≥98%(HPLC) for sensitive chromatographic and analytical workflows requiring minimal baseline interference.
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Storage & shipping
Room temperature Ships Normal Check lot-specific COA for exact specifications.
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Quality documents
SDS, COA, datasheet, and spec sheet available for download. Lot-specific COA accessible via lot number lookup.
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Literature proof
Cited in 710 peer-reviewed publications across chromatography, organic synthesis, and cross-coupling reactions.
Specifications
Sinónimos
Brasilamina congo 4B | Mitsui Congo Red | NSC 7232 | C.I. Direct Red 28, disodium salt (8CI) | Cotton Red L | disodium 3,3'-(biphenyl-4,4'-diyldidiazene-2,1-diyl)bis(4-aminonaphthalene-1-sulfonate) | Hispamin Congo 4B | DTXSID4022816 | C32H22N6O6S2.2Na |
Especificaciones y pureza
≥98%(HPLC)
Mecanismos bioquímicos y fisiológicos
Amyloid fibril-binding dye. Caspase inhibitor. Red shifts on aggregation and binding to amyloid fibrils. Blue shifts on binding to cellulose. Shows apple green birefringence under polarized light in the presence of amyloid fibrils.
Condiciones de almacenamiento de almacenamiento
Room temperature
Enviado en
Normal
Nota
Wherever possible, you should prepare and use solutions on the same day. However, if you need to make up stock solutions in advance, we recommend that you store the solution as aliquots in tightly sealed vials at -20°C. Generally, these will be useable for up to one month. Before use, and prior to opening the vial we recommend that you allow your product to equilibrate to room temperature for at least 1 hour. Need more advice on solubility, usage and handling? Please visit our frequently asked questions (FAQ) page for more details.
This compound belongs to the class of organic compounds known as benzidines. These are organic compounds containing the benzidine skeleton, made up of a biphenyl ring system substituted at the 4- and 4'-positions with a unsubstituted amine group.
External Descriptors
azo compound
1. Djoumbou Feunang Y, Eisner R, Knox C, Chepelev L, Hastings J, Owen G, Fahy E, Steinbeck C, Subramanian S, Bolton E, Greiner R, and Wishart DS. ClassyFire: Automated Chemical Classification With A Comprehensive, Computable Taxonomy. Journal of Cheminformatics, 2016, 8:61.
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Application Protocols
The following protocols are provided as general literature-based examples for research use only. Optimize for your system and consult primary references.
Indicator stock preparation (general)
Dissolve 0.5 g Congo red in 100 mL of deionized water or 50% ethanol/water (0.5% w/v). Filter through 0.45 μm. Store in amber bottle at room temperature. Use a few drops per 50–100 mL sample for pH ~3–5 transitions.
Supramolecular indicator-displacement assay (IDA)
Prepare 20 μM Congo red in 10 mM phosphate buffer, pH 7.2, 50 mM NaCl. Add cationic host (e.g., polymer) to achieve ~50% absorbance change at λmax (~500 nm). Titrate analyte; acquire full spectra after each addition. Fit to a competitive binding model to extract Kd.
Polymer/cellulose staining and quantification (research)
Stain: Immerse material in 0.1% Congo red for 10–20 min at ambient temperature. Rinse with water until wash is nearly colorless.
Quantify: Desorb retained dye in 0.1 M Na2CO3 (or 50% ethanol/water, pH ~10) for 30 min with gentle agitation. Measure A500 and calculate dye amount using a calibration curve prepared in the same medium.
Amyloid fibril visualization in vitro (research, non-diagnostic)
Prepare 0.5% Congo red in 50% ethanol, adjusted slightly alkaline (e.g., with a small amount of NaOH). Incubate fibril-containing samples 10–20 min, rinse with ethanol/water. Record spectra or examine birefringence under polarized light. Do not use for clinical diagnosis.
Biological Roles
Congo red is not a native metabolite; its "biological roles" relate to noncovalent binding behaviors exploited in biochemical research (literature/general context, for research use only).
