fluorescein, organic compound of molecular formula C20H12O5 that has wide use as a synthetic colouring agent. It is prepared by heating phthalic anhydride and resorcinol over a zinc catalyst, and it crystallizes as a deep red powder with a melting point in the range of 314° to 316° C (597° to 601° F). Fluorescein was named for the intense green fluorescence it imparts to alkaline solutions—a colour visible even at dilutions of 1:50,000,000. It is used as a dye to colour liquids in analytic instruments, in cosmetics, and as a water tracer or marker. Halogenated derivatives made from fluorescein also include eosin and erythrosin.