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The Chemistry of Indigo, page 1 of 2

Figure 1. C₁₆ H₁₀ N₂ O₂

Indigo is a powder, insoluble in water, with a melting point higher than 300C. It absorbs light in the yellow region of the spectrum (maximum at 602 nm), which gives it its intense blue colour. The indigo molecule is relatively small with molecular weight of 262.27 atomic units of mass. In the molecular models illustrated on this page carbon is shown in grey, oxygen in bright red, nitrogen in blue, bromine in deep red and hydrogen in white. Conventionally, hydrogen atoms are not shown in diagrams, as their position and number is implied by the valence of carbon and other atoms, to which they are attached. Figures 1, and 2 show indigo molecules in this abbreviated form. Indigo is derived from two indoxyl rings joined at position 7 on each ring. Derivatives, where hydrogen substitutions in positions 6,6' for bromine or in positions 5,5' for SO3.Na result in related dye molecules giving purple or carmine colour.

Blue indigo does not react with and will not bind chemically to fabric fibres, as most dyestuffs do. However, in the presence of a reducing agent (e.g. alkaline pH) it can undergo a two stage reduction (gain of electrons) on the oxygen atoms at positions 3 and 3' to a soluble form. Oxidation - reduction occurs when electrons are transferred from a donor (oxidising agent) to an acceptor (reducing agent). An indigo dye vat provides a reducing environment to keep the dye in solution. In this form indigo will penetrate and adhere to fabric fibres. When the fabric is removed from the vat in-situ oxidation of indigo occurs and the oxidation traps the molecule in the matrix of the fabric. This process is dramatic to watch because the soluble form of indigo is yellow and, as it contacts air, oxidation occurs that changes the colour to deep blue in a few moments. A single step dyeing process such as the one described here does not require an alkaline dye environment because the dye is disposed of once oxidised and will not need to be re-used.

Oxidation of indigo to its blue insoluble form is facilitated by exposure to air, heat and strong light. Once oxidised, indigo molecules are trapped inside the fibres of the surface, to which they were applied. Fading occurs as the small molecules drop out of the cellulose or protein bundles of the fabric and the three dimensional structure that holds them. Wear and flexing promote this loss of indigo dye.

By the end of the 19th century the demand for blue indigo dye began to exceed the supply available from the indigo plantations of India. The 1905 Nobel Prize in Chemistry was awarded the German chemist Johann Friedrich Wilhelm Adolf von Baeyer the first person to determine the molecular structure for indigo.

Safety Notes: Irritant, do not inhale.

Tyrian Purple

Tyrian Purple, also known as Royal purple, was made from a secretion gathered from various marine molluscs, such as Murex trunculus and Murex brandaris. The dye is costly to harvest, requiring 12,000 molluscs for just 1.4 grams of pigment, thus in many cultures purple was reserved only for the use of the noble classes that could afford it. Tyrian Purple can also be used to make a blue dye but the plant form of indigo is much cheaper and the colour identical.

Figure 2. 6,6'-dibromoindigo

Tyrian Purple, is so closely related to indigo that the two molecules are almost identical. The chemical name for Tyrian Purple is 6,6'-dibromoindigo. The difference between the two dye molecules is the substitution of two hydrogen atoms for two bromines, as shown in the diagram below.

Figure 3. Hydrogen Substitution

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