Congo Red

Congo red is aniline dye or coal tar dyes, and it has many types with very wide application. Its drawback is easy to fade by sunlight; aniline blue, bright green, methyl green are more easily fade. Be careful to handle the pH, and avoid direct sunlight when flaking, thus it will keep a few years without fading.
Congo red is an acid dye, a purplish red powder. It is soluble in water and alcohol, turning blue when in acid. It can be used as a dye as well as an indicator. In the production of plant slide, it is often used as a liner agent for hematoxylin or other cell dyes. When used in cytoplasm dyes, it can dye the cytosol or fibrin red. In the process of tissue slicing, it is used to dye nerve axis, elastic fibers, embryonic materials, etc. In addition, Congo red can be used for double staining with hematoxylin; it can also be used for amyloid staining. For it is soluble in water and alcohol, so washing and dehydration treatment should be quick.

Congo red is red-brown powder, soluble in hot water and the cold water 10 times of its weight; the color of the solution is yellow-red; soluble in ethanol and the solution is orange; slightly soluble in acetone, while almost insoluble in ether. The solution is dark blue when in sulfuric acid, turning to light blue accompanied with the appearance of blue precipitate when diluted.
Its aqueous solution will form a blue floc when in concentrated hydrochloric acid. When added with acetic acid, the precipitate turned blue with a little purple to a precipitate with some red color.  It is insoluble in the concentration of caustic soda solution; susceptible to acid and salt which means even if its absorption of carbon dioxide from the air will make the color blue and dark, while dilute soda solution can restore the original color.
Information on the chemical properties, uses and production methods of artificial dyes Congo Red is compiled and edited by Chemcialbook.

1. It has been widely used in cotton, viscose dyeing. Because it can easily be transformed into blue when in acid, while there is no appropriate treatment to improve, the usage amount is gradually decreased since the advent of insoluble azo dyes and acid-resistant red 4BS, however, the amount used in the paper industry is quite large. It can also be used as indicators of acid-base, namely Congo red test paper. The measurable range pH is 3.0 (blue)-5.0 (red). It is also used for biological staining, analysis reagents.
2. An indicator frequently used in lab as well as a carcinogenic disperse dyes; used to standard test.
3. Used as an acid-base indicator, adsorption indicator and biological stain, etc.
4. Acid-base indicator, pH 3.0 (blue violet)~5.0 (red); detect the acidity of paper and hydrochloric acid in the gastric juice; test boric acid, cyanide; biological media additives; biological staining, such as embryo slices, plant mucin, cellulose, elastic tissue dyeing.
5. Mainly used for dyeing cotton, hemp, silk and other textile and paper products; it can also be used as an indicator.

It is obtained by the coupling with  sodium naphthionate after the diazotization with benzidine, and then by salting out, filtration and drying in the system. The details are as follows: add 48.4 benzidine into 300ml of hydrochloric acid take 48.4g benzidine dissolved in 300ml hydrochloric acid (including 20ml of concentrated hydrochloric acid 20ml), add ice into the solution until the temperature is below 5 ℃; add 10% aqueous solutions prepared by 14.4g sodium nitrite after adding 30ml of concentrated hydrochloric acid.Add the diazonium salt solution slowly into the solution prepared by 150g sodium naphthionate and a little water after diazotization. Add 35g sodium carbonate slowly under the sufficient mixing after 30 minutes' standing, thus appearing an alkaline solution. Heating to 80℃, cooling, filtering, washing with saturated brine and drying can obtain the product. The materials consumption (kg/t) in industrial production are as follows: benzidine (100%) 200, sodium naphthionate (100%) 520, sodium nitrite (industry) 160, hydrochloric acid (31%) 330, soda ash (industrial) 340, fine salt 2500, turkey red oil 9, ethanol 60, sodium acetate 35, sodium sulfate 50.

C.I. Direct red 28(Congo Red) is an odorless,brownish-red powder. Molecular weight =698.72. Meltingpoint =359℃. Soluble in water; solubility =25 g/L at25℃.

brown-red powder

C.I. Direct red 28(Congo Red) is an odorless, brownish-red powder.

Dye, medicine (diagnostic aid), indicator, biological stain.

Congo red is useful for staining and visual detection of amyloid deposits. It inhibits the accumulation of the scrapie-associated, protease-resistant isoform of protein PrP without affecting the metabolism of the normal isoform.

ChEBI: An indicator dye that is blue-violet at pH 3.0 and red at pH 5.0.

Congo red is a benzidine-based anionic diazo dye. It is a histological dye which binds to many amyloid proteins and is used for the quantification of amyloid β-peptide aggregation. Congo red also interacts with β-D-glucans, polysaccharides containing continuous β-(1→4)-linked D-glucopyranosyl units and some hemicellulosic galactoglucomannans.

Human poison by ingestion with cardovascular effects. Experimental poison by intravenous route. An experimental teratogen. Experimental reproductive effects. An eye irritant. Mutation data reported. When heated to decomposition it emits very toxic fumes of NOx, SOx, and Na2O

It is used as an indicator dye, a biological stain; a diagnostic aid in medicine; and a dye for fabric and paper.

If this chemical gets into the eyes, remove anycontact lenses at once and irrigate immediately for at least15 min, occasionally lifting upper and lower lids. Seek medical attention immediately. If this chemical contacts theskin, remove contaminated clothing and wash immediatelywith soap and water. Seek medical attention immediately. Ifthis chemical has been inhaled, remove from exposure,begin rescue breathing (using universal precautions, including resuscitation mask) if breathing has stopped and CPR ifheart action has stopped. Transfer promptly to a medicalfacility. When this chemical has been swallowed, get medical attention. If victim is conscious, administer water ormilk. Do not induce vomiting.

Room temperature

UN3143 Dyes, solid, toxic, n.o.s. or Dye intermediates, solid, toxic, n.o.s., Hazard Class: 6.1; Labels: 6.1-Poisonous materials, Technical Name Required

Crystallise the dye from aqueous EtOH (1:3). Dry it in air. [Beilstein 6 I 342.]

Contact with oxidizers may cause fire and explosion hazard. Incompatible with strong acids; reducing agents.