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|below pH 9.3||above pH 10.5|
|below pH <0||above pH 9.3|
Thymolphthalein is a pH indicator. It has two transition ranges:
- Below pH 0, it is red. At pH 0 and above, it is colorless.
- The second transition is around pH 9.3–10.5. Below this level, Thymolphthalein is colorless; above, it is blue. The molar extinction coefficient for the blue Thymolphthalein dianion is 38,000 M−1 cm−1 at 595 nanometers.