from
The Collaborative International Dictionary of English v.0.48
Phenolphthalein \Phe`nol*phthal"e*in\, or Phenol phthalein
\Phenol phthalein\, n. (Chem.)
A white or yellowish white crystalline substance, {C20H14O4},
formed by condensation of the anhydride of phthalic acid and
phenol. Its solution in alkalies is brilliant red, but is
decolorized by acids. This reaction, being very delicate, is
used as a pH indicator. It is sometimes used as a laxative in
veterinary medicine.
[Webster 1913 Suppl. +PJC]