text: Congo red is an organic compound, the sodium salt of 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-aminonaphthalene-1-sulfonic acid). It is an azo dye. Congo red is water-soluble, yielding a red colloidal solution; its solubility is greater in organic solvents.  The use of Congo red in the textile industry has long been abandoned, primarily because of its carcinogenic properties, but it is still used for histological staining.