Indigo carmine powder is an important chemical substance with a wide range of applications. It is also called acid indigo, indigo carmine, indigo disulfonate sodium, Acid Blue 74, Indigotindisulfonate Sodium, etc. The chemical formula is C16H8N2Na2O8S2, dark blue powder with a copper-like luster. At 25°C, 1g is soluble in 100ml of water, slightly soluble in ethanol, and almost insoluble in other organic solvents. It is very sensitive to light and oxidants, easily faded by nitric acid and chlorate, and its aqueous solution can also fade after long-term storage. In my country, indigo carmine powder can be used in red and green silk, fruit juice (flavored) beverages, carbonated beverages, blended wine, candy, pastry decoration, dyed cherry canned food (for decoration), green plums and other foods, but the amount of use is strictly limited. It can be used for coloring cosmetic essences, toothpaste, floral water, etc., but it cannot be used in eye cosmetics. In the medical field, it can be used to determine renal function by observing the change in urine color after intravenous or intramuscular injection to determine whether renal function is normal. Indigo carmine powder can be used as an acid-base indicator and redox indicator, playing an important role in chemical experiments.
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| Chemical Formula | C16H11N2NaO8S2 |
| Molecular Weight | 446.38 |
| Melting point | >300°C |
| Storage conditions | room temp |
| Form | Powder |
| Color | Very dark blue to purple |
| Solubility | Stable. Incompatible with strong oxidizing agents. |
| Water solubility | 1 g/100 mL (25 ºC) |
As a versatile chemical, indigo carmine has a wide range of applications in many fields. The following is an overview of its main application areas
Food Industry: Indigo carmine powder is a synthetic pigment commonly used in the food industry, mainly used to add blue or green tones to food. It can be used in a variety of foods, such as red and green silk, juice (flavored) beverages, carbonated beverages, blended wine, candy, pastry decoration, dyed cherry cans (decoration), green plums, etc. However, its use is strictly limited to ensure food safety and compliance.
Cosmetics: In the cosmetics industry, indigo carmine powder is used as a colorant and can be used for coloring cosmetic fragrances, toothpaste, floral water and other products. However, it must not be used in eye cosmetics to avoid potential safety risks.
Medical Field: In medicine, indigo carmine powder can be used as a biological stain to help doctors highlight tissue changes during examinations. It is particularly suitable for observing subtle changes in mucous membranes, such as gaps, cysts and yellow spots. In addition, indigo carmine powder can also be used to determine renal function, which can be assessed by observing the change in urine color after a specific injection method.
Chemical indicator: As a chemical indicator, indigo carmine powder plays an important role in acid-base titration and redox reactions. It can change color according to the change in the pH value or redox state of the solution, thereby helping scientists and experimenters to accurately judge the progress and results of the reaction.
Analytical chemistry: In analytical chemistry, indigo carmine powder can be used to test the presence of substances such as nitrates, sulfites, and chlorates. Its specific chemical reaction properties make it an important tool in these analytical processes.
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The main uses of Indigo Carmine can be summarized as follows:
Food colorant: This is one of the most widely used areas of Indigo Carmine. It is mainly used to add blue or green tones to food to enhance the visual appeal of food and consumers' desire to buy. In the food industry, it can be used in a variety of foods such as red and green silk, juice (flavored) drinks, carbonated beverages, mixed wines, candies, pastry decoration, dyed cherry cans, etc. However, its use is strictly limited to ensure food safety and compliance with relevant regulations.
Cosmetic colorant: Indigo Carmine is also commonly used for coloring cosmetics. It can provide stable colors for cosmetic fragrances, toothpaste, floral water and other products to meet consumers' needs for product appearance. However, it is worth noting that it must not be used in eye cosmetics to avoid potential safety risks.
Biological dye: In the medical field, Indigo Carmine can be used as a biological dye. It helps doctors highlight subtle changes in tissues during examinations, such as gaps in mucous membranes, cysts and yellow spots. This dye can help doctors more accurately judge the condition and formulate treatment plans.
Chemical indicators: Indigo Carmine can be used as an acid-base indicator and redox indicator in chemical experiments. It can change color according to the change of pH value or redox state of the solution, thus helping scientists and experimenters to accurately judge the reaction process and results. This indicator plays an important role in chemical analysis and laboratory research.
Substance detection: In analytical chemistry, Indigo Carmine can be used to detect the presence of specific substances. Its specific chemical reaction properties make it an important tool in the detection process of substances such as nitrates, sulfites, and chlorates. By utilizing the chemical reaction between Indigo Carmine and these substances, scientists can accurately determine their presence and concentration.
In summary, the main uses of Indigo Carmine include as a food colorant, cosmetic colorant, biological stain, chemical indicator, and substance detection tool. However, due to its potential safety risks and usage restrictions, it needs to strictly comply with relevant laws and regulations when using it to ensure its safety and compliance.
