The molecule is an organosulfonate oxoanion obtained by deprotonation of the sulfonic acid groups of naphthalene blue black CS (acid form). It is a conjugate base of a naphthalene blue black CS (acid form).
4-amino-5-hydroxy-3-[(4-nitrophenyl)diazenyl]-6-[(4-sulfonatophenyl)diazenyl]naphthalene-2,7-disulfonate