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Tincture of iodine or iodine tincture is an antiseptic, it is also called weak iodine solution. Usually 2–7% elemental iodine, along with potassium iodide or sodium iodide, dissolved in a mixture of ethanol and water. It was used from 1908 in pre-operative skin preparation by surgeon Antonio Grossich.
As in the case of Lugol's iodine, the role of iodide and water in the solution is to increase the solubility of the elemental iodine, by turning it to the soluble triiodide anion I3−. However, since iodine has moderate solubility in ethanol, it is also assisted by this solvent directly.
Thus, iodine tincture solutions are characterized by the presence of alcohol, and about the same amount (weight % content) of elemental iodine as iodide. Lugol's iodine, by contrast, has no alcohol, and has twice the amount of iodide as elemental iodine.
USP Tincture of Iodine is defined in the U.S. National Formulary (NF) as containing in each 100 mL, 1.8 to 2.2 grams of elemental iodine, and 2.1 to 2.6 grams of sodium iodide. Alcohol is 50 ml and the balance is purified water. This "2% free iodine" solution provides about one mg of free iodine per drop.
USP Strong Iodine Tincture is defined in the NF as containing in each 100 mL, 6.8 to 7.5 gram of iodine, and 4.7 to 5.5 gram of potassium iodide. Purified water is 50 mL and the balance is alcohol. This 7% tincture solution is about 3.5 times more concentrated than USP 2% tincture.
As both USP solutions contain elemental iodine, which is moderately toxic when ingested in amounts larger than those required to disinfect water, tincture of iodine is sold labelled "for external use only," and used primarily as a disinfectant.
Tincture of iodine is often found in emergency survival kits, used both to disinfect wounds and to sanitize surface water for drinking. When an alcohol solution is not desirable for this purpose, the alcohol-free Lugol's iodine, an aqueous solution of iodine in potassium iodide solution, or else povidone iodine (Betadine), a PVPI solution, can be used.
Small amounts may be added to suspect drinking water as a disinfectant (typically 5 mg free iodine per liter, or 5 drops of 2% tincture).
Iodine tincture is not a recommended source of solely-nutritional iodine. Nutritional iodine is better supplied in the form of the less toxic iodide (see SSKI) or iodate salts, which the body can easily convert to thyroid hormone.
Nevertheless, the iodide in tincture of iodine used as a water disinfectant does supply more than adequate nutritional iodine, perhaps 30 or more times the recommended dietary allowance per liter or quart. Application of tincture or Lugol's to the skin also results in absorption and bioavailability of some moderate fraction of the iodine.