chemistry

Indigo carmine

Indigo carmine is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Indigo carmine rather than just read about it. In short: Indigo carmine, or 5,5′-indigodisulfonic acid sodium salt, is an organic salt derived from indigo by aromatic sulfonation, which renders the compound soluble in water. Like indigo, it produces a blue color, and is used in food and other consumables, cosmetics, and as a medical contrast agent and staining agent; it also acts as a pH indicator.

Key takeaways

Indigo carmine belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Indigo carmine to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Indigo carmine from memory before moving on to harder problems.

Reference excerpt

Indigo carmine, or 5,5′-indigodisulfonic acid sodium salt, is an organic salt derived from indigo by aromatic sulfonation, which renders the compound soluble in water. Like indigo, it produces a blue color, and is used in food and other consumables, cosmetics, and as a medical contrast agent and staining agent; it also acts as a pH indicator. It is approved for human consumption in the United States and European Union. It has the E number E132, and is named Blue No. 2 by the US Federal Food, Drug, and Cosmetic Act.

Uses

Indigo carmine in a 0.2% aqueous solution is blue at pH 11.4 and yellow at 13.0. Indigo carmine is also a redox indicator, turning yellow upon reduction. Another use is as a dissolved ozone indicator through the conversion to isatin-5-sulfonic acid. This reaction has been shown not to be specific to ozone: it also detects superoxide, an important distinction in cell physiology. It is also used as a dye in the manufacturing of pharmaceutical capsules.

Medical uses Indigotindisulfonate sodium, sold under the brand name Bludigo, is used as a contrast agent during surgical procedures. It is indicated for use in cystoscopy in adults following urological and gynecological procedures. It was approved for medical use in the United States in July 2022. In obstetric surgery, it may be used to detect amniotic fluid leaks. In urologic surgery, intravenous indigo carmine can be used to highlight portions of the urinary tract. The dye is filtered rapidly by the kidneys from the blood, and colors the urine blue. However, the dye can cause a potentially dangerous acute increase in blood pressure in some cases. Indigo carmine stain is not absorbed into cells, so it is applied to tissues to enhance the visibility of mucosa. This leads to its use for examination and diagnosis of benign and malignant lesions and growths on mucosal surfaces of the body.

Food, pharmaceutical, cosmetic, and scientific uses Indigo carmine is one of the few blue food colorants. Others include the anthocyanidins and rare substances such as variegatic acid and popolohuanone.

Safety and regulation

Indigo carmine shows "genotoxicity, developmental toxicity or modifications of haematological parameters in chronic toxicity studies". Only at 17 mg/kg of body weight per day were effects on testes observed.

References

Illustrations

Worked examples

Example 1 — a first encounter with Indigo carmine

Start with the simplest possible case. Write down what Indigo carmine claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Indigo carmine before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Indigo carmine ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Indigo carmine

In research: Indigo carmine appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Indigo carmine in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Indigo carmine is common in secondary-school and first-year university syllabi. It links to neighbouring topics Acid dyes, E-number additives, Enones, so understanding it makes those chapters shorter.
In everyday life: Look for Indigo carmine outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Indigo carmine in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Indigo carmine means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Indigo carmine out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Indigo carmine in simple terms?

Why does Indigo carmine matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Indigo carmine?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Indigo carmine.