Congo red

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. Congo red was first synthesized in 1883 by Paul Böttiger, who had been employed at Friedrich Bayer Company in Elberfeld, Germany. He was looking for textile dyes that did not require a mordant step. The company which had a right of first refusal to his inventions was not interested in this bright red color, so he filed the patent under his own name and sold it to the AGFA company of Berlin. AGFA marketed the dye under the name "Congo red", a catchy name in Germany at the time of the 1884 Berlin West Africa Conference, an important event in the Colonisation of Africa. The dye was a major commercial success for AGFA. In the following years, for the same reason, other dyes were marketed using the "Congo" name: Congo rubine, Congo corinth, brilliant Congo, Congo orange, Congo brown, and Congo blue. Once of economic significance, Congo red has fallen into disuse as have all benzidine-derived dyes, owing to their carcinogenic activity. It is prepared by azo coupling of the bis(diazonium) derivative of benzidine with naphthionic acid. Congo blue, however, is in widespread international use, in gel sheet form, as a filter to place in front of theatrical, motion picture, television, church, and live event lighting instruments. It is sold under the item name "181 Congo Blue" by Lee Filters. It emits a deep rich saturated blue color with elements of red. Depending upon the color temperature of the source lamp, the light from a lighting instrument with a Congo Blue filter reflected from a white surface can vary from very saturated blue to purple or violet.