CoachA colour indicator is a weak acid–base pair HIn/In whose two forms have different colours. Its colour changes over the turning zone, the pH range where neither form clearly dominates, about .
| indicator | turning zone | colours (acid / base) | |
|---|---|---|---|
| methyl orange | 3.5 | 2.5–4.5 | red / yellow |
| methyl red | 4.8 | 3.8–5.8 | red / yellow |
| phenol red | 7.9 | 6.9–8.9 | yellow / red |
| thymol blue (second change) | 9.2 | 8.2–10.2 | yellow / blue |