Congo Red
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This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition
Issued and maintained by the United States Pharmacopeial Convention (USP)
C32H22N6Na2O6S2 696.67 CAS RN®: 573-58-0.—A dark red or reddish-brown powder. Decomposes on exposure to acid fumes. Its solutions have a pH of about 8 to 9.5. One g dissolves in about 30 mL of water. Is slightly soluble in alcohol.
• Loss on Drying〈731〉: Dry it at 105° for 4 hours: it loses not more than 3.0% of its weight.
• Residue on Ignition: Accurately weigh about 1 g, previously dried at 105° for 4 hours, and place it in a porcelain dish or crucible. Carefully ignite until well charred, cool, add 2 mL of sulfuric acid, and carefully ignite until the residue is white or practically so. Cool, add 0.5 mL of sulfuric acid and 1 mL of nitric acid, evaporate, and again ignite to constant weight: the weight of the sodium sulfate so obtained is between 20.0% and 24.0% of the weight of the dried specimen taken.
• Sensitiveness: To 50 mL of carbon dioxide-free water add 0.1 mL of congo red solution (1 in 1000). The red color of the solution is changed to violet by the addition of 0.05 mL of 0.10 N hydrochloric acid and is restored by the subsequent addition of 0.05 mL of 0.10 N sodium hydroxide.
