Congo red

Congo red is the sodium salt of benzidinediazo-bis-1-naphtylamine-4-sulfonic acid. It is a secondary diazo dye. Congo red is water soluble, yielding a red colloidal solution. It is used for the dyeing of cotton, despite its sensitivity to light and acids. In biochemistry and histology, congo red is used to stain microscopic preparates, especially as a cytoplasm and erythrocyte stain. Green birefringence of congo red stained preparates under polarized light is indicative for the presence of amyloid fibrils.

Due to a color change from red to blue at pH 3.0-5.2, congo red can be used as a pH indicator.