Phenolphthalein, fee nohl THAL een, is a chemical compound used as an indicator of alkalinity or acidity, and as a laxative. It is also used in making dyes. It is prepared by heating phenol and phthalic anhydride with sulfuric acid. Pure phenolphthalein forms small crystals that dissolve in alcohol or ether. It turns red in the presence of an alkaline substance. As a laxative, phenolphthalein is the active ingredient in many advertised medicines. It is one of the least poisonous of common laxatives, but some people are allergic to it. Thus, it must be used with care.