Question

indigo dye is an example of vat dyeing in which the dye is applied in a colorless form and subsequent chemical reaction produces the color. Explain how vat dyeing produces the caracteristic bluish color of indig

Answer 1

Colourless form is the reduced form of the dye. It is water soluble.
Coloured form is the oxidized form of the dye. It is water insoluble.
Thus vat dyes are solubilized in water by using appropriate reducing agent such as sodium dithionite (Na_2S_2O_4). Then the water soluble reduced form of the dye is applied to the fabric. This is followed by air oxidation to convert dye into coloured water insoluble form.
During the process, Indigo dye changes from yellow, in the dyebath to green and then blue due to air oxidation.