Congo red

As suggested by its intense red color, congo red has important spectrophotometric properties.

Congo red has a propensity to aggregate in aqueous and organic solutions.

Using Congo red, only one-fourth of senile plaques present were noticeably stained.

With diazotized benzidine it gives Congo red.

Sections of 5 m thickness were stained with haematoxylin and eosin, Congo red, and azan stain.

Brain amyloid can be shown by staining brain sections with thioflavin S or Congo red.

Congo red is used as a test for achlorhydria in the stomach, as it changes colour from red to black at a pH less than 3.

Congo red is water soluble, yielding a red colloidal solution; its solubility is better in organic solvents such as ethanol.

In biochemistry and histology, congo red is used to stain microscopic preparates, especially as a cytoplasm and erythrocyte stain.

Clinically, amyloid diseases are typically identified by a change in the fluorescence intensity of planar aromatic dyes such as thioflavin T or congo red.

For a comparative evaluation of amyload protein, potassium permanganate (KMnO 4 ) treatment was performed before staining with Congo red.

Senile plaques are visible in light microscopy using the staining techniques of silver, Congo red, Thioflavin, cresyl violet, and PAS-reaction.

Congo red was first synthesized in 1883 by Paul Bottiger who was working then for the Friedrich Bayer Company in Elberfeld, Germany.

A simpler, but not always predictable, technique is to soak grease-free surfaces in aqueous solutions of 0 5 g malachite green, congo red or methylene blue in 250 ml water.

Congo red also shows a fluorescent activity when bound to amyloid fibrils, which tends to be used as a sensitive diagnosis tool for amyloidosis, instead of the traditional histological birefringence test.

The term congophilic is used because the presence of the abnormal aggregations of amyloid can be demonstrated by microscopic examination of brain tissue after application of a special stain called Congo red.

In this test a few drops of aldehyde containing specimen is dissolved in ethanol, the sulfonamide is added together with some sodium hydroxide solution and then the solution is acidified to Congo red.

Additionally, congo red is used in microbiological epidemiology to rapidly identify the presence of virulent serotype 2a Shigella flexneri, where the dye binds the bacterium's unique lipopolysaccharide (LPS) structure.

However, the use of congo red in the cellulose industries (cotton textile, wood pulp & paper) has long been abandoned, primarily because of its toxicity and tendency to run and change color when touched by sweaty fingers.

Since this color change is an approximate inverse of that of litmus, it can be used with litmus paper in a simple parlor trick: add a drop or two of congo red to both an acid solution and a base solution.