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Industrial Applications
Food Industry
As a permitted synthetic food colorant (E132 in the EU, FD&C Blue #2 in the US), Indigo Carmine is used to enhance the visual appeal of:
Confectionery: Chocolates, candies, and icing (max 100 mg/kg).
Beverages: Carbonated drinks, fruit juices, and alcoholic mixers (max 0.1 g/kg).
Processed foods: Pickled vegetables (max 0.01 g/kg), canned cherries, and preserved plums.
Its instability under acidic conditions limits its use to alkaline or neutral formulations. Recent studies highlight challenges in detecting residual Indigo Carmine in complex food matrices, necessitating advanced extraction techniques like aqueous biphasic chromatography (ABEC) and electrospun nanofiber adsorption.
Medical and Diagnostic Uses
Indigo Carmine's biocompatibility and vivid color make it invaluable in:
Endoscopy: As a contrast agent for mucosal visualization, highlighting lesions in inflammatory bowel disease (IBD) and gastrointestinal cancers. Administered intravenously, it transiently stains urine blue, aiding in ureteral identification during pelvic surgeries.
Renal Function Tests: Acting as a redox indicator, its color shift from blue (oxidized) to pink (reduced) helps assess glomerular filtration rates.
Histology: Staining cell nuclei and connective tissues in microscopic examinations.
Biotechnology and Research
Enzyme Assays: Serving as a substrate for peroxidase and laccase activity measurements.
Cell Culture: Tracking cell migration in wound-healing assays via colorimetric imaging.
Microbiology: Differentiating bacterial colonies on agar plates through differential staining.
The synthesis of Indigo Carmine is a relatively complex process, usually involving multiple steps and chemical reactions. The following is a generalized synthesis route.
- Overview of the synthesis route
The synthesis of Indigo Carmine usually starts from indigo, and its sodium sulfonate salt form, i.e., indigo carmine powder, is obtained through sulfonation, neutralization, salting out and other steps.
Sulfonation of indigo
Indigo is subjected to sulfonation reaction with concentrated sulfuric acid or fuming sulfuric acid. Sulfonation is a reaction that introduces sulfonic acid groups (-SO₃H) or sulfonyl chloride groups (-SO₂Cl) into organic compounds, mainly introducing sulfonic acid groups here.
After the sulfonation reaction, some hydrogen atoms in the indigo molecule are replaced by sulfonic acid groups to form sulfonated indigo.
Neutralization reaction
The sulfonated indigo solution is diluted with water, and then an alkaline substance such as sodium carbonate or sodium hydroxide is added for neutralization.
The purpose of the neutralization reaction is to convert the sulfonic acid group into the sodium sulfonate salt form, while adjusting the pH value of the solution to an appropriate range.
Salting out and refining
Add salts such as sodium chloride to the neutralized solution for salting out, so that indigo carmine powder is precipitated in the form of sodium salt.
The finished indigo carmine powder is obtained by filtering, washing with water, drying and other steps.
- Precautions
During the synthesis process, the reaction conditions such as temperature, pressure, reaction time, etc. need to be strictly controlled to ensure the quality and yield of the product.
Sulfonation reaction is an exothermic reaction, and appropriate cooling measures need to be taken to prevent overheating of the reaction.
After the neutralization reaction, the pH value of the solution should be carefully adjusted to avoid product decomposition or impurities.
Salting out and refining steps are essential to improve the purity and stability of the product.
Innovations and Future Directions
► Green Synthesis
Researchers are exploring enzymatic sulfonation of indigo using sulfotransferases, reducing reliance on hazardous sulfuric acid. A 2023 study demonstrated 85% yield using immobilized bacterial enzymes, offering a sustainable alternative to chemical methods.
► Advanced Detection Methods
Electrochemical Sensors: N-ethylaniline-modified carbon paste electrodes achieve detection limits of 0.36 μM, enabling real-time monitoring in food and water.
Nanotechnology: Gold nanoparticle-based colorimetric assays detect Indigo Carmine at parts-per-trillion levels, crucial for environmental compliance testing.
► Biodegradable Alternatives
Natural dyes like spirulina extract (phycocyanin) and gardenia blue are gaining traction, though challenges remain in cost, stability, and scalability. Hybrid approaches combining natural pigments with microencapsulation technologies show promise for high-performance applications.
Indigo carmine powder is a versatile and widely used synthetic dye with applications spanning food, pharmaceuticals, cosmetics, textiles, and medical diagnostics. Its vibrant blue color, water solubility, and stability make it indispensable in many industries. While generally safe when used within regulatory limits, careful handling and storage are essential to minimize risks. As research advances, indigo carmine may find new applications in biomedical imaging, smart materials, and sustainable manufacturing. By adhering to strict quality and safety standards, the industry can ensure the continued relevance and safety of this iconic blue dye. Always consult regulatory guidelines and safety data sheets before using indigo carmine in any application.
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