What is "congo red"

Congo red \Congo red\ (Chem.) An artificial red dye from which the Congo group received its name. It is also widely used either in aqueous solution or as test paper (

Congo paper) for the detection of free acid, which turns it blue.

n. a red-brown azo dye especially as a chemical pH indicator (congo red is red in basic and blue in acidic solutions)

Congo red is the sodium salt of 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-aminonaphthalene-1-sulfonic acid) (formula: CHNNaOS; 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 noncovalent, affinity to cellulose fibers. However, the use of Congo red in the cellulose industries ( cotton textile, wood pulp, and paper) has long been abandoned, primarily because of its toxicity and tendency to run and change color when touched by sweaty fingers.