Congo Red

Congo red is the sodium salt of 3,3'-(-4,4'-diyl)bis(4-aminonaphthalene-1-sulfonic acid)(formula: C₃₂H₂₂N₆Na₂O₆S₂; molecular weight: 696.66 g/mol). It is a secondary diazo dye. Congo red is water soluble, yielding a red colloidal solution; its solubility is better in organic solvents such as ethanol.

It has a strong, though apparently non-covalent affinity to cellulose fibers. However, the use of congo red in the cellulose industries (cotton textile, wood pulp & paper) has long been abandoned, primarily because of its tendency to change color when touched by sweaty fingers, to run, and because of its toxicity.