In strongly alkaline solutions, phenolphthalein's pink color undergoes a rather slow fading reaction and becomes colorless again. In other words the molecule has three forms:
H2P <-> P2- <-> POH3- acidic alkaline strong alkaline no color pink no colorThe fading reaction is sometimes used in undergraduate classes for the study of reaction kinetics.
Phenolphthalein is insoluble in water, and is usually dissolved in alcohols for use in experiments. It is itself a weak acid, which can lose an H+ ions in solution. The phenolphthalein atom is colourless. However, the ion is a pink/purple colour. When an alkaline is added to the phenolphthalein, the atom<->ions equilibrium shifts to the ionisation because H+ ions are removed, (by Le Chatelier's principle).
Phenolphthalein has been used for over a century as a laxative, but is now being removed from the market because of concerns over carcinogenity. However, the small amounts usually used in experiments are harmless.
Spelling tip: Phenolphthalein has two pHs in it!