Iodine

Description

The halogen your thyroid cannot function without — and the reason table salt is iodised.

Physical Properties

Atomic Properties

Discovery & History

Bernard Courtois: The Accidental Discovery (1811)

The discovery of iodine reads like a perfect example of scientific serendipity. Bernard Courtois , a French chemist and saltpeter manufacturer, made this discovery quite by accident while trying to extract sodium and potassium compounds from seaweed ash during the Napoleonic Wars.

Courtois was processing kelp ash to produce saltpeter (potassium nitrate) for gunpowder when Napoleon's continental blockade made traditional sources scarce. While adding sulfuric acid to concentrated seaweed ash, he accidentally used too much acid. To his amazement, beautiful violet vapors rose from the mixture, condensing into dark, metallic crystals with a metallic luster.

The striking purple vapor was unlike anything Courtois had seen before. Being a careful observer, he immediately recognized that he had discovered something new. However, lacking the resources for extensive investigation during wartime, he shared samples with fellow chemists for further study.

Scientific Investigation and Naming

Joseph Louis Gay-Lussac and Humphry Davy independently studied Courtois' mysterious substance in 1813-1814. Both quickly recognized it as a new element due to its unique chemical properties and behavior. Gay-Lussac conducted detailed experiments proving its elemental nature, while Davy demonstrated its similarity to chlorine.

The name "iodine" comes from the Greek word "iodes," meaning violet-colored , referring to the distinctive purple vapor the element produces when heated. Gay-Lussac coined this name in 1814, though some early texts also used "iodinium." The French preference for "iode" reflects Courtois' nationality and his priority in the discovery.

Early Applications and Understanding

Within a few years of its discovery, iodine found its first practical applications. Swiss physician Jean-François Coindet first used iodine to treat goiter in 1820, though he didn't understand the underlying mechanism. This represented the first medical use of the element that would later prove essential for human health.

Photography pioneers like Louis Daguerre incorporated silver iodide into their early photographic processes by the 1840s. The light-sensitive properties of iodine compounds made them crucial for capturing permanent images, launching the entire field of photography.

The Thyroid Connection

The connection between iodine and thyroid function wasn't understood until the late 19th century. Eugen Baumann first isolated iodine from thyroid tissue in 1896, proving that this mysterious gland concentrated the element. This discovery explained why iodine could treat goiter and revealed its fundamental role in human metabolism.

The development of iodized salt in the 1920s represents one of public health's greatest triumphs. David Marine and others demonstrated that adding tiny amounts of iodine to salt could eliminate goiter and cretinism. This simple intervention transformed the health of millions in iodine-deficient regions worldwide.

Isotopes

Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.

Applications & Uses

Medical and Healthcare Applications

Iodine plays a critical role in human health as an essential component of thyroid hormones. The thyroid gland uses iodine to produce thyroxine (T4) and triiodothyronine (T3) , which regulate metabolism, growth, and development. Iodine deficiency can lead to goiter, hypothyroidism, and severe developmental issues in children.

Medical imaging relies heavily on iodine-based contrast agents. Iodinated contrast media contains iodine atoms that absorb X-rays effectively, making blood vessels, organs, and tissues visible in CT scans and angiography. These contrast agents have transformed medical diagnostics, allowing doctors to detect heart disease, stroke, and cancer with high clarity.

In nuclear medicine , radioactive isotopes like 131 I are used to treat thyroid cancer and hyperthyroidism. The iodine concentrates specifically in thyroid tissue, delivering targeted radiation therapy while minimizing damage to healthy organs.

Antiseptic and Disinfection

Povidone-iodine (Betadine) is one of the most effective broad-spectrum antiseptics available. It kills bacteria, viruses, fungi, and spores on contact, making it invaluable for surgical preparation, wound care, and infection prevention. Unlike alcohol-based antiseptics, iodine solutions maintain effectiveness even in the presence of organic matter.

Water purification tablets containing iodine provide emergency disinfection for hikers, travelers, and disaster relief operations. These tablets kill most bacteria, viruses and Giardia, but they do not reliably inactivate Cryptosporidium, which also resists chlorine.

Industrial Manufacturing

The pharmaceutical industry uses iodine as a key ingredient in numerous medications beyond thyroid treatments. Iodine-containing compounds serve as intermediates in synthesizing antibiotics, anti-inflammatory drugs, and cardiovascular medications.

LCD manufacturing requires ultra-pure iodine for polarizing films. These films control light transmission in liquid crystal displays, making modern smartphones, computers, and televisions possible. The required purity is very high - even trace impurities can create visible defects in displays.

In photography , silver iodide crystals form the light-sensitive emulsion in traditional film. When exposed to light, these crystals undergo chemical changes that create the latent image, later developed into photographs through chemical processing.

Advanced Applications

Catalysis applications use iodine compounds to accelerate chemical reactions in pharmaceutical synthesis and polymer production. Iodine-based catalysts enable more efficient, environmentally friendly manufacturing processes.

Analytical chemistry employs iodometric titrations to determine the concentration of oxidizing agents, vitamin C content in foods, and water quality parameters. The distinctive blue-black color formed with starch makes iodine an ideal indicator for these precise measurements.

Natural Sources

Seaweed, Chilean saltpeter

Abundance