Phenolphthalein (3,3-bis (4-hydroxyphenyl) phthalide) - triphenylmethane dye, an acid-base indicator changes color from colorless (at pH <8,2) to reddish purple, "raspberry "(alkaline); but in concentrated alkali - again colorless. In concentrated sulfuric acid, affords a pink cation.
The substance is a colorless crystals, poorly water soluble, but good - alcohol and diethyl ether.
Phenolphthalein changes color depending on the pH of the medium. It can exist in several forms, which are converted into each other by changing the pH.