Congo red (CR) is a synthetic azo dye used as a biological stain and pH indicator. It binds to cellulose and amyloid fibrils, producing a characteristic red color under the microscope.
| IUPAC Name | disodium 3,3′-(4,4′-biphenylylenebis(azo))bis(4-aminonaphthalene-1-sulfonate) |
| Molecular Formula | C32H22N6Na2O6S2 |
| CAS Number | 573-58-0 |
| Synonyms | Congo red B, Direct Red 28, C.I. 22120, C.I. Direct Red 28, C.I. Direct Red 28:1, C.I. 22120 |
| InChI | InChI=1S/C32H24N6O6S2.2Na/c33-29-17-9-21-13-25(37)19-23(21)27(39)15-31(41(35,36) 38) 7-1-3-11- 5-4-10-12-6-2-8-22-14-26(38)20-24(18-29)28(40)16-32(42(43)44) 30-19-19/h1- 18H,33-34H2,(H,35,36,38)(H,43,44)/q+2;;/p-2 |
Congo Red Molar Mass
CR has a molar mass of approximately 696.67 g/mol. Molar mass is the mass of one mole of a substance, which is determined by adding up the atomic masses of all the atoms in the molecule. In the case of CR, the molecule is made up of 32 carbon atoms, 22 nitrogen atoms, 6 oxygen atoms, 2 sulfur atoms, and 2 sodium atoms. The molar mass of CR is an important factor in determining the amount of the substance needed for a particular application or experiment.
Congo Red Boiling Point
CR does not have a boiling point in the traditional sense because it decomposes before it boils. However, the temperature at which CR decomposes is typically around 350-360°C. The decomposition of CR is characterized by the release of toxic gases, so it is important to handle the substance with care when heating it.
Congo Red Melting Point
CR has a melting point of approximately 240°C. The melting point is the temperature at which a substance changes from a solid to a liquid state. The melting point of CR is important in determining the conditions under which the substance can be melted and used in various applications.
Congo Red Density g/ml
CR has a density of approximately 1.5 g/mL. Density is the measure of the mass per unit volume of a substance. The density of Congo red is an important factor in determining how the substance will behave in various applications, such as in biological staining procedures or in pH indicator solutions.
Congo Red Molecular Weight
CR has a molecular weight of approximately 696.67 g/mol. The molecular weight of a substance is the sum of the atomic weights of all the atoms in a molecule. The molecular weight of CR is an important factor in determining the substance’s physical and chemical properties, such as its boiling and melting points, density, and solubility.
Congo Red Structure
CR has a complex structure that consists of two naphthalene rings linked by a biphenyl unit. The molecule also contains two azo groups (-N=N-) and two sulfonate groups (-SO3^-) that are attached to the naphthalene rings. The two sulfonate groups are negatively charged, while the two sodium atoms are positively charged, giving the molecule an overall negative charge.
Congo Red Formula
The chemical formula of CR is C32H22N6Na2O6S2. The formula indicates the number and types of atoms in the molecule. The presence of two sodium atoms in the formula reflects the fact that Congo red is a salt of the disodium cation and the CR anion. The chemical formula of CR is important in understanding its chemical and physical properties, such as its solubility in water and its reactivity with other chemicals.
| Appearance | Red powder |
| Specific Gravity | 1.5 g/mL |
| Color | Red |
| Odor | Odorless |
| Molar Mass | 696.67 g/mol |
| Density | 1.5 g/mL |
| Melting Point | Approx. 240°C |
| Boiling Point | Decomposes before boiling |
| Flash Point | Not applicable |
| Water Solubility | Soluble |
| Solubility | Soluble in water, ethanol, acetone |
| Vapour Pressure | Not applicable |
| Vapour Density | Not applicable |
| pKa | 2.5 – 4.0 (sulfonic acid) |
| pH | 3.0 – 5.0 (in water) |
Congo Red Safety and Hazards
CR can pose several safety hazards if not handled with care. The substance can cause irritation if it comes into contact with the skin or eyes, and it can also cause respiratory problems if inhaled. In addition, CR is combustible and can release toxic gases when heated to decomposition. Proper protective equipment, such as gloves and goggles, should be worn when handling CR, and the substance should only be used in well-ventilated areas. It is also important to follow proper disposal procedures for CR and any contaminated materials to prevent environmental contamination.
