Congo red, indicator

Congo red is the sodium salt of 3,3'-([1,1'-biphenyl]-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.

C 2465 (OTTO) Congo red, indicator Cas 573-58-0 - used with litmus paper in a simple parlor trick: add a drop or two of Congo red to both an acid solution and a base solution.