Iodine

Iodine was discovered in 1811 by the French chemist Bernard Courtois while he was extracting sodium and potassium compounds from seaweed. When Courtois added sulfuric acid to the seaweed ash, he observed the formation of a violet vapor, which condensed into dark crystals. The substance was later named iodine, derived from the Greek word 'iodes,' meaning violet. The element was further studied and classified by other chemists, notably by Humphry Davy, who confirmed its elemental nature in 1813. Iodine gained recognition for its unique properties and color, and over time, its importance in nutrition and medicine was realized, especially in relation to thyroid health.

Iodine is relatively rare in the Earth's crust, with an average concentration of about 0.45 parts per million. It is primarily derived from seawater, where it exists as iodide ions (I⁻) due to the dissolution of iodine present in marine organisms and the leaching of soils. The highest concentrations are found in certain types of seaweed, particularly brown algae, which can accumulate iodine from seawater. Additionally, iodine is found in mineral deposits, brines, and certain types of sediments. Major sources of iodine for commercial extraction include salt brines and ocean water, accounting for the vast majority of the world's iodine supply.

Iodine is an essential element for human health, primarily playing a critical role in the synthesis of thyroid hormones, such as thyroxine (T4) and triiodothyronine (T3), which regulate metabolism, growth, and development. The thyroid gland requires iodine to produce these hormones, and a deficiency can lead to a range of health issues, including goiter (enlargement of the thyroid gland) and hypothyroidism. Additionally, iodine is particularly important during pregnancy and early childhood, as it contributes to proper cognitive development. The recommended dietary allowance for iodine varies by age and physiological condition, but inadequate iodine intake remains a global health concern, leading to initiatives aimed at iodizing salt and promoting dietary education.

Iodine is a dark gray to purplish-black solid at room temperature, possessing a metallic luster. It has a relatively low melting point of 113.7 degrees Celsius and boils at 184.3 degrees Celsius. Iodine is sublimes readily, transitioning from solid to gas without passing through a liquid state. Chemically, iodine is reactive, particularly with alkali metals and alkaline earth metals, forming iodides. In its elemental form, iodine exists as diatomic molecules (I₂). It exhibits strong oxidizing properties and is soluble in many organic solvents, such as ethanol and chloroform, but is only slightly soluble in water. Iodine's strong color in solution and its capability to form complex molecules make it valuable in various chemical processes.

Iodine has a wide range of applications across different fields. In medicine, it is used as an antiseptic and disinfectant, particularly in the form of povidone-iodine, a compound effective against bacteria, viruses, and fungi. Iodine is also critical in radiology, where it helps visualize organs and metabolic processes through its use in contrast agents. In nutrition, iodine is crucial for thyroid health, as mentioned earlier, leading to its use in fortified salt products globally. Industrially, iodine is employed in the manufacture of dyes, photographic film, and as a catalyst in certain chemical reactions. Iodine is also pivotal in the production of iodinated compounds used in pharmaceuticals and in agriculture to enhance crop growth.

• Iodine is the heaviest of the stable halogens and is the only one that is solid at room temperature.
• When iodine vapors are exposed to starch, they create a deep blue color, a reaction that is used in various chemical experiments.
• The first medically recognized use of iodine was in the form of tincture, created by dissolving iodine in alcohol, introduced in the 19th century.
• Iodine is essential for marine life, influencing their metabolism and growth, particularly in seaweeds.
• In medieval times, iodine's presence in coastal areas was marked as a sacred element, believed to ward off infections.