Acid dyes are highly soluble in water and have good light fastness compared to simple basic dyes. The acid dyes are most effective for the protein fibers like silk, wool, modified acrylics, and Nylon. They have sulphonic acid groups that are normally present as the sodium sulphonate salts. It increases the solubility in water and gives the dye molecules a negative charge. So as a group, acid dyes are divided into two subgroups – acid leveling or acid milling.