C6H4.(CO2H)2

from The Collaborative International Dictionary of English v.0.48

Phthalic \Phthal"ic\, a. [Naphthalene + -ic.] (Chem.)
   Pertaining to, or designating, a dibasic acid obtained by the
   oxidation of naphthalene and allied substances.
   [1913 Webster]

   {Phthalic acid} (Chem.), a white crystalline substance,
      {C6H4.(CO2H)2}, analogous to benzoic acid, and employed in
      the brilliant dyestuffs called the phthaleins.
      [1913 Webster]