A hydrochloride that is the dihydrochloride of 4,4'-[1,3-phenylenebis(diazene-2,1-diyl)]di(benzene-1,3-diamine). Bismarck brown Y is a metachromatic dye which stains acid mucins yellow. It is also a constituent of Papanicolaou's EA solutions, used for cervical, and other smears.