Thymolphthalein is used in disappearing ink, to make color-change bubbles, and as a pH indicator. It has an inky blue color and readily changes to its colorless form from exposure to air or carbon dioxide from breath.

Thymolphthalein is a phthalein dye used as an acid–base (pH) indicator. Its transition range is around pH 9.3–10.5. Below this pH, it is colorless; above, it is blue.

Thymolphthalein is also known to have use as a laxative and for disappearing ink.

Dissolve 0.04 g thymolphthalein in 50 mL of 95% ethanol. Dilute this solution to 100 mL with distilled water.

Thymolphthalein is used in disappearing ink, to make color-change bubbles, and as a pH indicator. It has an inky blue color and readily changes to its colorless form from exposure to air or carbon dioxide from breath.

THYMOLPHTHALEIN INDICATOR SOLUTION