**4. Classification of dyes based on their constitution**

#### **4.1 Azo dyes**

V.**Fibre reactive dyes:** These dyes contain a reactive group which combines directly with the hydroxyl or amino group of the fibre. As a permanent bond formed between the fibre and dye in this case so the colour of the dyed

VI.**Ingrain dyes or insoluble azo dyes:** These constitute about 60% of total dyes used. These dyes are obtained by the coupling of phenols, naphthols, arylamines, aminophenols. These dyes are adsorbed only on the surface of the fabric, colouring by this dye is not so fast. These can be used to dye cellulose, silk, nylon and leather [22, 23]. Para red (**Figure 8**) is an example of such a class of dye. Para red (**Figure 8**) dye can be prepared as follow: These dyes are also used in foodstuffs, cosmetics, drugs and as an indicator

VII.**Vat dyes:** Vat dyes are insoluble in water so they cannot be used directly for dyeing. They are first reduced to soluble colourless form (leuco form) with a reducing agent such as an alkaline solution of sodium hydrosulphite. Under these conditions, the leucoform develops an affinity for cellulose fibres [22, 23]. Hence these dyes are mainly used to dye cotton fibre. The

mordant to dye the fabric directly come under this category. The mordant

Metal ions are used as mordants for the acid dyes while tannic acid is used as the mordant for basic dyes. The desired fabric is first soaked in the suitable metal salt and then this soaked fibre is dipped in the solution of dye when insoluble coloured complexes formed on the fabric. These insoluble coloured complexes are called

VIII.**Mordant dyes:** Those dyes which do not bind directly but require a

act as the binding agent between the fibre and the dye.

fabric is very fast and has a long life [22, 23].

*Dyes and Pigments - Novel Applications and Waste Treatment*

example of vat dyes is indigo (**Figure 9**).

in chemical analysis.

**Figure 6.** *Chrysodine G.*

**Figure 7.** *Martius yellow.*

**Figure 8.** *Para red.*

**136**

These dyes constitute the largest part of the synthetic dyes. The chromophore of the azo dyes is aromatic system joined to the azo group and auxochromes are NH2, NR2, OH.

Azo dyes are classified as the number of azo group in the molecule such as monoazo, diazo and triazole etc. The characteristic of two important azo dyes methyl orange (**Figure 11)** and congo red can be described as follows:

### *4.1.1 Methyl orange*

It is obtained by coupling of N, N-dimethylaniline with diazotised sulphanilic acid. **Properties:** Methyl orange is a colouring dye for the wool and silk but its colour fades on the exposure to the light and washing. Usually, it is not used as a dye but

**Figure 11.** *Methyl Orange.*

used as an indicator in the acid–base titrations. The pH range of methyl orange is 3.1–4.4. It is yellow in basic solutions (above pH 4.4) while red in acidic solutions (below pH 3.1) [24–26]. The colour change takes place because of the change in the structure of ions in acidic and basic medium. In acidic medium, the ion contains pquinonoid chromophore while in basic medium ion contains azo chromophore (**Figure 12**).

*4.2.1 Malachite green*

*4.2.2 Crystal violet*

**4.3 Phthalein dyes**

*4.3.1 Phenolphthalein*

*4.3.2 Fluorescein*

(**Figure 17**).

*4.3.3 Alizarin*

**Figure 14.** *Malachite green.*

**Figure 15.** *Crystal violet.*

**139**

It is used for dying wool and silk directly and cotton mordanted with tannin. The

A weakly acidic solution of it is purple, a strongly acidic solution is green and

It is insoluble in water but dissolves in alkalies to form a deep red solution. When

colourless again. Because of the colour change, it is used as an indicator in acid–base

It is a xanthan derivative. In properties, it more closely resembles phthalein dyes. It is a red powder insoluble in water. It dissolves in alkalies to give a reddishbrown solution which, on dilution, gives a strong yellowish-green fluorescence

It is one of the most important anthraquinone dye. It occurs in madder root in

an excess of strong alkali is added, the solution of phenolphthalein becomes

colour of it fades slowly on the addition of acid and base (**Figure 14**).

still more acidic solution is yellow (**Figure 15**).

*Structure and Properties of Dyes and Pigments DOI: http://dx.doi.org/10.5772/intechopen.97104*

titrations. It is also a powerful laxative (**Figure 16**).

form of its glucoside called, ruberthyric acid.

#### *4.1.2 Congo red*

Congo red (**Figure 13**) is the example of diazo dye as it contains two azo groups. It is obtained by the coupling of tetrazotised benzidine with two molecules of napthionic acid (4-aminonapthalene-1-sulphonic acid).

