Definition

1. Noun:
   • A red-brown azo dye used as a chemical pH indicator: Congo red is a synthetic dye that changes color depending on the acidity or alkalinity (pH) of a solution. It is red in basic (alkaline) solutions and turns blue in acidic solutions.

Usage Examples

• Noun:
  • The laboratory used congo red to test the pH of the unknown liquid.
  • Under the microscope, the amyloid proteins were stained with congo red and exhibited apple-green birefringence.
  • A drop of congo red was added to the solution, and its color change confirmed the sample was acidic.

Advanced Usage

• "Congo red staining": A common histological and laboratory technique.
  • Congo red staining is a standard method for identifying amyloid deposits in tissue samples.
• "Congo red agar": A specialized growth medium used in microbiology.
  • The ability of the bacteria to bind the dye is tested on congo red agar.

Variants and Related Words

• Azo dye (n): A class of synthetic dyes to which congo red belongs, characterized by the presence of an azo group (–N=N–).
• pH indicator (n): A substance, like congo red, that changes color to show the pH of a solution.
• Direct Red 28 (n): An alternative chemical name for congo red.

Synonyms

• Chemical indicator: A substance used to show a condition, such as acidity.
• Biological stain: A dye used to color tissues or cells for microscopic examination.

Related Phrases

• Congo red test: A specific diagnostic or analytical procedure using this dye.
  • The Congo red test was positive, indicating the presence of amyloidosis.
• Congo red binding: Refers to the property of certain substances, like amyloid fibrils, to attach to this dye.
  • The assay measures the congo red binding capacity of the protein sample.

Related Idioms

(This term is highly technical and scientific; it is not commonly used in idiomatic expressions.)

Noun

1. a red-brown azo dye especially as a chemical pH indicator (congo red is red in basic and blue in acidic solutions)