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Pale yellow powder that is used as a pH indicator. Phenolphthalein 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 and solutions above pH 9 are red.

Chemical structure


Synonyms and Related Terms

3,3-bis(p-hydroxyphenyl)phthalide; 3,3-bis(4-hydroxyphenyl)-1(3H)-isobenzofuranore


  • Toxic by ingestion, even small amounts will cause illness.
  • Suspected carcinogen.
  • Contact may cause irritation.
  • ThermoFisher: SDS

Physical and Chemical Properties

Soluble in ethanol, ether and alkalis. Insoluble in water.

Solution: Dissolve 1 g phenolphthalein in 50 ml ethanol and add 50 ml water.

Composition (C6H4OH)2C2O2C6H4
CAS 77-09-8
Melting Point 258-262 C
Density 1.299 g/ml
Molecular Weight mol. wt. = 318.33

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.