Thymolphthalein is an acid-base indicator. Its chemical formula is C28H30O4. Thymolphthalein has two color stages in an aqueous solution.

There are 11 test tubes containing the pH indicator thymolphthalein and a solution. The solutions are between pH 3 and 13. The solutions between pH 3 and 9 are colorless. The solutions between pH 10 and 13 change from light blue at pH 10 to dark blue at pH 13.

Figure: The color of thymolphthalein in solution at various pH levels. At pH 0 - 9.3 the solution is colorless. At a pH of 9.3 - 10.5 it changes from light blue to blue.