Dyes

Colored substances used for dyeing fabrics are called dyes. A true dye must:

• Have a suitable color

• Be able to attach itself to the material from solution or be capable of being fixed on it

• Be fast to light and washing when fixed. For this it must be resistant to water, acid and alkali

Graebe and Liebermann in 1868 observed that organic coloring matter on reduction gave a colorless product. This product regained its original color on oxidation. Otto Witt, a German chemist proved the relationship between color and constitution in 1876. He introduced the concept of chromophore and auxochromes.

Classification of dyes

Dyes can also be classified as natural and synthetic dyes. Compounds extracted from plants are called natural dyes. These were used in olden days to color fabrics. Alizarin (red) and indigo (blue) are two examples. Synthetic dyes came into being to provide more varieties of colors.

Alizarin belongs to the anthraquinone class of dyes. Indigo belongs to the indigoid type of dyes.

Classification of dyes based on application

Direct or Substantive Dyes

These can be directly applied by immersing the cloth in a hot solution of the dye in water. They can be again classified into acid and basic dyes.

Acid dyes are sodium salts of sulphonic acid and nitrophenols. They are used for dyeing animal fibers (wool and silk) but not vegetable fibers (cotton). The dye solution is acidified with sulphuric or acetic acid.

Basic dyes are salts of color bases with hydrochloric acid or zinc chloride. They can directly dye animal fibers. They need a fixing agent called mordant (tannin) to dye vegetable fibers. These are used for dyeing silk and cotton.

Mordant or Adjective Dyes

Mordant is any substance that can be fixed to fiber and later dyed on. Hydroxides or basic salts of chromium aluminium or iron are examples. Tannic acid is a suitable mordant for basic dyes. The fabric is first dipped into the solution of mordant and then in the dye solution. An insoluble colored complex called lake is obtained. It is insoluble and fast to washing.

Ingrain Dyes

They are produced in the fiber itself during dyeing. For example, a cloth is soaked in an alkaline solution of b - naphthol and dipped in a diazonium salt solution. Azodye is produced on the fiber due to coupling.

Vat Dyes

These are water insoluble colored compounds. They can be reduced to colorless (leuco) compounds, that are soluble in alkali and are easily reoxidized to give the dye. These dyes dye both animal and vegetable fibers directly. Mostly they are used for cotton fibers. The cloth treated with alkali is oxidised by air which makes the dye return to the insoluble form.

Example: Indigo

Methyl orange as a dye

This belongs to the azodyes. It is prepared by coupling diazotized sulphanilic acid with dimethylaniline.

This acid dye is used for wool and silk. It imparts an orange color, as the name suggests though the color is not fast to light or washing.

It is used in acid-alkali titrations, as an indicator and gives a yellow color with alkali and pink with acid. When the structure of the ion changes, the end point is reached.

Aniline yellow as a dye

This is another azodye and has little value as a dye. This is because it is sensitive to acids. This is the simplest basic azo dye. This can be obtained by coupling benzene diazomium chloride with aniline.

Malachite green as a dye

Belongs to the triphenyl methane dyes. Prepared by condensing 1 molecule of benzaldehyde with 2 molecules of dimethylaniline (1:2 ratio) in presence of con H₂SO₄. The leuco base is oxidised with lead dioxide and HCl to color base which further reacts with HCl to give the dye.