Eosin is a fluorescent acidic / negative compound that binds to and forms salts with basic, or eosinophilic, compounds containing positive charges (such as proteins that are basic / positive due to the presence of amino acid residues such as arginine and lysine) and stains them dark red or pink as a result of the actions of bromine on fluorescein.
E 1245 (OTTO) Eosine yellow, spirit soluble Cas 17372-87-1 - used as a counterstain to hematoxylin in H&E (haematoxylin and eosin) staining. H&E staining is one of the most commonly used techniques in histology. Tissue stained with haematoxylin and eosin shows cytoplasm stained pink-orange and nuclei stained darkly, either blue or purple. Eosin also stains red blood cells intensely red.