Amyloid-binding dye
Binds to cross-β sheet structures in amyloid fibrils via planar aromatic stacking and hydrogen-bond templating. This yields diagnostic spectral and birefringent properties in stained preparations. In research, it serves as a qualitative probe of amyloid architecture in vitro.
Polysaccharide interactions
Associates with cationic biopolymers and modified polysaccharides through electrostatic attraction (dye sulfonates to cationic sites) and π–π interactions; useful for probing charge density and binding capacities.
Protein and surfactant systems
Forms complexes with cationic proteins/surfactants, shifting absorption; can report on micellization and protein unfolding under controlled conditions.
Enzyme susceptibility
Azo linkages can be reduced by certain microbial azoreductases, leading to decolorization; this behavior is leveraged in environmental microbiology studies of dye degradation.
Note: No therapeutic or clinical claims are made. Use is limited to laboratory research, biophysical characterization, and qualitative visualization.
Buffer Applications
Congo red is not a buffer reagent; it does not provide significant buffering capacity. However, it is commonly used as an acid–base indicator within buffered systems (literature/general guidance).
Indicator range and preparation
Effective color transition range: approximately pH 3.0–5.2 (literature). Below pH ~3, solutions appear blue-violet; above ~5, red/orange predominates.
Typical indicator stock: 0.1–1.0% (w/v) in water or 50% ethanol/water. Filter to remove insolubles; store protected from light.
Use in buffer titrations
Suitable for monitoring acidification endpoints in weakly acidic ranges; pair with standard buffers (e.g., acetate, citrate) when visual endpoint detection is acceptable.
Practical notes
Ionic strength and solvent composition shift λmax and perceived color; standardize conditions for reproducible endpoints.
For spectrophotometric work, record full spectra across the pH range of interest and fit to Henderson–Hasselbalch models to determine apparent pKa under your exact ionic strength.
Recipes (example, general)
Prepare 0.5% (w/v) Congo red in 50% ethanol; add a few drops per 50–100 mL of analyte solution near expected endpoint. For precise pH control, rely on a pH meter; the dye provides a qualitative color cue.
Green Alternatives
Sustainability considerations focus on dye persistence, toxicity profile, and waste minimization.
Environmental profile (general)
Congo red is a synthetic azo dye with high color strength and persistence; wastewater treatment may require adsorption or advanced oxidation. Minimizing discharge is critical.
For pH indication (pH ~3–6): consider natural or lower-toxicity indicators where feasible, e.g., litmus, red cabbage anthocyanins (for demonstrations), or bromocresol green (depending on range). These may offer improved biodegradability but can be less stable or precise.
For supramolecular indicator assays: alternatives such as methyl orange or neutral red may provide similar reporter behavior with different environmental profiles; selection depends on host-binding, pKa, and spectral needs.
For cellulose/polysaccharide visualization: natural dyes (e.g., curcumin) or anionic food dyes (e.g., FD&C dyes) can sometimes substitute, with tradeoffs in binding specificity and lightfastness.
Process improvements
Use microplate or cuvette-scale assays to reduce dye mass.
Implement closed-loop rinse and activated carbon treatment for colored waste streams.
Validate reusability of dye-loaded adsorbents in materials testing.
Comparison summary (general)
Congo red: high water solubility, sharp visible band, robust signal; environmental persistence.
Alternative azo indicators: tunable ranges; similar persistence concerns—choose based on minimal mass use and ease of removal.
Pharmaceutical Uses
No clinical use claims are made. Congo red is not typically used as a pharmaceutical active. In the context of pharmaceutical research/manufacturing (general literature context):
Process visualization and QC (research/manufacturing)
As a highly colored tracer, Congo red can qualitatively assess filtration efficiency, resin binding characteristics, or visualize flow paths in equipment cleaning studies.
Excipient status
Not listed among common pharmacopeial colorants for internal use; selection of dyes in formulations is strictly regulated. When a colorant is needed, pharmacopeia-compliant dyes (e.g., specific FD&C dyes) are preferred.
Analytical applications
Potential use as a probe in indicator displacement assays or in surface characterization (e.g., quantifying cationic site density on modified excipients) under research conditions.
Any application involving drug products must comply with regulatory guidance on color additives and impurities. This product is supplied strictly for research use only, as noted by the manufacturer.
