Congo red

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Congo red
Identifiers
CAS number	573-58-0
PubChem	11313
MeSH	Congo+red
Properties
Molecular formula	C32H22N6Na2O6S2
Molar mass	696.665
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

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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 3.0		above 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.

v • d • e WikiDoc Research Resources for Congo red
Articles on Congo red	Most recent articles on Congo red • Most cited articles on Congo red • Review articles on Congo red • Articles on Congo red in N Eng J Med, Lancet, BMJ
Media (Slides, Video, Images, MP3) on Congo red	Powerpoint slides on Congo red • Images of Congo red • Photos of Congo red • Podcasts & MP3s on Congo red • Videos on Congo red
Evidence Based Medicine Regarding Congo red	Cochrane Collaboration on Congo red • Bandolier on Congo red • TRIP on Congo red
Cost Effectiveness of Congo red	Cost Effectiveness of Congo red
Clinical Trials Involving Congo red	Ongoing Trials on Congo red at Clinical Trials.gov • Trial results on Congo red • Clinical Trials on Congo red at Google
Guidelines / Policies / Government Resources (FDA/CDC) Regarding Congo red	US National Guidelines Clearinghouse on Congo red • NICE Guidance on Congo red • NHS PRODIGY Guidance • FDA on Congo red • CDC on Congo red
Textbook Information on Congo red	Books and Textbook Information on Congo red
Pharmacology Resources on Congo red	Dosing of Congo red • Drug interactions with Congo red • Side effects of Congo red • Allergic reactions to Congo red • Overdose information on Congo red • Carcinogenicity information on Congo red • Congo red in pregnancy • Pharmacokinetics of Congo red •
Genetics, Pharmacogenomics, and Proteinomics of Congo red	Genetics of Congo red • Pharmacogenomics of Congo red • Proteomics of Congo red
Newstories on Congo red	Congo red in the news • Be alerted to news on Congo red • News trends on Congo red
Commentary on Congo red	Blogs on Congo red
Patient Resources on Congo red	Patient resources on Congo red • Discussion groups on Congo red • Patient Handouts on Congo red • Directions to Hospitals Treating Congo red • Risk calculators and risk factors for Congo red
Healthcare Provider Resources on Congo red	Symptoms of Congo red • Causes & Risk Factors for Congo red • Diagnostic studies for Congo red • Treatment of Congo red
Continuing Medical Education (CME) Programs on Congo red	CME Programs on Congo red
International Resources on Congo red	Congo red en Espanol • Congo red en Francais
Business Resources on Congo red	Congo red in the Marketplace • Patents on Congo red
Informatics Resources on Congo red	List of terms related to Congo red

de:Kongorot