Congo Red indicator solution

Congo Red indicator solution is an aqueous or ethanolic solution of Congo Red dye (C.I. 22120) that changes color from blue-violet (pH <3.0) to red (pH >5.2), serving as an acid-base indicator for titrations and qualitative pH testing.​

Composition and Preparation

Typical formulation: 0.1–1% Congo Red in water/ethanol (e.g., dissolve 0.5 g dye in 50 mL water + 50 mL ethanol; or 1 g in 100 mL water).​

Appearance: Reddish-brown to red clear solution.​

Properties

pH transition: Blue-violet (acidic, pH 3.0) ⇌ red (alkaline, pH 5.2).​

Solubility: Water-soluble; stable but light-sensitive.​

Uses

pH indicator in acid-base titrations (inverse of litmus).​

Microscopy/histology stain for amyloid, cellulose, and bacteria.​

Handling

Store in amber bottles away from light; potential carcinogen—use PPE.​

For catalog: “Congo Red indicator solution, 0.1–1% red liquid, pH 3.0 (blue-violet) to 5.2 (red), for titrations/microscopy; light-sensitive.”