Physical Properties
Item-specific specifications
Appearance: Not specified for this item; refer to CoA/Spec Sheet.
Grade/Purity: Not specified for this item; refer to CoA/Spec Sheet.
Literature/general values (for reference only; not item specifications)
Physical form: typically a dark red to brownish-red crystalline powder (dye).
Solubility: highly soluble in water; soluble in polar organics such as ethanol; poorly soluble in nonpolar solvents.
Acid–base/color transition (indicator): pH transition range roughly 3.0–5.2 (literature); color shifts from blue-violet in strong acid to red in neutral/weakly basic media due to protonation state changes at the aniline nitrogens and conjugation perturbations.
UV–Vis: strong absorption in the visible (orange-red region); λmax commonly reported near 497–505 nm in water (literature, concentration- and pH-dependent) with additional bands in the UV due to the extended azo–aromatic chromophore.
Thermal behavior: dyes of this class generally do not have a sharp boiling point; they darken and decompose upon strong heating rather than distilling (literature).
Partitioning: expected to be highly polar (disulfonate salt), with very low logP and strong hydration; tends to associate with cationic surfactants and polymeric cations via electrostatic interactions (literature).
Note: For exact numerical specifications (mp, λmax at stated conditions, water content, UV cutoffs, metal levels), consult the item’s CoA/Spec Sheet.
Quality and Grades
Item-specific grade/purity: Not specified for this item; refer to CoA/Spec Sheet.
How to interpret grades for dyes (general guidance)
Technical/industrial dye grade: typically targeted to textile applications; may contain inorganic salts or isomeric/byproduct dyes; color strength and shade consistency are primary specifications.
Laboratory/reagent grade: controlled for identity and dye content; may include absorbance/λmax checks and limits on insolubles.
HPLC/spectrophotometric grade (when offered): tighter control of UV–Vis background, low insoluble matter, and defined extinction coefficients under stated conditions—useful for quantitative spectrophotometry and indicator assays.
Stabilizers/impurities
Congo red is generally supplied without stabilizers. Trace salt content (e.g., NaCl) and moisture can influence extinction readings and solution density; high-precision analytical work benefits from drying under vacuum at ambient temperature and referencing to a primary standard.
Recommendation
For quantitative colorimetric uses (e.g., binding/indicator displacement assays), request and rely on the CoA parameters (e.g., dye content by absorbance at λmax, insolubles, ash) to establish calibration accuracy and reproducibility.
Reaction and Applications
While Congo red is not commonly used as a stoichiometric reagent, it is a valuable chromophoric probe and indicator in synthetic and materials chemistry.
Indicator and probe uses (literature/general)
Acid–base indicator around pH 3–5.2: convenient visual and spectrophotometric monitoring of acidification/neutralization steps.
Supramolecular chemistry: employed in indicator-displacement assays (IDA) to quantify binding affinities for cationic hosts (e.g., cyclodextrins, cucurbiturils, cationic polymers) via competitive binding and color/absorbance change.
Polymer/surfactant systems: used to determine critical micelle concentration (CMC) and polymer–surfactant complexation through spectral shifts.
Materials/analytical applications
Cellulose and polysaccharide interactions: qualitative assessment of cationic modification of cellulose or chitosan by dye uptake/washfastness.
Solid-phase visualization: staining of polymeric resins, gels, or paper to map functional group distributions.
Practical tips
Work at low micromolar to sub-millimolar concentrations to minimize aggregation; record full spectra to detect hypsochromic/bathochromic shifts.
Control ionic strength; high salt screens electrostatic interactions and can suppress binding-induced shifts.
Calibrate extinction coefficient under your exact solvent/pH/ionic strength; do not assume literature ε without verification.
Limitations
Strong oxidants can bleach the dye; azoreductases or reducing conditions may cleave –N=N– bonds, altering signal.
Reaction Conditions
Because Congo red serves mainly as a chromogenic probe/indicator, “reaction conditions” here refer to assay and analytical conditions (literature/general guidance).
Spectrophotometric assays
Solvent: water or buffered aqueous media; optional co-solvent (up to 50% ethanol) to aid wetting.
