Multiple choiceQuestion 1 · Recall
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By the end you can explain how indicators work, state the pH ranges of common indicators, and select the appropriate indicator for a given titration based on the vertical jump region of its titration curve.
An indicator is a weak acid (or weak base) where the acid form (HIn) and conjugate base form (In⁻) have different colours.
Colour A Colour B
Indicator transition range
The pH range over which an indicator changes colour, typically pKa ± 1. The end point is the pH at which the colour change is observed (usually the midpoint of the range).
| Indicator | pH Range | Acid Colour | Alkaline Colour | pKa (approx) |
|---|---|---|---|---|
| Methyl orange | 3.1 – 4.4 | Red | Yellow | 3.7 |
| Bromothymol blue | 6.0 – 7.6 | Yellow | Blue | 7.0 |
| Phenolphthalein | 8.2 – 10.0 | Colourless | Pink/Fuchsia | 9.3 |
| Thymolphthalein | 9.3 – 10.5 | Colourless | Blue | 9.9 |
| Litmus | 5.0 – 8.0 | Red | Blue | 6.5 |
Methyl orange: good for strong acid + weak base (equivalence pH ~4–6). Phenolphthalein: good for weak acid + strong base (equivalence pH ~8–10). Bromothymol blue: good for strong acid + strong base (equivalence pH = 7).