phenol phthalein

 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 an indicator. It is sometimes used as a laxative in veterinary medicine.  [Webster 1913 Suppl.]