Tetrasodium (3-hydroxy-4-((2-hydroxy-1-naphthyl)azo)-7-nitronaphthalene-1-sulphonato(3-))(3-hydroxy-4-((2-hydroxy-6-sulpho-1-naphthyl)azo)-7-nitronaphthalene-1-sulphonato(4-))chromate(4-)