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
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.
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.
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.
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
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.