congo red

Information about congo red

Congo red
Identifiers
CAS number573-58-0
PubChem11313
MeSHCongo+red
Properties
Molecular formulaC32H22N6Na2O6S2
Molar mass696.665
Except where noted otherwise, data are given for
materials in their standard state
(at 25 C, 100 kPa)
Congo red is the sodium salt of benzidinediazo-bis-1-naphtylamine-4-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 fibres. However, the use of congo red in the cellulose industries (cotton textile, wood pulp & paper) has long been abandoned, mainly because of its toxicity.

Behaviour in solution

Congo red (pH indicator)
below pH 3.0above pH 5.2
3.0↔5.2
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.

Congo red has a propensity to aggregate in aqueous and organic solutions. The proposed mechanisms suggest hydrophobic interactions between the aromatic rings of the dye molecules, leading to a pi-pi stacking phenomenon. Although these aggregates are present under various sizes and shapes, the "ribbon-like micelles" of a few molecules seem to be the predominant form (even if the "micelle" term is not totally appropriate here). This aggregation phenomenon is more important for high congo red concentrations, at high salinity and/or low pH.

Dyeing activity

As suggested by its intense red color, congo red has important spectrophotometric properties. Indeed, its UV-visible absorption spectrum shows a characteristic, intense peak around 498 nm in aqueous solution, at low dye concentration. Congo red molar extinction coefficient is about 45000 [L]/[mol].[cm] in these conditions. Aggregation of the dye tends to red-shift the absorption spectrum, whereas binding to cellulose fibres or amyloid fibrils has the opposite effect. Congo red also shows a fluorescent activity when bound to amyloid fibrils, which tends to be used as a sensitive diagnosis tool for amyloidosis, instead of the traditional histological birefringence test.

Diagnostic use

In biochemistry and histology, congo red is used to stain microscopic preparates, especially as a cytoplasm and erythrocyte stain. Apple-green birefringence of Congo red stained preparates under polarized light is indicative for the presence of amyloid fibrils.
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standard state of a material is its state at 1 bar (100 kilopascals exactly). This pressure was changed from 1 atm (101.325 kilopascals) by IUPAC in 1990.[1] The standard state of a material can be defined at any given temperature, most commonly 25 degrees Celsius,
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Azo compounds refer to synthetic inorganic chemical compounds bearing the functional group R-N=N-R', in which R and R' can be either aryl or alkyl. The N=N group is called an azo or diimide.
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Cellulose is an organic compound with the formula (C6H10O5)n. It is a structural polysaccharide derived from beta-glucose.[1][2] Cellulose is the primary structural component of green plants.
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Cotton is a soft fibre that grows around the seeds of the cotton plant (Gossypium sp.), a shrub native to tropical and subtropical regions around the world, including the Americas, India, and Africa.
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    A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH (acidity or alkalinity) of the solution can be determined easily.
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      pH is a measure of the acidity or alkalinity of a solution. Aqueous solutions at 25 ℃ with a pH less than seven are considered acidic, while those with a pH greater than seven are considered basic (alkaline). The pH of 7.
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        A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH (acidity or alkalinity) of the solution can be determined easily.
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        • Litmus test (chemistry), a common pH test
        • Litmus Zine , a web magazine devoted to creative science writing
        • Litmus (Mozilla), a test case management tool maintained by Mozilla

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        Stacking in supramolecular chemistry refers to a stacked arrangement of aromatic molecules, which interact through aromatic interactions. The most popular example of a stacked system is found for consecutive base pairs in DNA.
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        A micelle (rarely micella, plural micellae) is an aggregate of surfactant molecules dispersed in a liquid colloid.
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        In physics, spectrophotometry is the quantitative study of electromagnetic spectra. It is more specific than the general term electromagnetic spectroscopy in that spectrophotometry deals with visible light, near-ultraviolet, and near-infrared.
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        The molar extinction coefficient, also known as molar absorptivity, is a measure of how strongly a chemical species at a given wavelength absorbs light at that wavelength.
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        Fluorescence is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of another photon with a longer wavelength.
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        MeSH D000686

        In medicine, amyloidosis refers to a variety of conditions in which amyloid proteins are abnormally deposited in organs and/or tissues, causing disease.
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        Birefringence, or double refraction, is the decomposition of a ray of light into two rays (the ordinary ray and the extraordinary ray) when it passes through certain types of material, such as calcite crystals or boron nitride, depending on the polarization of
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        Biochemistry is the study of the chemical processes in living organisms.[1] The word "biochemistry" comes from the Greek word βιοχημεία biochēmeia, which means "the chemistry of life.
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        Histology (from the Greek ἱστός) is the study of tissue sectioned as a thin slice, using a microtome. It can be described as microscopic anatomy.
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        Staining is a biochemical technique of adding a class-specific (DNA, proteins, lipids, carbohydrates) dye to a substrate to qualify or quantify the presence of a specific compound. It is similar to fluorescent tagging.
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        Cytoplasm is a gelatinous, semi-transparent fluid that fills most cells. Eukaryotic cells contain a nucleus that is kept separate from the cytoplasm by a double membrane layer. The cytoplasm has three major elements; the cytosol, organelles and inclusions.
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        Red blood cells are the most common type of blood cell and the vertebrate body's principal means of delivering oxygen from the lungs or gills to body tissues via the blood.
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        Birefringence, or double refraction, is the decomposition of a ray of light into two rays (the ordinary ray and the extraordinary ray) when it passes through certain types of material, such as calcite crystals or boron nitride, depending on the polarization of
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