Eosin Y also called Acid Red 87 isdye is an organic sodium salt that is 2',4',5',7'-tetrabromofluorescein in which the carboxy group and the phenolic hydroxy group have been deprotonated and the resulting charge is neutralised by two sodium ions. eosin most commonly used in histology, most notably in the H&E (Haematoxylin and Eosin) stain.