What is Congo Red?

Updated: 4/27/2022
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12y ago

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Congo red is the sodium salt of benzidinediazo-bis-1-naphtylamine-4-sulfonic acid (formula: C32H22N6Na2O6S2). 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.

Conga Red is an acidic stain primarily used to stain bacterial cells to test for capsules. The Conga Red stain is used to stain a microscope slide background against the colorless cell. Then a basic stain like Maneval's Stain is used to stain the cell. The capsule surrounding some cells will repel the stains and appear colorless against a red (Conga Red) background. The cell inside the capsule will be stained light purple from the Maneval's Stain.

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Q: What is Congo Red?
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