A colourless crystalline dicarboxylic acid, C6H4(COOH)2; r.d. 1.6; m.p. 207°C. The two –COOH groups are substituted on adjacent carbon atoms of the ring, the technical name being benzene-1,2-dicarboxylic acid. The acid is made from phthalic anhydride (benzene-1,2-dicarboxylic anhydride, C8H4O3), which is made by the catalytic oxidation of naphthalene. The anhydride is used in making plasticizers and polyester resins.