Phenolphthalein is a chemical compound with the formula C20H14O4 and is often used as an indicator in acid-base titrations. It belongs to the class of dyes known as phthalein dyes and is slightly soluble in water, usually dissolved in alcohols in experiments. Phenolphthalein is a weak acid that can lose H+ ions in solution, and the non-ionized molecule is colorless while the double deprotonated phenolphthalein ion is fuchsia. It turns colorless in acidic solutions and pink in basic solutions, making it useful as an indicator in acid-base titrations. Phenolphthalein is also used as a component of universal indicator, together with methyl red, bromothymol blue, and thymol blue.

Phenolphthalein has other uses, such as in toys, for example as a component of disappearing inks, or disappearing dye on the "Hollywood Hair" Barbie hair. In the ink, it is mixed with sodium hydroxide, which reacts with carbon dioxide in the air, leading to the pH falling below the color change threshold as hydrogen ions are released by the reaction. Phenolphthalein is also used as a laxative, although its use has been largely discontinued due to potential carcinogenicity.

In an acid-base titration, phenolphthalein solution is used as the indicator to signal that the end point has been reached. Phenolphthalein is clear in acidic solutions and turns pink in basic solutions, making it useful for detecting the endpoint of the titration.

In summary, phenolphthalein is a weak acid that is widely used as an indicator in acid-base titrations, turning colorless in acidic solutions and pink in basic solutions. It is also used in toys, as a laxative, and as a component of universal indicator.