| Hazard Symbols | Xi, N |
| Safety Description | Avoid breathing dust/fume/gas/mist/vapors/spray. Wear protective gloves/protective clothing/eye protection/face protection. Dispose of contents/container in accordance with local/regional/national/international regulations. |
| UN ID | UN2811 |
| HS Code | 3204.13.00 |
| Hazard Class | 6.1 (Toxic substances) |
| Packing Group | III |
| Toxicity | Oral rat LD50: 400 mg/kg; Inhalation rat LC50: > 16 mg/L (4 hours); Dermal rabbit LD50: > 2000 mg/kg |
Congo Red Synthesis Methods
To synthesize CR, one can use a multi-step process that involves reacting sulfanilic acid with 2-naphthol. Next, the resulting product undergoes diazotization and couples with β-naphthol-3,6-disulfonic acid to create CR.
The first step in the synthesis of CR is the reaction between sulfanilic acid and 2-naphthol in the presence of hydrochloric acid. The reaction produces 4,4′-diaminodiphenyl-2,2′-disulfonic acid, which undergoes oxidation with sodium nitrite and hydrochloric acid to form the diazonium salt.
To produce CR, the reaction involves coupling the diazonium salt with β-naphthol-3,6-disulfonic acid in the presence of sodium carbonate. The reaction is carefully controlled in terms of temperature and pH to achieve a high yield of product.
Another method of synthesizing CR involves the reaction of 2-naphthol-3,6-disulfonic acid with 4-aminobenzenesulfonic acid in the presence of sodium nitrite and hydrochloric acid. To form CR, the chemist couples the resulting diazonium salt with 2-naphthol-6,8-disulfonic acid.
People use CR as a red, water-soluble dye in histology and as an indicator in chemical reactions. It forms through both methods.
Congo Red Uses
CR has a variety of uses in different fields. Some of the common uses of Congo red are:
- Histology: In histology, CR stains amyloid protein deposits that are linked with various diseases like Alzheimer’s and Parkinson’s.
- Chemical indicator: Used as an indicator in chemical reactions. Congo red changes its color from red to blue as the pH of a solution changes from acidic to alkaline.
- Microbiology: Used in microbiology to differentiate between bacterial strains. Used in agar plates to distinguish between bacteria that produce cellulase, which is responsible for breaking down cellulose.
- Textile industry: Used in the textile industry as a dye for cotton, silk, and wool. Used in the production of carpets and textiles.
- Photography: Used in photography as a filter to enhance the contrast between different colors in photographs.
- Research: Used in scientific research to study the binding of proteins and other molecules.
- Education: Used in education as a teaching aid to demonstrate chemical reactions and properties of dyes.
Overall, CR is a versatile substance with various applications in different fields, ranging from medical research to industrial production.
Questions:
Q: Did you notice any significant changes when the Congo red fabric was treated with acid?
A: I am an artificial intelligence language model and do not have the ability to conduct experiments or observations. However, it is known that CR can change color from red to blue as the pH of a solution is changed from acidic to alkaline.
Q: Is Congo red acidic or basic?
A: CR is an acidic dye and has a pH of around 4.0 – 5.0 in aqueous solution.
Q: What is the purpose of Congo red in capsule staining?
A: CR is used in capsule staining to visualize the capsule surrounding bacterial cells. The dye binds to the capsule, which is typically composed of polysaccharides, and allows the capsule to be seen under a microscope. Capsule staining is commonly used in microbiology to identify the presence of bacterial capsules and is particularly useful in identifying pathogenic strains.