By Rosann Kozlowski, Updated Aug 30, 2022
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Phenolphthalein (C20H14O4) is a weakly acidic, white to yellow crystalline solid. It dissolves readily in alcohols and is slightly soluble in water. This organic molecule is the cornerstone of many acid–base assays in chemistry laboratories.
In solution, phenolphthalein exists in equilibrium between a colorless protonated form and a pink deprotonated ion. When a solution is acidic, hydrogen ions shift the equilibrium toward the colorless form, rendering the indicator transparent. In basic media, hydroxide ions promote the formation of the pink ion, giving the solution its characteristic hue. This shift is a direct consequence of electron delocalization that occurs upon ionization.
The protonated (acidic) and deprotonated (alkaline) forms differ structurally. The deprotonated ion features an extended π‑electron system that absorbs visible light around 553 nm (green region), which the human eye perceives as pink. The more alkaline the solution, the greater the degree of ionization and the deeper the pink coloration.
The pH scale ranges from 0 to 14, with 7 as neutral. Phenolphthalein remains colorless below pH 8.2. It begins to turn faint pink at pH 8.2, intensifying to a bright magenta by pH 10 and above, making it an ideal indicator for weakly basic solutions.
Discovered in 1871 by German chemist Adolf von Baeyer, phenolphthalein is produced by fusing phenol with phthalic anhydride in the presence of sulfuric acid or zinc chloride—processes that are still standard today. In the laboratory, it is predominantly used in acid–base titrations: a known concentration of base is titrated against an unknown acidic solution, and the moment the color changes from colorless to pink signals the equivalence point. Historically, phenolphthalein served as a laxative in over‑the‑counter preparations such as Ex‑Lax, but it was banned in the United States in 1999 after studies linked it to cancer risk.
