Thymolphthalein
Summary: Thymolphthalein is an indicator used in acid-base titrations. It is colourless in acidic and neutral solutions and turns blue in alkaline solutions. Its colour change occurs at pH 9.3–10.5. Tags: igcse chemistry definition acids-bases indicators Created: 2026-07-14 Last Updated: 2026-07-23
Thymolphthalein is a synthetic indicator used in acid-base chemistry. It is specifically mentioned in the IGCSE Chemistry 0620 syllabus as one of the indicators affected by acids and alkalis.
Colour Changes
| Solution Type | Colour of Thymolphthalein |
|---|---|
| Acidic (pH < 9.3) | Colourless |
| Neutral (pH = 7) | Colourless |
| Alkaline (pH > 10.5) | Blue |
The colour change occurs over the range pH 9.3–10.5.
Comparison with Other Indicators
| Indicator | Acid Colour | Neutral Colour | Alkali Colour | pH Range |
|---|---|---|---|---|
| Thymolphthalein | Colourless | Colourless | Blue | 9.3–10.5 |
| Phenolphthalein | Colourless | Colourless | Pink | 8.3–10.0 |
| Litmus | Red | Purple | Blue | 5–8 |
| Methyl orange | Red | Orange | Yellow | 3.1–4.4 |
Why Use Thymolphthalein?
Thymolphthalein is sometimes preferred over phenolphthalein for titrations involving weak acids and strong bases because its colour change occurs at a slightly higher pH, closer to the equivalence point of such titrations.
Key Facts
- Thymolphthalein is colourless in acid and blue in alkali
- Colour change range: pH 9.3–10.5 (similar to phenolphthalein but slightly higher)
- Named in the IGCSE 0620 syllabus as an indicator affected by acids and alkalis
- Used in titrations where the end point is in the alkaline range
See Also
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.