Phenolphthalein, an organic chemical compound. The light-yellow, crystalline powder is insoluble in water, but dissolves in alcohol and ether. It turns red in a mild alkali solution, but remains colorless in a neutral or acid solution. Phenolphthalein is used in dyes, laboratory reagents, and laxatives.

Chemical formula: C20H1404. Specific gravity: 1.2765. Melting point: 501.8 F. (261 C.).