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. Additionally, Congo Red is used in microbiological epidemiology to rapidly identify the presence of virulent serotype 2a Shigella flexneri, where the dye binds the bacterium’s unique lipopolysaccharide (LPS) structure.

Biological Dye
Amyloid Stain
pH Indicator

C.I. 22120
Molecular Formula: C32H22N6S2Na2
Molecular Weight: 696.66 g/mol
Melting Point: 360°C
Absorptivity (@lambda max): 585nm
Loss on drying: <2.5%
Transition Range (3.5-5): Blue-red

Hazard Phrases: Causes serious eye irritation. May cause cancer.

Prec Phrases: Obtain special instructions before use. Use personal protective equipment as required. IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing. IF exposed or concerned: Get medical advice/attention.



Cas No: 573-58-0
Einecs No: 209-358-4