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

Acid-Base Indicators

• 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

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• The endpoint of the reaction is where there is a balance between [HIn] and [In^–]. At this point these two concentrations are equal:

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• 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