Liquid iodine is an amazingly useful substance, finding applications around the home, in the research laboratory and at modern hospitals. Pure iodine is a bluish-black crystal, although it is rarely found in this form. Iodine is very soluble in nonpolar liquids such as hexane; however, iodine's solubility in water is limited. To increase solubility, iodine is mixed with an iodophor like povidone or an iodide such as potassium iodide.
Liquid iodine is an extremely effective disinfectant, capable of destroying both viruses and bacteria. Liquid iodine disinfectants work by directly destroying viral and bacterial proteins, making it both effective and immune to the evolved resistance that commonly plagues antibiotics. The oldest form of liquid iodine, known as tincture of iodine, is a mixture of liquid iodine and ethanol. Newer versions utilize an iodophore called povidone. Povidone acts like a cage that holds iodine molecules, slowly releasing them to the surrounding area.
Liquid iodine is often used in chemistry labs, in both elementary schools and advanced university labs, as a detector for starches. A solution of iodine and potassium iodide is dripped on a test substance. If the substance contains starches, a dark blue-black color will appear. Starches contain a sugar called amylose, which is shaped like a tight coil. When an iodine solution is added, iodine slips inside of this coil and gets stuck--causing the appearance of the deep blue-black color.
Colposcopy Contrast Stain
Liquid iodine is also utilized in modern colposcopies, a procedure that examines the interior of a woman's vagina to detect precancerous tumors and lesions. Liquid iodine is applied to the vulva, cervix and vaginal tissue. The vaginal tissues are high in glycogens and produce a color reaction similar to starches after iodine application. Precancerous growths often lack these glycogens and therefore do not stain brown, providing a clear indicator of tissue to biopsy.