6,6'-[(3,3'-Dimethyl[1,1'-biphenyl]-4,4'-diyl)bis(2,1-diazenediyl)]bis[4-amino-5-hydroxy-1,3-naphthalenedisulfonic acid]

Evans blue free acid is a naphthalenesulfonic acid that is the free acid form of the dye Evans blue. The tetrasodium salt is used as a counterstain, especially in fluorescent methods to suppress background autofluorescence. It has a role as a sodium channel blocker and a teratogenic agent. It is a member of azobenzenes, a member of biphenyls, a member of naphthols, a naphthalenesulfonic acid, an aminonaphthalene, a bis(azo) compound and a primary arylamine. It is a conjugate acid of an Evans blue(4-).