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]

The Collaborative International Dictionary of English v.0.48:

Naphthalic \Naph*tha"lic\, a. (Chem.)
   (a) Pertaining to, derived from, or related to, naphthalene;
       -- formerly used to denote any one of a series of acids
       derived from naphthalene, and called naphthalene acids,
       now specifically referring to 1,8-naphthalenedicaboxylic
       acid, C12H8O4.
   (b) Formerly, designating an acid probably identical with
       phthalic acid.
       [1913 Webster +PJC]