Phthalein

noun

1. Any of a group of chemical compounds formed by a reaction of phthalic anhydride with a phenol, from which certain synthetic dyes are derived.
2. One of the group of coloring-matters formed by the condensation of phthalic anhydrid with phenols. Some of the phthaleins have a purgative action when taken internally. They are of particular importance in the color-chemistry, and certain of their derivatives are valuable dyestuffs. See phenolphthalein and phthalic anhydrid.
3. One of a series of artificial organic dyes made as condensation products of the phenols with phthalic acid, and well represented by phenol phthaleïn. Their alkaline solutions are fluorescent.
4. a white or yellowish white crystalline substance made from phthalic acid and phenol. Its solution in alkalies is brilliant red, but is decolorized by acids, and as this reaction is exceedingly delicate it is used as an indicator.
5. any of a range of lactones formed by the reaction of phenols with phthalic anhydride