**Properties:** it is a direct dye and sodium salt of this dye gives red colour on the applied cotton. As on addition of acid, its colour changes so it is also not used as a dye generally. Mostly it is used as an indicator. It is blue in acidic solution (below pH 3) and red in solutions (above pH 3). The change in colour from red to blue in the acidic solution is due to the resonance among charged canonical structures.

#### **4.2 Triphenylmethane dyes**

These are the derivatives of -NH2, -NR2, and -OH groups in the rings. The compound obtained by this method are generally colourless and called leuco bases. On oxidation, these are converted to the corresponding colourless tertiary alcohols called carbinol bases. These carbinol bases readily change from the colourless benzenoid form to the coloured quinonoid forms in the presence of acids due to salt formation, which is known to be dye [26–30]. The structure and properties of two important triphenylmethane dyes can be discussed as follows;

**Figure 12.** *Red (acidic medium) yellow (basic medium).*

**Figure 13.** *Congo red.*

*Structure and Properties of Dyes and Pigments DOI: http://dx.doi.org/10.5772/intechopen.97104*

#### *4.2.1 Malachite green*

used as an indicator in the acid–base titrations. The pH range of methyl orange is 3.1–4.4. It is yellow in basic solutions (above pH 4.4) while red in acidic solutions (below pH 3.1) [24–26]. The colour change takes place because of the change in the structure of ions in acidic and basic medium. In acidic medium, the ion contains pquinonoid chromophore while in basic medium ion contains azo chromophore

Congo red (**Figure 13**) is the example of diazo dye as it contains two azo groups.

**Properties:** it is a direct dye and sodium salt of this dye gives red colour on the applied cotton. As on addition of acid, its colour changes so it is also not used as a dye generally. Mostly it is used as an indicator. It is blue in acidic solution (below pH 3) and red in solutions (above pH 3). The change in colour from red to blue in the acidic solution is due to the resonance among charged canonical structures.

These are the derivatives of -NH2, -NR2, and -OH groups in the rings. The compound obtained by this method are generally colourless and called leuco bases. On oxidation, these are converted to the corresponding colourless tertiary alcohols called carbinol bases. These carbinol bases readily change from the colourless benzenoid form to the coloured quinonoid forms in the presence of acids due to salt formation, which is known to be dye [26–30]. The structure and properties of two

It is obtained by the coupling of tetrazotised benzidine with two molecules of

napthionic acid (4-aminonapthalene-1-sulphonic acid).

*Dyes and Pigments - Novel Applications and Waste Treatment*

important triphenylmethane dyes can be discussed as follows;

(**Figure 12**).

*4.1.2 Congo red*

**Figure 12.**

**Figure 13.** *Congo red.*

**138**

*Red (acidic medium) yellow (basic medium).*

**4.2 Triphenylmethane dyes**

It is used for dying wool and silk directly and cotton mordanted with tannin. The colour of it fades slowly on the addition of acid and base (**Figure 14**).

#### *4.2.2 Crystal violet*

A weakly acidic solution of it is purple, a strongly acidic solution is green and still more acidic solution is yellow (**Figure 15**).

#### **4.3 Phthalein dyes**

#### *4.3.1 Phenolphthalein*

It is insoluble in water but dissolves in alkalies to form a deep red solution. When an excess of strong alkali is added, the solution of phenolphthalein becomes colourless again. Because of the colour change, it is used as an indicator in acid–base titrations. It is also a powerful laxative (**Figure 16**).

#### *4.3.2 Fluorescein*

It is a xanthan derivative. In properties, it more closely resembles phthalein dyes. It is a red powder insoluble in water. It dissolves in alkalies to give a reddishbrown solution which, on dilution, gives a strong yellowish-green fluorescence (**Figure 17**).

#### *4.3.3 Alizarin*

It is one of the most important anthraquinone dye. It occurs in madder root in form of its glucoside called, ruberthyric acid.

**Figure 14.** *Malachite green.*

**Figure 15.** *Crystal violet.*

**5. Pigments**

**Figure 19.** *Indigo.*

**5.1 History of pigments**

*Structure and Properties of Dyes and Pigments DOI: http://dx.doi.org/10.5772/intechopen.97104*

Those organic and inorganic substances which are widely used as surface coatings, employed in ink, plastic, rubber etc. to impart colour. A large number of

Pigments are believed to be 3.5 lakh to 4 lakh years old. They have been reported in a cave at Twin Rivers, near Lusaka, Zambia. Blue pigment was derived from lapis lazuli. Pigments based on minerals and clays often bear the name of the city or region where they were originally mined. Synthetic pigments are believed to be introduced in early second millennium BCE. White lead 2PbCO3. Pb(OH)2 and blue frit (Egyptian Blue) also known as calcium copper sulphate CaCuSi4O10 are the two

