Thymolphthalein - acid-base indicator. At pH 9,3-10,5 it is colorless, and above this value becomes blue.
Thymolphthalein is practically insoluble in water, soluble vetanole, acetone and chloroform.
In analytical chemistry used in the acid-base titration and determination of pH value during titration. Thymolphthalein may also be used as an invisible ink.