4-(1-Hydroxy-2-naphtylazo)-3-hydroxy-1-naphthalenesulfonic acid

3-hydroxy-4-[(1-hydroxy-2-naphthyl)azo]naphthalene-1-sulphonic acid

3-Hydroxy-4-[(1-hydroxynaphthalen-2-yl)azo]-1-naphthalenesulfonic acid