Acid dyes are water-soluble anionic dyes that are mainly used to color protein-based fibers such as wool, silk, and nylon, along with leather and certain types of paper. They contain sulfonic acid groups that enhance their solubility in water, allowing them to easily penetrate fibers. Unlike reactive dyes that form covalent bonds, acid dyes adhere to fibers through ionic bonds with the positively charged sites in the material. This bonding provides vibrant colors and excellent light-fastness, although wash-fastness may be moderate in some cases. The dyeing process requires acidic conditions, usually with a pH between 2 and 6, achieved using acetic acid or formic acid. Heat is also applied to ensure proper fixation, with temperatures ranging between 60–100°C depending on the fiber type. Acid dyes are highly popular in industries that work with wool and silk, such as carpet manufacturing, silk sari production, and luxury textile processing. They are also used in leather finishing and certain food applications when produced as food-grade dyes.While acid dyes are affordable and widely available, their wastewater effluents can be harmful if not treated properly, as they increase the chemical load in water bodies. However, modern innovations have introduced eco-friendly acid dyes with better wash-fastness and reduced environmental impact, making them more sustainable for industrial use.