Acid-Base Indicators (HL) (HL IB Chemistry)

• An acid-base indicator is a weak acid which dissociates to give an anion of a different colour
• Consider a weak acid, HIn:

HIn (aq)   ⇌ H⁺ (aq) + In^– (aq)

colour 1        ⇌         colour 2

• HIn and its conjugate base In^– are different colours
• The colour of the solution depends on the relative concentrations of the two species
  • If the solution is acidic, the above equilibrium will be shifted to the left and more HIn will be present
    • Colour 1 will dominate
  • If the solution is alkaline, the above equilibrium will shift to the right and more In^– will be present
    • Colour 2 will dominate
• The colour does not change suddenly at a certain pH, but changes gradually over a pH range
• The colour of the indicator depends on the ratio of [HIn] to [In^–]
• The colour of the indicator depends on the pH of the solution
• The pH at which these transitions will occur depends on the K_a  of the indicator
  • 
• The endpoint of the reaction is where there is a balance between [HIn] and [In^–]. At this point these two concentrations are equal:
  • 
• Taking negative logs of both sides
  • pK_a  = pH
• This means the pK_a of an indicator is the same as the pH of its endpoint
• The colour change for most indicators takes place over a range of pH = pK_a ± 1

Weak bases as indicators

• An indicator can also be a weak base:

             BOH (aq) ⇌ B⁺ (aq) + OH^- (aq) 

colour 1    ⇌     colour 2

• For such indicators:
  • Colour 1 is observed in alkaline conditions
  • Colour 2 is observed in acidic conditions