Indigo dye

Indigo dye is an important dyestuff with a distinctive blue color (see indigo). The natural dye comes from several species of plant, but nearly all indigo produced today is synthetic. Among other uses, it is used in the production of denim cloth for blue jeans.

Chemical properties

Indigo is a dark blue crystalline powder that melts at 390°–392°C. It is insoluble in water, alcohol, or ether but soluble in chloroform, nitrobenzene, or concentrated sulfuric acid. The chemical structure of indigo corresponds to the formula C16H10N2O2.

Related Topics:
Alcohol - Ether - Chloroform - Nitrobenzene - Sulfuric acid - C - H - N - O

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The naturally occurring substance is indican, which is colorless and soluble in water. Indican can easily be hydrolyzed to glucose and indoxyl. Mild oxidation, such as by exposure to air, converts indoxyl to indigo.

Related Topics:
Indican - Hydrolyzed - Glucose - Oxidation

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The manufacturing process developed in the late 1800s is still in use throughout the world. In this process, indoxyl is synthesized by the fusion of sodium phenylglycinate in a mixture of sodium hydroxide and sodamide.

Related Topics:
Sodium - Sodium hydroxide

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Several simpler compounds can be produced by decomposing indigo; these compounds include aniline and picric acid. The only chemical reaction of practical importance is its reduction by urea to indigo white. The indigo white is reoxidized to indigo after it has been applied to the fabric.

Related Topics:
Aniline - Picric acid - Urea

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Indigo treated with sulfuric acid produces a blue-green color. It became available in the mid-1700s. Sulfonated indigo is also referred to as Saxon blue or indigo carmine.

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Tyrian purple was a valuable purple dye in antiquity. It was made from excretions of a common Mediterranean Sea snail. In 1909 its structure was shown to be 6,6′-dibromoindigo. It has never been produced synthetically on a commercial basis.

Related Topics:
Tyrian purple - Mediterranean Sea - 1909

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