colourless, crystalline organic compound ordinarily produced and sold in the form of its anhydride. The annual production of phthalic anhydride exceeded 1,000,000 metric tons in the late 20th century; most of it was used as an ingredient of polyesters, including alkyd resins (vehicles for paints and enamels), and simple esters used as plasticizers for polyvinyl chloride and other polymers. Smaller quantities were consumed in the manufacture of anthraquinone (a dye intermediate), phenolphthalein (a laxative and acid-base indicator), and phthalocyanine pigments.