Phenolphthalein (/fɛˈnɒl(f)θəliːn/[citation needed] 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.

Wittke reported in 1983 that it exists in protonated form (H3In+) under strongly acidic conditions, providing an orange coloration.

In strongly basic solutions, phenolphthalein is converted to its In(OH)3− form, and its pink color undergoes a rather slow fading reaction[6] and becomes completely colorless when pH is greater than 13.

[8] In a highly basic solution, phenolphthalein's slow change from pink to colorless as it is converted to its Ph(OH)3− form is used in chemistry classes for the study of reaction kinetics.

This reaction leads to the pH falling below the color change threshold as hydrogen ions are released by the reaction: To develop the hair and "magic" graphical patterns, the ink is sprayed with a solution of hydroxide, which leads to the appearance of the hidden graphics by the same mechanism described above for color change in alkaline solution.

If the sample contains hemoglobin, it will turn pink immediately upon addition of the peroxide, because of the generation of phenolphthalein.

Despite concerns regarding its carcinogenicity based on rodent studies, the use of phenolphthalein as a laxative is unlikely to cause ovarian cancer.

This is effected by its inhibiting thrombin and thapsigargin, two activators of SOCE that increase intracellular free calcium.

[17] Phenolphthalein has been added to the European Chemicals Agency's candidate list for substance of very high concern (SVHC).

[19] The discovery of phenolphthalein's laxative effect was due to an attempt by the Hungarian government to label[clarification needed] genuine local white wine with the substance in 1900.

Max Kiss, a Hungarian-born pharmacist residing in New York, heard about the news and launched Ex-Lax in 1906.