Indigo Carmine (Acid Blue 74)

Indigo Carmine

Other Names: Acid Blue 74, Indigotine, Disodium indigotindisulfonate, Sodium indigotindisulfonate
Chemical Formula: C₁₆H₈N₂Na₂O₈S₂
Molar Mass: 466.4 g/mol
CAS Number: 860-22-0
Appearance: Dark blue to blue-violet powder
SMILES: C1=CC2=C(C=C1S(=O)(=O)[O-])C(=C(N2)C3=NC4=C(C3=O)C=C(C=C4)S(=O)(=O)[O-])O.[Na+].[Na+]

Indigo Carmine is a water-soluble synthetic organic dye belonging to the indigoid class of compounds. Chemically, it is the disodium salt of indigodisulfonic acid and contains a conjugated indigo chromophore substituted with two sulfonate groups. The extended conjugated electronic system is responsible for its intense blue coloration, while the negatively charged sulfonate groups substantially increase its compatibility with aqueous media compared with unsubstituted indigo. Indigo Carmine has been extensively investigated in spectroscopy, photochemistry, electrochemistry and environmental chemistry. Quantum-chemical studies have examined its molecular geometry, vibrational spectrum, electronic structure, charge distribution and HOMO-LUMO characteristics, demonstrating that its conjugated structure supports significant intramolecular electronic delocalization.

Chemical and physical properties

Indigo Carmine is an ionic organic compound with a molecular mass of approximately 466.4 g/mol. The presence of two sulfonate groups and two sodium counterions gives the molecule strongly polar and ionic characteristics and promotes dissolution in aqueous systems.

The characteristic blue color of Indigo Carmine results from absorption in the visible region associated with electronic transitions within its extended conjugated chromophore. Spectroscopic investigations of aqueous Indigo Carmine solutions have reported a principal visible absorption maximum at approximately 610 nm, although the precise spectrum can depend on solution composition, concentration, ionic strength and chemical environment.

Indigo Carmine exhibits electrochemical activity and can participate in oxidation-reduction reactions. Its electrochemical behavior has therefore been investigated by cyclic voltammetry and other electroanalytical techniques. Changes in oxidation state alter the conjugated electronic structure of the dye and consequently its optical properties, making it useful as a model compound in studies combining electrochemistry and spectroscopy.

The conjugated chromophore can be chemically, electrochemically or photochemically degraded. Scientific investigations have shown that oxidative treatment and semiconductor photocatalysis can disrupt the conjugated structure, causing decolorization followed by further degradation of the organic molecule. TiO₂-based photocatalytic studies have demonstrated extensive oxidation of Indigo Carmine in aqueous systems and have been used to investigate reaction intermediates and degradation pathways.

Applications

Indigo Carmine is used as a blue dye in textile-related applications and is widely employed as a model indigoid dye in research on dye adsorption, oxidation, photodegradation and wastewater-treatment processes. Its intense visible absorption enables convenient monitoring of concentration changes by UV-Vis spectroscopy.

In analytical chemistry, Indigo Carmine is employed as an oxidation-reduction and acid-base indicator. Its reversible or chemically induced spectral changes make it useful for studying electron-transfer processes, reaction kinetics and electrochemical sensing systems.

Indigo Carmine is also used as a staining compound in microscopy and biological laboratory procedures. Its strong visible coloration and aqueous compatibility support its use in experimental visualization and staining techniques.

In materials and photochemistry research, Indigo Carmine has been investigated as a visible-light-absorbing molecular component in photochemical systems. Studies have demonstrated its use as a photoinitiating component for polymerization and high-resolution three-dimensional printing, taking advantage of its strong absorption in the visible spectral region and its ability to participate in photoinduced electron-transfer reactions.

Indigo Carmine is extensively used as a model contaminant in advanced oxidation and photocatalysis research. TiO₂, MnO₂, metal oxides, semiconductor heterojunctions and other catalytic materials have been evaluated for its degradation, allowing researchers to investigate photocatalytic mechanisms, reactive oxygen species, adsorption behavior and mineralization of persistent organic dyes in aqueous media.

Scientific references

1. PubChem, National Center for Biotechnology Information. Indigo Carmine, CID 2723854. Molecular formula, molecular mass, identifiers, structure and SMILES.
2. Manivannan, S.; Dhanuskodi, S. Quantum chemical studies on structural, vibrational, nonlinear optical properties and chemical reactivity of indigo carmine dye. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2017, 183, 284-290. DOI: 10.1016/j.saa.2017.04.047.
3. Vautier, M.; Guillard, C.; Herrmann, J.-M. Photocatalytic Degradation of Dyes in Water: Case Study of Indigo and of Indigo Carmine. Journal of Catalysis, 2001, 201(1), 46-59. DOI: 10.1006/jcat.2001.3232.
4. Zhan, W.; Du, Y.; Lan, J.; Lei, R.; Li, R.; Du, D.; Zhang, T. C. Electrochemical degradation of indigo carmine by low voltage pulse electrolysis. Journal of Molecular Liquids, 2022, 348, 118006. DOI: 10.1016/j.molliq.2021.118006.
5. de Andrade, F. V.; de Lima, G. M.; Augusti, R.; Coelho, M. G.; Ardisson, J. D.; Romero, O. B. A versatile approach to treat aqueous residues of textile industry: The photocatalytic degradation of Indigo Carmine dye employing the autoclaved cellular concrete/Fe₂O₃ system. Chemical Engineering Journal, 2012, 180, 25-31. DOI: 10.1016/j.cej.2011.10.089.
6. Zhu, D.; Peng, X.; Xiao, P. Indigo carmine: A base and neutral electrolyte-mediated photoinitiator for 3D printing in high fidelity. Additive Manufacturing, 2022. Study of visible-light absorption and photochemical behavior of Indigo Carmine.
7. Abdel-Maksoud, Y. K.; Imam, E.; Ramadan, A. R. MnO₂ efficiently removes indigo carmine dyes from polluted water. Heliyon, 2018, 4(11), e00897. DOI: 10.1016/j.heliyon.2018.e00897.

Safety

Precautionary Statements Prevention Avoid breathing dust/fume/gas/mist/vapors/spray Contaminated work clothing should not be allowed out of the workplace Response IF ON SKIN: Wash with plenty of soap and water If skin irritation or rash occurs: Get medical advice/attention Take off contaminated clothing and wash it before reuse Disposal Dispose of contents/container to an approved waste disposal plant Other Hazards Light sensitive