Dyes: Classification and Preparation

Dyes: Classification and Synthesis



Dyes are colored substance that chemically bonds to the substrate to which it is being applied. Dyes are generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fiber. To be commercial significance, a dye must be show fastness to light, washing, heat and bleaching.
The color of a dye is dependent upon the ability of the substance to absorb light within the visible region of the electromagnetic spectrum (380-750 nm).

Classification of Dyes

Dyes are classified into two categories i.e. Natural dyes and Synthetic dyes.

Natural Dyes

Natural dyes are obtained from natural sources and is used for all types of textile dyeing and printing. Natural dyes are classified in to three types based on their source of origin namely vegetable dyes, animal dyes and mineral dyes.

Vegetable Dyes

Vegetable dyes are obtained from different parts of plants such as leaves, flowers, fruits, pods, bark etc. These vegetable dyes can be applied directly or with different mordants. Examples of vegetable dyes are-
Marigold produces lemon or orange colour.
Henna produces yellowish orange colour.
Tea produces brown shades.
Onion produces yellowish color.
Indigo produces deep blue color.
Indian Madder produces red shades.
Turmeric produces yellow

Animal Dyes

Dyes extracted from certain insects and invertebrates are called as animal dyes. Various shades of red and purple were obtained from animal origin.

Mineral Dyes

Mineral Dyes are extracted from mineral sources. Most widely used mineral dyes are Iron, which produces yellowish brown shades, chrome yellow, prussian blue and manganese brown. Mineral dyes may be poisonous and hence are not being used commercially.

Synthetic Dyes

Synthetic Dyes are produced chemically and these are classified based on the chemical composition of the dye.

Direct Dyes

Direct dye colours the fabric directly without the help of any fixing agent. Direct dyes are water soluble, easy to produce, simple to apply, cheap in cost of production, anionic in nature and have greater affinity for cellulosic fibres.

Reactive Dyes

Reactive dyes react with the fibres and form covalent bonds. They become an integral part of the fibre. Reactive dyes are water soluble, fast in color and are used to dye cellulose, protein and polyamide fibres. They produce full range of bright shade across the spectrum.

Basic Dyes

Basic dyes are also known as cationic dyes. Basic dyes react with the acidic groups present in the fibres and form electrovalent bonds. These are soluble in alcohol but not easily soluble in water. Basic dyes are suitable for dyeing wool, silk and acrylic, but they have no affinity towards cellulosic fabrics.

Acid Dyes

Acid dyes require acid in dye bath to dye silk or wool. Acid dyes are similar to direct dyes but they cannot be applied to cellulosic fibre because of different structure. Acid dyes have greater affinity for protein and polyamide fibres. Acid dyes are fast in color.

Mordant or Chrome Dyes

Moderants are chemical substance that helps natural dyes and some synthetic dyes to color the fibres. These chemical substances are called mordants or mordant dyes. Mordant dyes have affinity for both fibre and dye and form a linkage between the dye molecule and the fibre.

Disperse Dyes

Disperse dyes are suitable for dyeing hydrophobic fibres like nylon, polyester, acrylic and other synthetic fibres. Disperse dyes are non -ionic or neutral in nature. They have an excellent fastness to washing and sunlight exposure. Disperse dyes are insoluble in water and solubility increases by increasing the temperature and by adding dispersing agents.

Vat Dyes

Vat dyes are insoluble in water but soluble in alkali. Indigo, an example of vat dyes. In the soluble form, vat dyes have excellent affinity for cellulosic fibres. Vat dyes are the fastest dyes for cotton, linen and rayon. They may also be applied to wool, nylon, polyester and acrylics with the use of a mordant. Vat dyes are resistant to light, acids and alkalies.

Sulphur Dyes

Like vat dyes, sulphur dyes are water insoluble and made soluble by the addition of reducing agents and alkali as solubiliser.

Azoic Dyes or Napthol Dyes

Unlike other colouring matters, the azoic dyes are not prepared as dye stuffs but have to be produced directly in the fibre by the combination of their constituents. They have good colour fastness to washing and light.

Pigment Dyes

Pigments are not true dyes because they have no affinity for the fibre. They are applied and held to the fabric with the help of adhesives and resins. Pigments are commonly used in dope dyeing and printing and it shows excellent light fastness.

Optical Brightners

These dyes are also called colourless dyes or fluorescent whiteners. These dyes absorb light at UV-region and emit blue light in visible region. It may be applied during bleaching or with the final finish. These dyes have affinity for cotton but are also used for wool, nylon, acetate and acrylics.

