Used in Gram staining to differentiate bacterial types. It acts as a mordant, helping trap the primary stain (crystal violet) inside bacterial cells. This allows distinction between Gram-positive bacteria, which retain the stain and appear purple, and Gram-negative bacteria, which do not retain it and appear pink after counterstaining. Essential in microbiology for bacterial identification and guiding antibiotic treatment. Also used in some antiseptic formulations and starch detection tests.