N-[5-[[2-Hydroxy-3,6-bis(sodiosulfo)-1-naphthalenyl]azo]-2-[[4'-[[2-hydroxy-3,6-bis(sodiosulfo)-1-naphthalenyl]azo]-3,3'-dimethoxy[1,1'-biphenyl]-4-yl]azo]phenyl]oxamidic acid sodium salt