Methyl Orange

Methyl Orange
Methyl Orange is one of the very common water-soluble azo dyes and acidic in nature that is extensively used in several industries including the textile, paper, printing, and food industries and mostly discharged in industrial waste water. It is commonly known as a pH indicator. In an acidic medium (below pH 3.1), methyl orange is red, and in a basic medium (above pH 4.4), it is yellow.
Methyl Orange pH Color Change

Method of Preparation of Methyl Orange

Coupling of diazotized sulphanilic acid with N,N-dimethylaniline gives helianthine which on treatment with NaOH gives Methyl orange.
Preparation of Methyl Orange

Congo Red

Congo Red
Congo red is a water-soluble azo dye and its solubility is greater in organic solvents.
Due to a color change from blue to red at pH 3.0–5.2, Congo red can be used as a pH indicator(i.e. acid-base indicator). It turns red in the presence of alkalies and blue when exposed to acids.
Congo red was formerly used to dye cotton but has been superseded by dyes more resistant to light and to washing. It is still used in histology to stain tissues for microscopic examination.

Method of Preparation of Congo Red

Congo Red is made by the coupling of tetrazotized benzidine with two molecules of naphthanoic acid.
Preparation of Congo Red

Malachite Green

The name Malachite Green comes from the fact that it has a deep blue green color resembling that of malachite a copper ore. Malachite green is also called aniline green, benzaldehyde green, or china green. It is used as a dye for acrylic fibers, leathers, paper and lacquers. Malachite green is effective against fungi and gram-positive bacteria. In the fish-breeding industry it has been used to control the fungus Saprolegnia, a water mold that kills the eggs and young fry.

Method of Preparation of Malachite Green

Malachite green is obtained by condensing one mole of benzaldehyde with two moles of N,N-dimethylaniline in the presence of concentrated sulphuric acid to give a leuco base of malachite green which on oxidation with lead peroxide followed by treatment with HCl yields the Malachite Green.
Preparation of Malachite Green

Crystal Violet

Crystal violet is a greenish-brown metallic luster. When dissolved in water gives blue-violet colour with an absorbance maximum at 590 nm. The colour of the dye depends on the acidity of the solution. At a pH of +1.0, the dye is green while in a strongly acidic solution (pH −1.0), the dye is yellow.
Crystal violet or gentian violet, also known as methyl violet 10B or hexamethyl pararosaniline chloride. Crystal violet has antibacterial, antifungal, and anthelmintic (vermicide) properties and was formerly important as a topical antiseptic. Crystal violet is used in the manufacture of inks, stamping pads, typewriter ribbons, ball-point pens, and inkjet printers. It is aslo used as an indicator for the determination of hydrogen ion concentration of solutions.

Method of Preparation of Crystal Violet

Crystal violet is obtained by condensing Michler's ketone with N,N-dimethyl aniline in the presence of POCl3.
Preparation of Crystal Violet


Phenolphthalein is not a dye. Phenolphthalein is a colorless solid and is insoluble in water but dissolves in alkalis to form deep red solutions. When excess of strong alkali is added , the solution becomes colorless because the loss of resonance and quinoid structure.
Sodium Salt of Phenolphthalein
Due to color change, phenolphthalein used as an indicator in acid-base titrations. The pKa values of phenolphthalein was found to be 9.05, 9.50 and 12.
Phenolphthalein is a powerfull laxative.

Method of Preparation of Phenolphthalein

Phenolphthalein is prepared by condensing two molecules of phenol with phthalic anhydride in the presence of zinc chloride (ZnCl2) at 120°C. Preparation of Phenolphthalein


Fluorescein is a dark orange/red powder which is slightly soluble in water and alcohol. It is widely used as a fluorescent tracer for many applications. The color of its aqueous solutions are green by reflection and orange by transmission. A dilute solution of fluorescein in NaOH gives a strong yellow-green fluorescence when exposed to light. It is used to trace pollution of water supplies by sewerage because a small quantity of it changes the color of water at some distance from the source. Fluorescein has an isosbestic point (equal absorption for all pH values) at 460 nm.
Sodium salt of fluorescein is called uranine and is used to dye wool and silk.

Method of Preparation of Fluorescein

Preparation of Fluorescein


It is one of the most important mordant dyestuffs belonging to anthraquinone group.It is found in the maddar root as a glycoside, ruberythric acid which on acidic or enzymatic hydrolysis gives alizarin. Alizarin forms dark red crystal. It is soluble in alkali and ethanol but sparingly soluble in water. Alizarin used to dye cotton, wool and silk fabrics. It is also used for making printing ink. Alizarin changes color depending on the pH of the solution it is in, thereby making it a pH indicator.

Method of Preparation of Alizarin

Alizarin can be synthesized by condensing phthalic anhydride and catechol in the presence of sulphuric acid at 130°C.
Preparation of Alizarin


Indigo is a dark blue crystalline compound. It is insoluble in water and most organic solvents but soluble in aniline, chloroform and nitrobenzene. Indigo is used very extensively for dyeing cotton and in printing ink. Indigo can dye all natural fibres. It gives beautiful shades of blue — from the palest summer sky to an almost purple black. The colour achieved depends on the type of vat, the concentration of indigo, and the number of dips.

Method of Preparation of Indigo

Indigo can be synthesized by the condensation of aniline with chloroacetic acid to give N-phenylgycine which is fused with sodium hydroxide and sodamide at 250°C to form indoxyl. Indoxyl on oxidation by air yields the dye Indigo.
Preparatoion of Indigo