Omar Bennett
Indigo dye is an organic compound with a distinctive blue colour. It is a natural dye obtained from the leaves of some plants of the Indigofera genus, in particular, Indigofera tinctoria. The chemical formula of indigo is C16H10N2O2. Indigo is insoluble in water, alcohol, or ether, but soluble in DMSO, chloroform, nitrobenzene, and concentrated sulfuric acid. Indigo can be converted to numerous simpler compounds, but the only chemical reaction of practical importance is its reduction to the soluble yellow leuco indigo, in which form it is applied to textile fibres and reoxidized to indigo.
| Characteristics | Values |
|---|---|
| Chemical Formula | C16H10N2O2 |
| Sublimation Point | 390-392 °C (734-738 °F) |
| Solubility | Insoluble in water, alcohol, or ether |
| Soluble in | DMSO, chloroform, nitrobenzene, and concentrated sulfuric acid |
| Absorption Spectrum | Orange part of the spectrum (λmax=613 nm) |
| Colour | Dark blue |
| Precursor | Indican |
| Natural Sources | Indigofera tinctoria, Indigofera suffruticosa, Indigofera arrecta, Isatis tinctoria, mollusks |
| Reduction Method | Dissolving in a heated vat with thermophilic, anaerobic bacteria |
| Reduction Agent | Sugars, some natural dyes or other plants, or chemicals |
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What You'll Learn
- Indigo is a natural dye obtained from the leaves of plants of the Indigofera genus
- The chemical formula of indigo is C16H10N2O2
- Indigo dye is insoluble in water, alcohol, or ether
- Indigo is a vat dye, which means it needs to be chemically altered before use
- Indigo is stabilized by intramolecular hydrogen bonding
Indigo is a natural dye obtained from the leaves of plants of the Indigofera genus
Indigo dye is an organic compound with a distinctive blue colour. Indigo is a natural dye primarily obtained from the leaves of plants of the Indigofera genus, in particular, Indigofera tinctoria. Indigofera plants were once common throughout the world, but today, most indigo dye is synthetic, with around 80,000 tonnes produced annually as of 2023.
Indigo is known for its deep blue colour and high potency compared to other blue dyes. Due to the rarity of usable blue pigments in nature, indigo has historically been a prized good, especially in the Indian subcontinent, where it was used for dying textiles for local use and international trade. It played a significant role in the global economy during the medieval and colonial periods. The Indigofera tinctoria species was domesticated in India, and the country was a major centre for its production and processing. British policies in Bengal, such as the Tinkathia system, made it mandatory for landowners to grow indigo, leading to exploitation and the Indigo Revolt of 1859.
The synthesis of indigo dye involves the extraction of indican, a water-soluble derivative of the amino acid tryptophan, from Indigofera leaves. The leaves are pressed into a vat, soaked, and hydrolyzed, releasing β-D-glucose and indoxyl. After fermentation, the indoxyl dimerizes to form the yellow, water-soluble leucoindigo, which, upon exposure to air, transforms into the blue, water-insoluble indigo dye. This process is repeated in a series of dips to achieve deeper shades of blue.
The chemical formula of indigo is C16H10N2O2, and it sublimates at 390-392 °C. Indigo is insoluble in water, alcohol, or ether but soluble in certain solvents like DMSO and chloroform. The compound owes its deep colour to the conjugation of double bonds within the molecule. The benzene rings in indigo can be modified to create different shades of dye, such as thioindigo and Tyrian purple.
Indigo is primarily used for dyeing cotton yarn, especially in the production of denim cloth and blue jeans. The denim fabric is then used to create items such as blue jeans, which require an average of 3 to 12 grams of dye per pair. Indigo is also used in techniques like shibori (tie-dye), kasuri, katazome, and tsutsugaki, as seen in the works of artists like Hokusai.
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The chemical formula of indigo is C16H10N2O2
Indigo dye is an organic compound with a distinctive blue colour. The chemical formula of indigo is C16H10N2O2, and it is a dark blue crystalline powder that sublimes at 390–392 °C (734–738 °F). Indigo is insoluble in water, alcohol, or ether, but soluble in DMSO, chloroform, nitrobenzene, and concentrated sulfuric acid. The molecule absorbs light in the orange part of the spectrum (λmax=613 nm).
The compound owes its deep colour to the conjugation of the double bonds, i.e. the double bonds within the molecule are adjacent and the molecule is planar. In indigo white, the conjugation is interrupted because the molecule is non-planar. Indigo is a natural dye obtained from the leaves of some plants of the Indigofera genus, in particular, Indigofera tinctoria. Indigofera leaves contain as much as 0.2–0.8% of a colorless, water-soluble derivative of the amino acid tryptophan called indican.
The dye-bearing Indigofera plants were once common throughout the world. It is now produced via chemical routes. Blue colorants are rare. Since indigo is insoluble, it is also referred to as a pigment (C.I. Pigment Blue 66, C.I.). Indigo is challenging to work with because it is not soluble in water. To dissolve it, indigo must undergo a chemical change (reduction). Reduction converts indigo into "white indigo" (leuco-indigo).
