Question

Indigo is an example of :
(A)- Vat dye
(B)- Mordant dye
(C)- Azo dye
(D)- Phthalein dye

Answer 1

Hint: A dye is a coloured substance which chemically bonds to the substrate to which it is applied. This differentiates dyes from its pigments which do not chemically bind to the fabric they colour. The dye is generally poured in an aqueous solution.

Complete step by step answer:
- Indigofera tinctoria, also known as true indigo, which is a kind of plant from the bean genre that was one of the actual sources of indigo dye. It is an organic compound with a proper distinctive blue colour.

-The harvested leaves of the Indigofera plant have to be poured in water to let the indican (an amino acid) contained therein produce glucose. The freed indican then starts to ferment to other plant enzymes that were present. Thus, the water turns yellow after a few days. The released product of this fermentation process is known as indoxyl and as it is kept to dry in the open air, further it oxidises and then finally coagulates into the proper blue indigo.

-After that, it is mixed with an alkaline solution and the producing sludge is then pressed into solid cakes and dried outside. The hard cakes lastly could be grinded into a fine powder which contained the dark blue pigment.

-Indigo is a dark blue coloured crystalline powder that generally sublimes at $390-392{}^\circ C(734-738{}^\circ F)$. It is thus insoluble in water, alcohol, or ether, and on the other hand, is soluble in chloroform, nitrobenzene, and concentrated sulfuric acid. The chemical formula of indigo is${{C}_{16}}{{H}_{10}}{{N}_{2}}{{O}_{\begin{smallmatrix}
 2 \\

\end{smallmatrix}}}$.
-A class of dyes that are classified because the method of Vat dyeing which is a process that refers to dyeing that takes place in a bucket or vat is known as Vat dyes.

-A substance used to set (that is bind) dyes on fibres by forming a coordination complex with the dyes is known as a mordant or dye fixative. The coordination complex formed later attached to the fabric or tissue. These types of dyes are used for dyeing fabrics or intensifying staining of cells or tissues.
-Azoic dyeing is a method in which an insoluble Azo dye is poured directly onto or within the fibre. This is done by creating a fibre with both diazo and coupling components.

-A class of dyes which are mainly used as pH indicators due to their ability to change colours in the changing pH are called Phthalein dyes. These dyes are formed by the reaction of phthalic anhydride with various phenols.

-Indigo dyestuff which is known as vat dye and has no affinity to the fibre. They have poor washing fastness which makes the colour of denim fabric to change naturally. It can never properly penetrate the fibre since its molecule is so bulky and it only adheres to the surface thus remains at the exterior layer of the fibre. The interior layer stays white. It fades gradually. This character of indigo makes the fabric have its final look with different kinds of washing and finishing applications.
So, the correct answer is “Option A”.

Note: Vat dyes are normally insoluble in water and are not capable of dyeing fibres directly. But, the reduction process in alkaline liquor makes the water-soluble alkali metal salt of the dye. This form is normally colourless.