Fluorescein

Fluorescein is an organic dye also known as D & C Yellow No. 7 and many other names. A. von Baeyer first synthesized it in 1871 from phthalic anhydride and resorcinol and called it resorcinphthalein. It is not soluble in water or many organic solvents, but its disodium salt (fluorescein sodium, uranine, D & C Yellow No. 8) is highly water-soluble.

Crystalline or powdered fluorescein is reddish orange, but in alkaline solution it has an intense yellow-green fluorescence. Because of its fluorescence, it was used to dye the Chicago River for St. Patrick’s Day, beginning in 1962. A few years later, it was replaced by a more environmentally friendly vegetable dye.