Human is always
fascinated by colours, because we are living in a colourful world. We could see
so many colours in plants and their flowers. We eat coloured food stuffs and
use numerous coloured materials in our daily life. Do you know how do they get
coloured? Because they contain some kind of chemicals in them which are called
The uses of colourants
by mankind for painting and dyeing dates back to the dawn of civilization.
Until the middle of the 19th century, all colourants applied were from natural
origin. For example, inorganic pigments such as soot, manganese oxide, hematite
were used as colourants. Organic natural colourants have also a timeless
history of application, especially for colouring textiles.
The organic compounds
that are used as colourants are called dyes. These dyes are all aromatic
compounds, originating from plants and also from insects, fungi and lichens.
After the evolution of
modern organic chemistry, many kinds of synthetic dyes were prepared and used
by mankind. Dye chemistry is the study of such kind of dyes. It
provides us information on theory, structure, synthesis and applications of
Not all the aromatic
compounds are coloured. Aromatic compounds which absorb light of wavelength
range 350 nm – 700 nm (visible light) only are coloured. This nature of
absorption of visible light by aromatic compound depends on their structure.
The relationship between the colour of an organic compound and its structure
was explained by a German scientist Otto Witt (1876) through the Chromophore
and Auxochrome theory. You will study about this theory in your higher
All coloured compounds
are not dyes. Dyes are those coloured compounds which can be firmly fixed in
fabrics by chemical or physical bonding.
So, a dye should have
the following characteristics:
It should have a suitable colour.
It should be able to fix itself or be capable of being fixed to
It should be fast to light.
It should be resistant to the action of water, dilute acids and
Now a days, practically
all the dyes are synthetic, and are prepared from aromatic compounds obtained
from coal tar. Therefore, such dyes are sometimes called as coal tar dyes. But
they may differ in their basic structure and the way of application. So dyes
are classified in two ways, one, based on the method of application and other
on their parent structure.
Acid dyes: These are acidic in
nature and used for dyeing animal fibres and synthetic fibres. These can
be used for protein fibre such as wool and silk. E.g. Picric acid, Naphthol
Basic dyes: These are basic dyes
containing basic group (-NH2,- NHR, - NR2). They
are used for dyeing animal fibres and plant fibres.
Mordant dyes or Indirect
dyes have a poor affinity for cotton fabrics and hence do not dye
directly. They require pretreatment of the fibre with a mordant. Mordant (latin
: mordere = to bite) is a substance which can be fixed to the fibre and then
can be combined with the dye to form an insoluble complex called lake.
Aluminium, chromium, and iron salts are widely used as mordants. E.g. alizarin.
Direct dyes: They have high affinity
for cotton, rayon and other cellulose fibre. So they are applied
directly as they fix firmly on the fabric. E.g. Congo red
Vat dyes: It can be used only on cotton and, not on silk and wool. This dyeing is a continuous process and is carried out in a large vessel called vat. So it is called as vat dye. E.g. Indigo
Based on the structure,
dyes are classified as below:
• Azo dyes
• Diphenyl methane dye
• Triphenyl methane dye
• Phthalein dye
• Anthraquinone dye
• Indigo dyes
• Phthalo cyanine dye
• Nitro and nitroso dyes