Phenolphthalein is often used as an indicator in acid–base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions. ... It is a weak acid, which can lose H+ ions in solution.

Application

Phenolphthalein, (C20H14O4), an organic compound of the phthalein family that is widely employed as an acid-base indicator. As an indicator of a solution's pH, phenolphthalein is colourless below pH 8.5 and attains a pink to deep red hue above pH 9.0.