Congo red (C.I. 22120; Direct Red 28) is an anionic metachromatic dye used in histology and laboratory research. In validated tissue-staining methods, it provides contrast for amyloid deposits and can also be used as a contrast stain or counterstain.
Polysciences Product 02736 is supplied as a solid laboratory reagent in a 25 g package. Its reported absorption maximum is 497 nm, providing a useful reference for dye identification and method-specific spectrophotometric work.
Congo red also functions as a pH indicator, with a color transition from blue near pH 3.0 to red near pH 5.0. Observed color and spectral properties can vary with pH, concentration, solvent, temperature, and measurement conditions.
Synonyms: Direct Red 28; C.I. Direct Red 28; C.I. 22120
Key Properties
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Product number: 02736
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CAS number: 573-58-0
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Colour Index number: C.I. 22120
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Alternative dye name: Direct Red 28
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Molecular formula: C32H22N6Na2O6S2
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Molecular weight: 696.67 g/mol
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Dye type: Anionic metachromatic dye
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Product form: Solid
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Absorption maximum: 497 nm
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pH indicator transition: Approximately pH 3.0, blue, to pH 5.0, red
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Package size: 25 g
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Storage: Room temperature
Applications
Amyloid Histology
Congo red is used in established histologic methods to provide contrast for amyloid deposits in prepared tissue sections. Staining and interpretation should follow a validated method appropriate for the specimen, laboratory, and intended examination.
Polarized-Light Evaluation
Congo red-stained amyloid deposits can exhibit characteristic birefringence when examined between crossed polarizers. The observed intensity and colors depend on section thickness, staining quality, tissue orientation, microscope configuration, and optical alignment.
Contrast Staining and Counterstaining
Congo red can be incorporated into validated contrast-staining and counterstaining procedures. Compatibility with other stains, fixatives, differentiation steps, and mounting media should be confirmed within the complete method.
Laboratory pH-Indicator Applications
Congo red changes from blue under more acidic conditions to red as pH increases through its transition interval of approximately pH 3.0–5.0. Indicator performance should be verified under the concentration, solvent, and sample conditions used.
Preparation and Staining Considerations
- Prepare the working stain according to an established method appropriate for the specimen and intended use.
- Use suitable reagents, clean glassware, and controlled solution conditions.
- Maintain consistent section thickness, fixation, staining time, differentiation, washing, and mounting procedures.
- Use appropriate positive and negative controls when evaluating staining performance.
- Inspect prepared solutions for contamination or precipitate before use.
- Verify polarizer alignment and microscope settings before evaluating birefringence.
- Interpret staining together with controls, morphology, and the laboratory’s established criteria.
Application Tips
- Record the product lot, solution preparation date, concentration, pH, and staining conditions.
- Evaluate each newly prepared working solution with suitable control material.
- Keep specimen preparation and section thickness consistent across comparative samples.
- Confirm that counterstains and differentiation steps do not obscure the intended staining pattern.
- For spectrophotometric work, record the solvent, concentration, pH, temperature, path length, and instrument settings.
- Use the reported 497 nm absorption maximum as a reference and verify spectral behavior under the conditions of the specific method.
Handling & Storage
- Review the current Safety Data Sheet before handling or use.
- Follow the product label, institutional safety procedures, and applicable regulations.
- Use appropriate engineering controls and personal protective equipment.
- Avoid generating or breathing airborne dust.
- Prevent contact with skin, eyes, and clothing.
- Store at room temperature in accordance with the current product label and SDS.
- Keep the container securely closed when not in use.
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Frequently Asked Questions
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What is Congo red?
Congo red is an anionic metachromatic dye used in histology, amyloid-related staining procedures, contrast staining, and laboratory pH-indicator applications. Its CAS number is 573-58-0.
Are Congo red and Direct Red 28 the same dye?
Yes. Direct Red 28 is another name for Congo red. The dye is also identified by Colour Index number C.I. 22120.
How is Congo red used in amyloid staining methods?
Congo red is used to provide contrast for amyloid deposits in prepared tissue sections. Stained deposits may exhibit characteristic birefringence under crossed polarizers. Results depend on the staining method, controls, specimen preparation, and microscope configuration.
What is the absorption maximum of Congo red?
The reported absorption maximum for Product 02736 is 497 nm. Measured values can vary with pH, concentration, solvent, temperature, path length, and instrument conditions.
Is Product 02736 a ready-to-use staining solution?
No. Product 02736 is supplied as a solid laboratory reagent in a 25 g package. Prepare the working solution according to an appropriate laboratory method.
Safety & Documentation
This laboratory reagent may pose health and safety hazards. Review the current Safety Data Sheet in full before handling, storage, or use. Follow the product label and all applicable institutional procedures for engineering controls, personal protective equipment, spill response, storage, and disposal.
Safety classifications and handling requirements may change. Always use the current product label and Safety Data Sheet for product-specific safety information.
