Rose bengal free acid is a xanthene dye that is fluorescein bearing bromine substituents at positions 2', 4', 5' and 7' (on the xanthene ring) and iodine substituents at position 2, 3, 4, and 5 (on the phenyl ring).The dipotassium salt is the biological stain 'rose bengal'. It has a role as a fluorochrome.