Amaranth free acid (Acid Red 27 free acid; Azorubin S free acid; FD & C Red Dye No. 2 free acid) serves as a versatile dye utilized across various fields. This dye is crucial in biological experiments for observing and analyzing cell structures, tracking biomolecules, evaluating cell functions, differentiating cell types, detecting biomolecules, studying tissue pathology, and monitoring microorganisms. Its usage extends from fundamental scientific research to clinical diagnostics. Additionally, Amaranth free acid is employed in traditional applications like textile dyeing and in innovative areas including functional textile processing, food pigments, and dye-sensitized solar cells.