7,7'-Ureylenebis[4-hydroxy-3-[(2-hydroxy-5-sulfophenyl)azo]-2-naphthalenesulfonic acid]tetrasodium salt

7,7'-(Carbonylbisimino)bis[4-hydroxy-3-[2-hydroxy-5-[(sodiooxy)sulfonyl]phenylazo]naphthalene-2-sulfonic acid sodium] salt