Phthal"e*in (?), n. [See Phthalic.] (Chem.) 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.

Phenol phthalein, 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.