominates.
  • In a strongly alkaline solution: the system will shift to the right meaning In (or Blue) dominates.

The colour change is not sudden at a particular pH but instead gradual over a pH range. The colour change is dependent on the ratio of [HIn] to [In]. In general:

  • when [HIn] / [In] > 10, Red dominates
  • when [HIn] / [In] < 10, Blue dominates

The pH at which these changes occur is dependent on the indicator’s KIn:

KIn = ([H3O+][In]) / [HIn] therefore [H3O+] = KIn[HIn] / [In]

Therefore:

  • when [H3O+] > 10 x KIn then [HIn] / [In] > 10 so Red dominates
  • when [H3O+] < 10 x KIn then [HIn] / [In] < 0.1 so Blue dominates
  • when there is an intermediate concentration of when H3O+ then neither colour dominates

A lot of the time the values of KIn are expressed in terms of pKIn in which pKIn = -log10KIn. Therefore, in terms of pH:

pH = pKIn -log10 [HIn] / [In]

So:

  • when [HIn] / [In] < 0.1 the pH is less then pK -1 and Red dominates
  • when [HIn] / [In] > 10 the pH is more than pK +1 and Blue dominates

Therefore: the colour of indicator changes over the pH range pKIn