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.
Phenolphthalein dissociate in basic medium
(colourless)HPh(aq)⇌H++(Pink)Ph−