Acid-Base Indicator
A weak acid (HIn) whose conjugate base (In⁻) has a different colour. The colour change occurs over the pH range: pKa ± 1.
How Indicators Work
\[\text{HIn (aq)} \rightleftharpoons \text{H}^+ \text{(aq)} + \text{In}^- \text{(aq)}\]
Colour A Colour B
Common IB Indicators
| Indicator | Acid Colour | Base Colour | pH Range | Best For |
|---|---|---|---|---|
| Methyl orange | Red | Yellow | 3.1 - 4.4 | SA + WB |
| Phenolphthalein | Colourless | Pink | 8.3 - 10.0 | WA + SB |
| Bromothymol blue | Yellow | Blue | 6.0 - 7.6 | SA + SB |
💡 Choosing the Right Indicator
The indicator's pKa must fall within the sharp vertical section of the pH curve (the equivalence point jump). This means the indicator range must overlap the equivalence point pH.
⚠️ Common Mistake
No indicator is suitable for WA + WB titrations because the pH change at equivalence is too gradual for any indicator to give a sharp endpoint.
📋 Exam Tip
For SA + SB titrations, any indicator will work because the sharp pH jump spans pH ~3 to ~11. For WA + SB, use phenolphthalein (range in the basic equivalence point). For SA + WB, use methyl orange (range in the acidic equivalence point).