Introduction of Acid Dyes

Posted by Tylor Keller on October 29th, 2018

Acid dye is generally referred to as a dye applied to textiles at low pH and composed of acidic molecules. Acid dyes are generally soluble in water and alcohol, and can ionize some colorless cations such as sodium ions, hydrogen ions, potassium ions and colored anions such as eosin and picric acid. Acid dyes have better light fastness than basic dyes. The acidic group of the acid dye is generally sulfonic acid group (-SO3H), which is present in the form of sodium sulfonate (-SO3Na) on the dye molecule. Acid dye is characterized by good water solubility, bright color, complete chromatogram, relatively simple molecular structure compared with other dyes.

History of Acid Dyes:

In 1868, the earliest acid dye triaromatic methane acid dye appeared, which had strong dyeing ability but poor fastness. In 1877, the first acid dye, Acid Red A, was synthesized for wool dyeing, and its basic structure was determined. After 1890, the acid dye of anthraquinone structure was invented. Up to now, hundreds of acid dyes have been widely used in the dyeing of fibers such as wool, silk and nylon.

Classification of Acid Dyes

1 Classification according to the molecular structure of the parent dyes:

Azo acid dyes: Most azo dyes are not acid dyes while many acid dyes are azo dyes. About 60% of the acid dyes are azo dyes. Many acid dyes of the azo type are red in color.

Triarylmethane acid dyes: There are mainly purple and green dyes applied to fibers that are related to triphenylmethane.

Anthraquinone dyes: The structure of an anthraquinone dye contains a quinone structure or a polycyclic ketone. The types of this dye ranks second in all dye types, just below the azo dye.

2 Classification according to the PH of the acid dyes:

Strong acid dye: It has a PH of 2.5-4. It has good light fastness but poor wet fastness.

Weak acid dye: It has a PH of 4-5. The proportion of sulfonic acid group in the molecular structure of the dye is slightly low, so the water solubility and leveling dyeing property of the acid dye is slightly poor. Its wet fastness is better than that of the strong acid dye.

Neutral acid dye: It has a PH of 6-7. The proportion of sulfonic acid group in the molecular structure is lower than that of weak acid dye, so the dye solubility and leveling property of the neutral acid dye is poor but its wet processing fastness is good.

Applications of Acid Dyes

In textile dyeing, acid dyes can effectively dye protein fibers such as animal hair fibers. They are also applied in dyeing silk and nylon. In microscopy, acid dyes are used to stain basic tissue proteins for diagnosis or research. Certain substances in the cytoplasm often carry a positive charge and therefore can be colored by combining with colored anions in acid dyes. In contrast, basic dyes are used to stain the acidic components of the nucleus and other tissues. Some acid dyes are used as food colorants