However, they may differentiate broadly into two category organic & inorganic

inorganic pigments

Examples include phthalocyanines, quinacridones, dioxazenes, napthols, and other lab-made pigments

Topical cosmetics, as well as dirt, are used to make

Metal cations are present in an array shape with nonmetallic anions in the molecules of Inorganic Pigments. This arrangement prevents the pigments from dissolving in the solvent or the plastic**.**

Inorganic pigments have significantly lower values

pigments are used for commercial manufacture of paints.

examples of early synthetic pigments [17–30].

Pigments are mainly classified into two types:

**S.No. Organic pigments Inorganic pigments**

**5.** Organic pigments are transparent Most inorganic pigments are opaque

**5.2 Classification of pigments**

1.White pigments

2.Coloured pigments

pigments (**Table 1**) [23–26].

**1.** Pthalocyanines, Quinacidrones, Dioxazenes, Napthols, and other pigments made in the lab are examples

**2.** Plants and plant products are the primary source of organic pigments

**3.** Carbon chains or carbon rings are still found in Organic Pigment molecules.

**4.** Organic pigments have a high index of

*Difference between Organic & Inorganic Pigments.*

refraction

**Table 1.**

**141**

**Figure 16.** *Phenolphthalein.*

**Figure 17.** *Fluorescein.*

On reduction with zinc dust, it gives anthracene. This implies that alizarin is a derivative of anthracene. It forms ruby red crystals, insoluble in water and alcohol but dissolves in alkalies to form purple solution, sublimes on heating.

It is a mordant dye, and the colour of the lake depends on the metal used. Aluminium gives a red lake, ferric salts give violet-black while chromium salts form a brown-violet lake. It is also used as a purgative (**Figure 18**).

#### *4.3.4 Indigo*

It is the oldest vat dye known. India is the birthplace of Indigo (**Figure 19**). A fusion of indigo at a low temperature produces anthranilic acid.

It is a dark blue powder, with coppered lustre, m.p. 663 K. It is insoluble in water and most organic solvents. It is widely used for dyeing cotton. The quality of colour is excellent and is stable to light, washing etc. It is also used in printing inks [26–30].

**Figure 18.** *Alizarin.*

*Structure and Properties of Dyes and Pigments DOI: http://dx.doi.org/10.5772/intechopen.97104*

**Figure 19.** *Indigo.*

On reduction with zinc dust, it gives anthracene. This implies that alizarin is a derivative of anthracene. It forms ruby red crystals, insoluble in water and alcohol

It is the oldest vat dye known. India is the birthplace of Indigo (**Figure 19**). A

It is a dark blue powder, with coppered lustre, m.p. 663 K. It is insoluble in water and most organic solvents. It is widely used for dyeing cotton. The quality of colour is excellent and is stable to light, washing etc. It is also used in printing

It is a mordant dye, and the colour of the lake depends on the metal used. Aluminium gives a red lake, ferric salts give violet-black while chromium salts form

but dissolves in alkalies to form purple solution, sublimes on heating.

a brown-violet lake. It is also used as a purgative (**Figure 18**).

*Dyes and Pigments - Novel Applications and Waste Treatment*

fusion of indigo at a low temperature produces anthranilic acid.

*4.3.4 Indigo*

**Figure 16.** *Phenolphthalein.*

**Figure 17.** *Fluorescein.*

inks [26–30].

**Figure 18.** *Alizarin.*

**140**

#### **5. Pigments**

Those organic and inorganic substances which are widely used as surface coatings, employed in ink, plastic, rubber etc. to impart colour. A large number of pigments are used for commercial manufacture of paints.

## **5.1 History of pigments**

Pigments are believed to be 3.5 lakh to 4 lakh years old. They have been reported in a cave at Twin Rivers, near Lusaka, Zambia. Blue pigment was derived from lapis lazuli. Pigments based on minerals and clays often bear the name of the city or region where they were originally mined. Synthetic pigments are believed to be introduced in early second millennium BCE. White lead 2PbCO3. Pb(OH)2 and blue frit (Egyptian Blue) also known as calcium copper sulphate CaCuSi4O10 are the two examples of early synthetic pigments [17–30].

#### **5.2 Classification of pigments**

Pigments are mainly classified into two types:


However, they may differentiate broadly into two category organic & inorganic pigments (**Table 1**) [23–26].


**Table 1.**

*Difference between Organic & Inorganic Pigments.*
