Thymolphthalein is a phthalein dye used as an acidâÂÂbase (pH) indicator. Its transition range is around pH 9.3âÂÂ10.5. Below this pH, it is colorless; above, it is blue. The molar extinction coefficient for the blue thymolphthalein dianion is 38,000 M<sup>âÂÂ1</sup> cm<sup>âÂÂ1</sup> at 595 nm.
Thymolphthalein is also known to have use as a laxative and for disappearing ink.
Thymolphthalein can be synthesized from thymol and phthalic anhydride.