Phenolphthalein
Description
Pale yellow powder that is widely used as a pH indicator. Phenolphthalein was first synthesized in 1871 by German chemist Adolf von Baeyer by reacting phenol and phthalic anhydride, initially intended as a dye. It is a triphenylmethane salt whose three aromatic rings produce a red chromophore in basic and neutral solutions. The conjugation is disrupted in acidic solutions resulting in a clear solution. Thus, phenolphthalein is an effective acid-base indicator for titrations of mineral acids, organic acids and alkalis. It is prepared as a 1% solution in ethanol. In solutions with a pH below 8.5 phenolphthalein is colorless; for solutions above pH 9 are bright pink-red.
Synonyms and Related Terms
3,3-bis(p-hydroxyphenyl)phthalide; 3,3-bis(4-hydroxyphenyl)-1(3H)-isobenzofuranore
Risks
- Toxic by ingestion, even small amounts will cause illness.
- Suspected carcinogen.
- Contact may cause irritation.
- ThermoFisher: SDS
Physical and Chemical Properties
- Appearance = white powder
- Soluble in ethanol, ether and alkalis.
- Insoluble in water, benzene, hexane.
- Solution: Dissolve 1 g phenolphthalein in 50 ml ethanol and add 50 ml water.
- Composition = (C6H4OH)2C2O2C6H4 (mol. wt. = 318.33 g.mol.)
- CAS = 77-09-8
- Melting Point = 258-262 C
- Density = 1.299 g/ml
Resources and Citations
- G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p. 184
- Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
- S.R.Trotman, E.R. Trotman, Textile Analysis, J.B. Lippincott Company, Philadelphia, 1932
- The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983
- Thomas B. Brill, Light Its Interaction with Art and Antiquities, Plenum Press, New York City, 1980
- CRC Handbook of Chemistry and Physics, Robert Weast (ed.), CRC Press, Boca Raton, Florida, v. 61, 1980 Comment: Solution: Dissolve 1 g phenolphthalein in 50 ml alcohol and add 50 ml water.
- Wikipedia: Phenolphthalein Accessed Feb. 2026