Congo red stain

Congo red

Names
IUPAC name disodium 4-amino-3-[4-[4-(1-amino-4-sulfonato-naphthalen-2-yl)diazenylphenyl]phenyl]diazenyl-naphthalene-1-sulfonate
Identifiers
CAS Number	573-58-0 Y
3D model (Jmol)	Interactive image
ChEMBL	ChEMBL429694 Y
ChemSpider	10838 Y
ECHA InfoCard	100.008.509
MeSH	Congo+red
PubChem	11313
UNII	3U05FHG59S Y
InChI InChI=1S/C32H24N6O6S2.2Na/c33-31-25-7-3-1-5-23(25)29(45(39,40)41)17-27(31)37-35-21-13-9-19(10-14-21)20-11-15-22(16-12-20)36-38-28-18-30(46(42,43)44)24-6-2-4-8-26(24)32(28)34;;/h1-18H,33-34H2,(H,39,40,41)(H,42,43,44);;/q;2*+1/p-2/b37-35+,38-36+;; Y Key: IQFVPQOLBLOTPF-HKXUKFGYSA-L Y InChI=1/C32H24N6O6S2.2Na/c33-31-25-7-3-1-5-23(25)29(45(39,40)41)17-27(31)37-35-21-13-9-19(10-14-21)20-11-15-22(16-12-20)36-38-28-18-30(46(42,43)44)24-6-2-4-8-26(24)32(28)34;;/h1-18H,33-34H2,(H,39,40,41)(H,42,43,44);;/q;2*+1/p-2/b37-35+,38-36+;; Key: IQFVPQOLBLOTPF-COCKEIKOBB
SMILES [Na+].[Na+].[O-]S(=O)(=O)c5cc(/N=N/c1ccc(cc1)c4ccc(/N=N/c3cc(c2ccccc2c3N)S([O-])(=O)=O)cc4)c(N)c6c5cccc6
Properties
Chemical formula	C32H22N6Na2O6S2
Molar mass	696.665
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
Infobox references

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.

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.

Congo red was first synthesized in 1883 by Paul Böttiger, who was working then for the Friedrich Bayer Company in Elberfeld, Germany. He was looking for textile dyes that did not require a mordant step. The company was not interested in this bright red color, so he filed the patent under his 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 reasons, other dyes were marketed using the "Congo" name: Congo rubine, Congo corinth, brilliant Congo, Congo orange, Congo brown, and Congo blue.

Due to a color change from blue to red at pH 3.0–5.2, Congo red can be used as a pH indicator. Since this color change is an approximate inverse of that of litmus, it can be 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. Dipping red litmus paper in the red solution will turn it blue, while dipping blue litmus paper in the blue solution will turn it red. This property gives Congo red a metachromatic property as a dye, both in strongly acidic solutions and with strongly acidophilic tissue.