Acid–Base Indicators HL Only
Quick Notes
- Indicators are weak acids that partially dissociate in water.
- General formula: HInd ⇌ H+ + Ind−
- Colour changes depend on the relative amounts of HInd and Ind−.
- At pH = pKa:
- [HInd] ≈ [Ind−] and the colour is a mix of both forms.
- The endpoint of a titration is close to the pKa of the indicator.
- Universal indicator is a mixture of indicators, giving a wide pH colour range.
Full Notes
Indicators as Weak Acids
Acid–base indicators function through an equilibrium system:
HInd (aq) ⇌ H+ (aq) + Ind− (aq)
- HInd = weak acid form (one colour)
- Ind− = conjugate base form (different colour)
As pH changes, the position of this equilibrium shifts, causing the colour to change:
- In acidic solutions:
high [H+] shifts equilibrium left, giving more HInd (acid colour). - In basic solutions:
low [H+] shifts equilibrium right, giving more Ind− (base colour).
pKa and Endpoint
The pKa of the indicator tells you the pH at which the colour change occurs.
When pH = pKa, the concentration of weak acid and conjugate base are equal.
Meaning [HInd] = [Ind−], giving an equal mix of both colours.
Indicators are chosen so their pKa matches the equivalence point of the titration as closely as possible.
Choosing an Appropriate Indicator
The indicator must change colour close to the equivalence point of the titration.
Examples
- Phenolphthalein: pH range ~8.3–10.0 (colourless to pink)

- Methyl orange: pH range ~3.1–4.4 (red to yellow)

Universal Indicator
Universal indicator is a mixture of many indicators with different pKa values.
As each component changes colour at a different pH, a continuous colour gradient is produced from pH 1 to 14.

Universal indicator is useful for estimating pH, but not suitable for precise titration endpoints.
Summary
- Indicators are weak acids that change colour depending on the ratio of HInd to Ind−.
- At pH = pKa, both forms are equal, giving a mixed colour.
- Indicators must be chosen to match the equivalence point of a titration.
- Universal indicator is a mixture giving a full pH colour range but is not precise enough for titrations.
Linked Course Question
What are some of the similarities and differences between indicators used in acid–base titrations and in redox titrations?
Both give a colour change at the endpoint. Acid–base indicators rely on pH changes caused by changes in H+ ion concentration. Redox indicators rely on electron transfer and colour changes due to changes in oxidation state.