Congo Red

Congo Red (Kongorot), a red-brown azo dye, easily soluble in water with a red color. Structure*. NaO₃S.(NH₂)C₁₀H₅.N:N<^>-<(^j>^N:N-.C₁₀H₅(NH₂).SO₃Na. An aqueous solution of C. is an indicator for mineral alkalis, since even traces of them turn it blue. In its pure form, solutions of C. are neutral, but the commercial product is usually alkaline due to soda impurities. In histological technique, it was introduced by Griesbach (1886) for staining axis cylinders. Very demonstrative results are obtained by staining sections of gastric mucosa with Congo Red. For this purpose, the material is fixed by Carnoy or mercuric chloride, sections are stained with hemalum and counterstained for 2-5 minutes or longer with a solution of Congo Red (3 cm³ of 1% aqueous solution of C. in 100 cm³ of distilled water), differentiated with water or 50% alcohol, and mounted in Canada balsam. As a result, chief cells are stained blue (acidity), while parietal cells are stained red-brown. Bennhold (1922) proposed for the recognition of amyloidosis in living patients to inject a 1% aqueous solution of Congo Red intravenously. In positive cases, the dye is deposited in amyloid-altered tissues and quickly disappears from the plasma. Herzenberg (1924) used C. for the vital staining of amyloid. White mice are injected intravenously several times with 1 cm³ of a 0.01-0.1% sterile solution. Congo Red also stains amyloid excellently in histological sections (embedded and frozen). For this, sections are placed in a 1% aqueous solution of Congo Red for 5-10 minutes or 15 seconds when heated over a flame, rinsed with water, placed in a weak ammonia solution, differentiated in 80% alcohol, counterstained with hematoxylin, and mounted in the usual way either in Canada balsam or in glycerin-gelatin. The amyloid substance retains Congo Red and appears colored in bright red. Congo Red has found practical application as an indicator (paper) for the presence of free HCl in gastric contents (see below), for determining the total volume of circulating blood, as well as for the recognition of amyloidosis during life in appropriate patients: when a 1% aqueous solution of Congo Red is injected intravenously, in positive cases the dye is deposited in amyloid-altered tissues and quickly disappears from the plasma (Bennhold). At autopsy in such cases, the amyloid substance, wherever it is deposited, is colored pink-red. This phenomenon has been verified and confirmed by experimental reproduction of vital staining of amyloid in mice (Herzenberg).-For staining elastic fibers (Matsuura), material fixed preferably in formalin is stained in a 1% alcoholic (96°) solution of Congo for 12-24 hours. The preparation is washed with absolute alcohol and transferred to a freshly prepared 1% solution of phosphomolybdic acid in absolute alcohol for 5 minutes (until the preparation turns blue). Differentiated in absolute alcohol for 1-2 minutes; cedar oil, neutral Canada balsam. Elastic fibers are red-violet, collagen fibers of connective tissue are green; muscles, glandular and epithelial cells are brown-red, nuclei are red, erythrocytes are orange-red, leukocytes are red-violet.-C. is also used for staining the cementing substance of young tooth enamel and for staining keratin (concentrated aqueous solution). Congo paper. As an indicator, filter paper impregnated with a 0.1-1% solution of C. is usually used. It is used for qualitative examination of gastric contents for the presence of free HCl: the paper turns blue already in a 0.04% solution of HCl. In the presence of organic acids, the paper takes on a faint grayish-blue color, never reaching the intensity it does with HCl, and disappears when the paper is immersed in ether, which does not happen when the paper turns blue from HCl. When prepared from harder grades of filter paper and impregnated with a 0.1% indicator solution, Congo paper can be used to determine pH between 2.5 and 4.0; it is necessary to consider the possibility of a large salt error in this case (see Buffer properties, Indicator method). The Congo paper prepared for use and dried should be stored in a dark, tightly closed jar.