Congo red

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A brownish-red powder, C₃₂H₂₂N₆Na₂O₆S₂, used in medicine and as a dye, indicator, and biological stain.

from the GNU version of the Collaborative International Dictionary of English.

• (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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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