C6H4CO2H2

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]