It can be used to make a disappearing ink. Dissolve the thymolphthalein in alcohol solution and add sodium hydroxide (NaOH) dropwise until it turns blue. It can then be sprayed onto cloth. After several minutes, it will turn colorless. (Carbon dioxide from the air will react with the basic solution. When the pH drops below 9.3, the thymolphthalein turns colorless.) Such inscription can be turned visible again by submerging it in liquid of pH higher than 10.5. A similar pH indicator is phenolphthalein, which is colorless in acidic and neutral pH ranges and bright fuchsia at high (basic) pH.
CAS Registry Number 125-20-2