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Lugol's iodine

life science Maturity 7-9 Next article →

Lugol's iodine is a specialized liquid solution used in medicine, science, and industry. It is also known as aqueous iodine or strong iodine solution. The solution is composed of elemental iodine and potassium iodide dissolved in water. It is highly valued for its ability to react with specific substances and act as a disinfectant. Because of its wide range of uses, the World Health Organization includes it on its List of Essential Medicines.

The chemistry of this solution relies on a specific balance between its ingredients. It is made by mixing two parts potassium iodide with one part elemental iodine in water. Potassium iodide is important because it makes the elemental iodine soluble in the water. It does this by forming what is called a triiodide ion. This process creates a solution that provides a steady source of free elemental iodine. This iodine is readily generated through the equilibrium between iodine molecules and the polyiodide ions in the liquid.

Medical professionals use Lugol's iodine for several distinct purposes. One common use is treating thyrotoxicosis, a condition involving an overactive thyroid. By taking it by mouth, patients can temporarily inhibit how the thyroid processes iodine. This helps manage the gland before surgery can be performed. It is also used to protect the thyroid gland from radioactive iodine. In cancer screening, doctors perform a Schiller's test. They apply the solution to the cervix during a colposcopy. Normal tissue contains high amounts of glycogen, which is a type of starch. This causes the tissue to stain brown. However, tissue suspicious for cancer does not stain and remains pale.

In scientific laboratories, Lugol's iodine serves as a powerful indicator and stain. It is used as a mordant during the Gram stain process. A mordant is a substance that helps fix a dye to a specimen. After staining with crystal violet, the solution is applied for one minute. This ensures that the peptidoglycan in gram-positive organisms remains stained for microscopy. The solution also acts as an indicator for the presence of starches. When it touches starches like amylose or amylopectin, it turns a dark-blue or black color. This happens because the iodine interacts with the coil structure of the polysaccharide.

The history of this solution dates back to 1829. A French physician named Jean Lugol first created the mixture. Since then, it has been used in many emergency situations. For example, it was distributed in the Polish People's Republic following the Chernobyl catastrophe. The government used it to help block the intake of radioactive iodine. Although it was not the most optimal agent, it was used because iodine tablets were unavailable. Today, the solution is available in various strengths, such as 1%, 2%, 5%, or 10% concentrations.

While very useful, Lugol's iodine must be handled with great care. If used without dilution, a 2% or 5% concentration can be irritating to the mucosa. This is the delicate lining of the esophagus and stomach. For example, 10 mL of undiluted 5% solution has been reported to cause gastric lesions. In the United States, the DEA regulates solutions containing more than 2.2% elemental iodine. This is because the solution could be used as a precursor in the illegal production of methamphetamine. Transactions of up to 30 ml are exempt from these specific regulations.

Beyond medicine and labs, the solution has unique applications in other fields. In the marine aquarium industry, it is used to benefit reef inhabitants and macroalgae. It provides a source of free iodine and iodide to corals. It is thought to help corals with better coloration and may prevent bleaching. It can even help certain corals, like Acropora species, show more intense blue colors. Additionally, the solution can act as a less toxic replacement for aqua regia. This allows for the dissolution of gold without using extremely strong acids.

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