phenolphthalein

[sastergri]

a laxative used in many preparations under various trade names; also used as an acid-base indicator in titrations involving weak acids and strong bases because it is brilliant red at high alkalinity and colorless below pH 8

[TimothynTep]

A quinone derivative used as a pH indicator, and once used as a laxative

[LindaArrit]

Phenolphthalein ( feh-NOL(F)-thə-leen) is a chemical compound with the formula C20H14O4 and is often written as HIn, HPh, phph or simply Ph in shorthand notation. Phenolphthalein is often used as an indicator in acid–base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions. It belongs to the class of dyes known as phthalein dyes.
Phenolphthalein is slightly soluble in water and usually is dissolved in alcohols for use in experiments. It is a weak acid, which can lose H ions in solution. The nonionized phenolphthalein molecule is colorless and the double deprotonated phenolphthalein ion is fuchsia. Further loss of proton in higher pH occurs slowly and leads to a colorless form. Phenolphthalein ion in concentrated sulfuric acid is orange red due to sulfonation.

[PussyCat]

An acid-base indicator which is colorless in acid solution, but turns pink to red as the solution becomes alkaline. It is used medicinally as a cathartic.