Concentration: 5–100 μM typical for cuvette measurements; lower for microplate assays (1–20 μM) to minimize aggregation.
pH: adjust within 2–8 depending on the targeted interaction; record spectra to capture pH-dependent shifts. Avoid strong acid (<pH 1) for routine work due to protonation-induced spectral complexity.
Ionic strength: 0–200 mM NaCl typical; higher ionic strength will screen electrostatics and can reduce binding-induced spectral changes.
Indicator displacement assays (example setup)
Mix host (cationic receptor/polymer) with Congo red at a fixed ratio; titrate guest analyte while recording Amax (≈500 nm in water) or full spectra. Fit to competitive binding models to extract Kd values.
Cellulose/polymer staining (research)
Immerse substrate in 0.05–0.5% dye solution for 5–30 min; rinse with water or mild electrolyte; quantify retained dye by desorption into known volume of base/solvent and measure absorbance.
Stability considerations
Protect solutions from strong light and oxidants; avoid prolonged storage at elevated temperatures; azoreductants will bleach the dye.
Safety and Handling
Authoritative safety data must be taken from the product’s SDS. The following are general, literature-based considerations for azo sulfonate dyes and Congo red specifically.
GHS and hazards (item-specific):
Signal word: Not specified for this item; refer to SDS.
H-Statements: Not specified for this item; refer to SDS.
GHS classification/pictograms: Not specified for this item; refer to SDS.
General precautions (literature/good practice)
PPE: lab coat, safety glasses, and suitable gloves (e.g., nitrile). Avoid creating dust and inhalation of particulates. Use in a fume hood if dust/aerosol may form.
Hygiene: prevent skin/eye contact; wash thoroughly after handling; avoid ingestion/inhalation.
Incompatibilities: strong oxidizers (may degrade the azo chromophore); strong acids can alter dye form/solubility and color; bases are usually compatible but can change spectral properties.
Environmental: dyes are intensely colored; avoid release to drains; collect solutions/solid waste for appropriate disposal per institutional and regulatory guidance.
First-aid overview (general)
Skin/eye contact: rinse with water for at least 15 minutes; remove contaminated clothing.
Inhalation: move to fresh air; seek medical attention if symptoms persist.
Ingestion: rinse mouth; do not induce vomiting; seek medical attention.
Special notes
Dust control is important due to staining and potential respiratory irritation.
Always defer to the specific SDS for classification, exposure limits, and spill/cleanup procedures.
Solvent Selection
Congo red is a dianionic disulfonated azo dye; solvent choice is governed by ionic character and supramolecular interactions.
Polarity and miscibility (literature/general)
Water: preferred solvent; high solubility due to sulfonate groups; forms intensely colored solutions.
Alcohols (e.g., ethanol, methanol): moderate to good solubility; often used for indicator stock solutions and histological stains.
Polar aprotics (DMSO, DMF): soluble; may be used for high-concentration stocks; note potential for different spectral profiles relative to water.
Spectra and color depend strongly on pH; protonation in strong acid shifts color toward blue-violet; neutral to basic solutions are red/orange. Buffer composition and ionic strength can affect aggregation and λmax.
Practical selection guidance
Aqueous buffer (pH 6–8): for indicator displacement assays, cellulose-binding assays, or general coloring.
Slightly alkaline 50% ethanol/water: common for histological staining protocols (research use) to enhance tissue penetration and control background.
DMSO stock (1–10 mg/mL) diluted into aqueous media: for high-throughput screening where minimal aqueous storage is desired.
Comparison notes
Versus non-sulfonated azo dyes: Congo red is far more water-soluble and exhibits stronger electrostatic interactions with cationic polymers/surfactants.
Versus food/natural dyes: Congo red offers sharper, intense absorbance bands but with less favorable environmental profile.
Storage and Reconstitution
Item-specific conditions
Storage Conditions: Room temperature (as provided by manufacturer).
Shipped In: Normal (as provided by manufacturer).
General guidance (literature/best practice)
Solid dye: store tightly capped in a dry place at ambient temperature. Protect from excessive light and moisture to preserve color strength. Avoid contamination that can alter absorbance characteristics.
