The Collaborative International Dictionary of English v.0.48:

Phthalein \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 phenolphthalein. Their alkaline solutions
   are fluorescent.
   [1913 Webster]

   Phenol phthalein, a white or yellowish white crystalline
      substance made from phthalic acid and phenol. Same as
      phenolphthalein.
      [1913 Webster + PJC]