Eosins

a group of triarylmethane dyes; halogen derivatives of fluorescein [(I), X = H], having the structural formula

The best known eosins are eosin itself (tetrabromofluorescein, in which X = Br) and erythrosine (tetraiodofluorescein, in which X = I). Eosins dissolve readily in water and less readily in alcohol to form fluorescent solutions.

Eosins are acidic dyes, used to give wool and silk a bright pink or red color—hence the term “eosin,” which derives from the Greek eos (dawn). The color is not stable, however. Eosins are used mainly in the manufacture of ink, colored pencils, lipstick, and rouge; they are also used to dye paper, food products, and biological preparations.