Shelf life: solid azo dyes are typically stable for years when kept dry and protected from light; verify performance periodically by UV–Vis at stated concentration.
Aqueous/stock solutions
Preparation: dissolve in deionized water or 50% ethanol/water; filter if necessary.
Concentration: common stocks 0.1–1.0% (w/v) or 1–10 mg/mL for spectrophotometry/assays.
Storage: room temperature or refrigerated (2–8°C) for extended longevity; protect from light. Avoid microbial growth by preparing fresh as needed or using sterile filtration; do not add biocides unless compatible with downstream use.
Stability: solutions may slowly degrade or support microbial growth; monitor by absorbance at λmax for drift. Discard if significant spectral changes or precipitation occur.
Note: For any item-specific stability limits, purity, water content, or extinction coefficient, consult the CoA/Spec Sheet. Product is for research use only.
Structure and Identity
Brief overview: Congo red is a classical diazo anionic dye (disodium salt) comprising two azo (–N=N–) linkages connecting para-aminonaphthalene-1-sulfonate units through a 4,4′-biphenyl core. It bears two sulfonate groups (as disodium sulfonates) that impart high water solubility.
Item-specific identifiers (from Product Data)
SKU: C128371
Product Name: Congo red
CAS: 573-58-0
PubChem CID: 11313
InChIKey: 147670 (as provided)
SMILES: Not specified for this item; refer to CoA/Spec Sheet.
Molecular Formula: Not specified for this item; refer to CoA/Spec Sheet.
Molecular Weight: Not specified for this item; refer to CoA/Spec Sheet.
Structural features (general/literature)
Core scaffold: 4,4′-biphenyl bridging two azo linkages.
Functional groups: two azo bonds (Ar–N=N–Ar), two primary anilines (–NH2), two sulfonate groups (–SO3−, as Na+ salts), extended polyaromatic system.
Charge state: dianionic at neutral pH due to sulfonates; acid–base behavior at the aniline nitrogens contributes to pH-dependent color.
2D description: two naphthalene rings (each bearing –NH2 and –SO3− at 1,4-positions) are each coupled via –N=N– to para positions of a central biphenyl, yielding an overall elongated, planar, conjugated chromophore.
Common synonyms (literature): Direct Red 28; C.I. 22120; Sodium salt of benzidinediazo-bis-1-naphthylamine-4-sulfonic acid.
Synthetic Utility
Congo red is principally a chromogenic probe rather than a building block. Nevertheless, its functional groups and supramolecular behavior can be synthetically useful in analytical/synthetic workflows (literature/general).
Functional group reactivity
Azo linkages (–N=N–): susceptible to reductive cleavage (e.g., with sodium dithionite, catalytic hydrogenation, or enzymatic azoreduction), allowing studies of reduction kinetics and as a surrogate substrate in method development.
Aniline groups: can participate in diazotization under strong acidic/nitrosating conditions, though the dye’s conjugation and sulfonation typically disfavor further derivatization without degradation.
Sulfonate groups: confer water solubility and strong binding to cationic matrices; useful for phase-transfer-like visualization and resin capacity assessments.
Roles in synthesis/analysis
Indicator in titrations and pH tracking during multistep aqueous workups.
Reporter for polymer functionalization: quantify cationic loading on polymers/resins via dye uptake and spectroscopic desorption.
Chromophore in interfacial assembly: visualize layer-by-layer polyelectrolyte deposition.
Limitations
Not typically incorporated into target molecules; the extended chromophore is sensitive to strong oxidants/UV, and reductants can destroy the color, limiting use under harsh redox conditions.
Target Specificity
Not applicable. This product is a small-molecule dye, not a biological targeting reagent (e.g., antibody, ligand with defined target selectivity provided by the manufacturer). No antigen/epitope, species reactivity, clone, or isotype information applies. For usage contexts where Congo red binds structured biomaterials (e.g., amyloid fibrils) or cationic polymers, see Reaction & Applications and Biological Roles for general literature-based behavior.
Preguntas frecuentes
How is this product shipped?
This product ships under standard ambient conditions. No temperature-controlled packaging is required.
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