Science, Tech, Math › Science Phenolphthalein Definition Share Flipboard Email Print This is the chemical structure of phenolphthalein. Ben Mills/PD Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry in Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate by Anne Marie Helmenstine, Ph.D. Updated March 08, 2017 Phenolphthalein Definition: Phenolphthalein is an organic compound often used as an pH indicator.Phenolphthalein turns pink in pH greater than 8.3 and colorless in acidic solutions.The chemical formula for phenolphthalein is C20H14O4.Make Phenolphthalein Indicator Continue Reading