Congo red

GNU Webster's 1913

1. (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.

WordNet 3.0

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