leucoindigo

Leucoindigo is the reduced, colorless form of indigo, the well-known blue vat dye used in textiles. It is produced by chemically reducing indigo to a leuco (white) state, which is soluble enough to diffuse into fibers. When exposed to air, leucoindigo is oxidized back to indigo, forming the blue pigment inside the fiber.

In industrial dyeing, indigo is not applied in its insoluble blue form. It is reduced in an alkaline bath using agents such as sodium dithionite (sodium hydrosulfite) to generate leucoindigo. The textile is then immersed in this soluble bath, allowing the dye to penetrate the material. After fixation, oxidation in air deposits the blue indigo within the fiber, yielding a durable color. Leucoindigo itself is typically colorless or pale and is relatively unstable in air, rapidly reverting to indigo upon oxidation.

Leucoindigo is central to vat dye chemistry and explains the characteristic process of vat dyeing, including denim production. The depth and shade of blue depend on the extent of reduction, the duration of dyeing, and the subsequent oxidation steps. The reversible redox chemistry allows repeated dyeing and fading cycles to adjust coloration, while the dyeing is generally noted for its wash-fast and light-fast properties once bound in the fiber.

Safety and handling considerations apply to the reducing agents used to form leucoindigo, such as sodium dithionite, which require appropriate precautions in industrial and laboratory settings. Microbial or enzymatic reductions have also been explored in research and wastewater contexts.