Congo red is used for staining in amyloidosis, and for the cell walls of plants and fungi, and for the outer membrane of Gram-negative bacteria. Apple-green birefringence of Congo red stained preparations under polarized light is indicative of the presence of amyloid fibrils.

Warm 0.1 g of cresol red in a mixture of 2.65 ml of 0.1 M sodium hydroxide and 20 ml of ethanol 

Congo Red is an indicator dye that is blue-violet at pH 3.0 and red at pH 5.0. It contains a 3,3'-(biphenyl-4,4'-diyldidiazene-2,1-diyl)bis(4-aminonaphthalene-1-sulfonate). It derives from a 3,3'-(biphenyl-4,4'-diyldidiazene-2,1-diyl)bis(4-aminonaphthalene-1-sulfonic acid).

CONGO RED INDICATOR SOLUTION