Most indigo dye produced today is synthetic, constituting around 80,000 tonnes each year, as of 2023. It is most commonly associated with the production of denim cloth and blue jeans, where its properties allow for effects such as stone washing and acid washing to be applied quickly.
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Indigo dye is insoluble in water, alcohol, or ether
Indigo dye is an organic compound with a distinctive blue colour. It is a natural dye obtained from the leaves of some plants of the Indigofera genus, in particular, Indigofera tinctoria. However, it is now mostly produced synthetically, with around 80,000 tonnes being produced annually as of 2023. Indigo is a challenging dye because it is insoluble in water, alcohol, or ether. This means that it cannot simply be dissolved in water and applied to fabrics. Instead, it must undergo a chemical change (reduction) to be used as a dye.
To make indigo soluble, it must be converted into a leuco compound through a process called reduction. This involves treating the indigo with sodium hydroxide and sodium hydrosulphite in the presence of caustic soda at around 60–65°C. This process turns the indigo into a water-soluble form called "white indigo" or "leuco-indigo", which has a pale yellow colour. The fabric is then submerged in this solution, allowing the soluble leuco salt to be absorbed into the fibres. When the fabric is removed from the solution and exposed to air, the leuco-indigo quickly combines with oxygen and reverts to its original insoluble form, resulting in the fabric being dyed the distinctive blue colour of indigo.
The process of converting indigo into a water-soluble form and then back to an insoluble form is complex and requires precise knowledge of chemistry. This complexity has been a challenge for dyers throughout history, including in medieval Europe and 16th-century Europe, where dyers struggled with the unique properties of indigo. Even today, with modern synthetic indigo, multiple dips and intermediate squeezing are often required to achieve the desired depth of colour.
While indigo is insoluble in water, alcohol, and ether, it is soluble in certain other substances, such as DMSO, chloroform, nitrobenzene, and concentrated sulfuric acid. This solubility in specific solvents allows for the creation of different shades of blue and purple when mixed with the indigo powder. Additionally, bacteria-driven reduction of indigo in fermentation vats has been investigated as a potential eco-friendly alternative to chemical methods of vat dye reduction.
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Indigo is a vat dye, which means it needs to be chemically altered before use
The indigo pigment molecule must first be put into liquid suspension, as it cannot be dissolved in water like salt. This is done by creating a reduction vat, which removes all oxygen from the water and makes it alkaline. The indigo powder can then be dissolved into the water. Once dissolved, the indigo vat can be used to dye clothing, countertops, rugs, utensils, or other property.
The indigo vat liquid is alkaline and may be drying or irritating to the skin, so it is important to wear gloves when dipping items into the vat. The vat water is also very hot, nearly 180°F (90°C), so pot holders should be used when heating the water and during the dipping process. It is also recommended to have adult supervision when using indigo dye.
The indigo vat serves to modify the indigo molecule so that it can chemically interact with and dye cloth. The vat must fulfill two conditions to properly modify the indigo. First, the vat must have an elevated pH, a condition known as alkalinity, in which the amount of OH− ions exceeds that of H+ ions in the solution. Second, the vat must be oxygen-free.
Once the correct conditions are created, a piece of fiber can be dipped into the vat. When the fiber is pulled out, oxygen in the air forces the dissolved indigo out of the solution, and it chemically bonds to the fiber. The number of dips in the vat will determine the shade of blue, with more dips resulting in a darker color.
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Indigo is stabilized by intramolecular hydrogen bonding
Indigo dye is a dark blue crystalline powder that sublimes at 390–392 °C (734–738 °F). It is an organic compound with the chemical formula C16H10N2O2. Indigofera tinctoria, a plant species, is a common natural source of indigo. The dye is produced when Indigofera leaves are pressed and soaked, hydrolyzing the compound indican, which is present in the leaves, and releasing β-D-glucose and indoxyl. After 12–15 hours of fermentation, this yields water-soluble leucoindigo, which, when exposed to air, forms the water-insoluble indigo dye.
Intramolecular hydrogen bonding occurs between the adjacent carbonyl groups and the imino groups of the indigo molecule. This is also referred to as the conjugation effect, which gives rise to the deep colour of the dye. The molecule is planar, meaning that the double bonds within it are adjacent. This planarity distinguishes indigo from similar compounds such as indirubin, which has a small deviation from planarity and only one intramolecular hydrogen bond.
Indigo is used almost exclusively in the dyeing of denim, where its distinctive colour and fading characteristics are highly valued.
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Frequently asked questions
Indigo dye is an organic compound with a distinctive blue colour. It is a natural dye obtained from the leaves of some plants of the Indigofera genus.
The chemical formula of indigo is C16H10N2O2.
Indigo is used exclusively in the dyeing of denim due to its brilliant blue colour and excellent fading characteristics after washing and abrasion. It is most commonly associated with the production of denim cloth and blue jeans.
Indigo powder is extracted from the leaves of the indigo plant. Indigo is a vat dye, meaning it cannot simply be mixed with water. Instead, it must be chemically altered with additions of other substances in a vat to change its properties.
Indigo dye is insoluble in water. It must first be made water-soluble by converting it into a leuco compound. The dyed yarn is then run in the air, during which the soluble leuco salt is oxidised back into the original insoluble indigo.
