'Congo red' definitions:

Definition of 'congo red'

(from WordNet)
noun
A red-brown azo dye especially as a chemical pH indicator (congo red is red in basic and blue in acidic solutions)

Definition of 'Congo red'

From: GCIDE
  • Congo red \Congo red\ (Chem.) An artificial red dye from which the Congo group received its name. It is also widely used either in aqueous solution or as test paper (
  • Congo paper) for the detection of free acid, which turns it blue. [Webster 1913 Suppl.]