Dyes or dyestuffs are colored substances capable of imparting their colors, fast to light, water, soap etc., to the fibers, foodstuff, etc. Since most of the synthetic dyestuffs are synthesized from a few starting materials, such as benzene, naphthalene, phenol, aniline, etc, which are obtained from coal-tar, the synthetic dye is also known as coal-tar dye.
There is hardly any walk of life where we do not need the organic compound. Further, synthetic chemistry (especially organic) has improved almost every aspect of life, viz. food, clothing, fuel, medicines, luxuries, etc.
What are dyes made of
A dye is made up of two kinds of parts, chromophores, and auxochromes.
- Presence of chromophores (Greek chroma-color, phoros-bearer). These are unsaturated groups the presence of which make the compound colored. The important chromophores are : — NO2, — N = N — , >C=C<, etc. Compounds having chromophores are known as chromogens.
- Presence of auxochromes (Greek Auxo- increase, chroma-color). As described earlier a dye should be fast to light, water, and soap so it must be attached to the fibers by means of stable chemical bonds.
These chemical bonds are formed by some groups which may be either acidic basic in nature. Such groups are known as auxochromes, some of which are: -OH, -COOH, — SO3H (acidic); —NH2, —NHR, —NR2 (basic). These groups also serve to deepen or intensify the color even though they may impart no color in the absence of chromophore, and hence they are known as auxochromes (color intensifying groups). A chromogen without an autochrome (color intensifying groups) can never act as a dye, e.g. azobenzene although red colored yet is not a dye; on the other hand, p-amino azobenzene (aniline yellow) is a dye.
Uses of dyes
The most important use is of course, in coloring the fibers. Other more important uses of dye are also technically important. Some of them are mentioned below.
- Certain dye is used as indicators, e.g. phenolphthalein, methyl orange, etc.
- Some dyes, e.g. cyanine dye are used in color photography.
- These are also used for differentiating staining in histology and bacteriology and thus aid in the cell morphology under the microscope.
- Certain dye is used as bactericidal and antiseptics.
Classification of dyes & Types
The classification of can be done on two basis
- On the basis of Application
- On the basis of Structure.
Let’s first discuss on the basis of application
These are salts, usually the sodium salts of color acids which usually contain sulphonic acid or phenolic groups. The color of an acid dye is in its negative ion. These are applied in the presence of acidic solutions. They are usually applied to wool, silk, nylon. Common examples are orange I, orange II, methyl red, methyl orange, Martius yellow etc.
These are salts usually the hydrochloride or the zinc chloride complexes, of color bases. The color of these dyes are in their positive ions. These are generally applied to wood, cotton, leather, paper, nylon, polyester, etc. Examples are azo and the triphenylmethane dye.
Direct dyes or substantive
These dyes are salts of color acids and are generally azo dyes. These are used to dye the fabric directly by placing it in a hot aqueous solution of the dye. These stick to the fiber through hydrogen bonding. Typical examples are congo red and direct black EW.
Mordant dyes or adjective
These dyes require a mordant which depends upon the nature of the dye. If the dye is acidic, the mordant must be basic (the most common basic mordants are the salts of Cr, Al, Sn and Fe); on the other hand, if the dye is basic, the mordant must be acidic (the most common acidic mordant is tannin or tannic acid containing some amount of tartar emetic). Examples are alizarin and azo dye. Mordant impart different colors in presence of different mordants. For example, Alizarin imparts rose red color, blue color, and violet color when mordanted with aluminum, barium and ferric salt solutions respectively.
Ingrain developed or azo dyes
These are synthesized thus produced within the fibers. This method of dyeing is used particularly with azo dyes. The color obtained by this type of dyeing is sometimes also known as ice-colors since diazotization and coupling reactions are always carried out at low temperatures. Para red is an important example of ingrain dyes.
These are insoluble in water but their reduced forms are soluble; hence these are applied in their reduced forms which are obtained by treating the compound with some reducing agent, commonly alkaline sodium hyposulphite (hydrosulfite) in a large vat. Vat dye are exceptionally fast and obtained in various attractive shades. Typical examples of vat are indigo and anthraquinone.
Classification of Dyes According to Structure
Dyes are classified into various groups on the basis of their chemical constitution.
- Nitro dyes. These are poly nitrophenols, e.g. picric acid.
- Azo dyes. This is the largest and most important class of organic dyes. These have an azo group (-N = N-) as the chromophore and the acidic or basic group as auxochromes. Important examples are methyl orange, orange I and II, Bismarck brown, congo red, etc.
- Triphenylmethane dye, e.g. malachite green, rosaniline, pararosaniline, crystal violet, etc.
- Phthalein dye, e.g. phenolphthalein, fluorescein, eosin, mercurochrome, erythrosin, etc
- Anthraquinone dye, e.g. alizarin.
- Indigo dye, e.g. indigo.