phenolphthalein

Phenolphthalein is an organic compound widely used as a pH indicator in acid–base titrations and, historically, as a laxative. Its IUPAC name is 3,3-bis(4-hydroxyphenyl)phthalide. In the laboratory, it is valued for a clear color change from colorless to pink as the solution becomes basic.

It exists in two forms depending on pH: a colorless closed lactone form in acidic solutions, and a pink to magenta open quinoid form in basic solutions. The transition occurs over a pH range of roughly 8.2 to 10.0, making it suitable for detecting the endpoint of titrations near pH 8–10. It is sparingly soluble in water and more soluble in alcohols.

Uses and safety: In titrations, phenolphthalein is common in strong acid–strong base titrations. It is less suitable for near-neutral endpoints. Historically used as a laxative, phenolphthalein is no longer approved for this use in many jurisdictions due to safety concerns; the U.S. FDA prohibited its use in over-the-counter laxatives. Handling requires standard lab precautions; it can act as an irritant and may be harmful if ingested or inhaled.

Chemistry and synthesis: It is a phthalein dye prepared by condensation of phthalic anhydride and phenol under catalytic conditions, followed by dehydration. It is a synthetic compound and not found naturally.