4-Amino-5-hydroxy-3,6-bis[(4-nitrophenyl)azo]-2,7-naphthalenedisulfonic acid disodium salt

(3Z)-5-amino-6-(4-nitrophenyl)diazenyl-3-[(4-nitrophenyl)hydrazinylidene]-4-oxo-naphthalene-2,7-disulfonic acid