Iodine (I): Guide to Properties, Uses, and Market Price

Why this purple-smoking element became humanity's most important public health intervention

Halogens are a group of highly reactive nonmetals occupying Group 17 of the periodic table. These elements exist as diatomic molecules in nature, meaning two atoms bond together to form stable compounds like I₂. Halogens are characterized by their seven valence electrons, making them one electron short of a complete octet, which drives their reactivity as they seek to gain an electron. The term halogen means "salt-producing," as these elements readily form salts when combined with metals. Moving down Group 17 from fluorine to iodine, reactivity decreases while atomic size increases.

In this article, we explore iodine's properties, its accidental discovery during wartime, life-saving role in human health, medical and industrial applications, and the ongoing challenge of iodine deficiency affecting billions globally.

Properties of iodine

What does iodine look like?

Iodine is a bluish-black or dark gray solid with a lustrous, shiny appearance and a slight metallic sheen. The crystalline form displays a distinctive violet-black color with visible crystal structure. When heated at ordinary temperatures or under standard conditions, iodine sublimes directly from solid to gas without passing through a liquid phase, producing a striking violet or purple vapor with an irritating odor. At 114 degrees Celsius under pressure, iodine melts to form a deep violet liquid before boiling at 184 degrees Celsius to produce violet gas. This remarkable color transformation from dark solid to brilliant purple vapor makes iodine one of the most visually distinctive elements.

Will we ever run out of iodine?

Iodine is present in seawater at concentrations of approximately 0.05 parts per million, which translates to roughly 76 billion pounds of iodine dissolved in the world's oceans. This vast oceanic reservoir makes complete depletion extremely unlikely. Additionally, approximately 400,000 tons of iodine escape from the oceans annually, mainly as elemental iodine and methyl iodide produced by marine organisms, creating a natural recycling process.

Current worldwide industrial production is approximately 13,000 tons per year, sourced primarily from underground brines associated with natural gas and oil deposits, and from caliche ore deposits containing sodium iodate. Chile and Japan lead global production. While land-based deposits are finite, the vast ocean supply combined with recycling from seaweed and industrial processes ensures long-term availability for essential applications in human health, medicine, and industry.

Can iodine be recycled

Yes, iodine can be recycled effectively, and recycling contributes to the global iodine supply. Recovery and reuse help conserve resources and reduce environmental impact from primary extraction.

Recycled iodine is commonly sourced from:

• X-ray contrast media and pharmaceutical manufacturing waste
• Photographic processing solutions and film recycling
• Industrial catalyst recovery from chemical manufacturing
• Seaweed processing residues used for sodium alginate production

Where can iodine be found?

Iodine is never found uncombined in nature and does not occur in sufficient concentrations to form independent minerals. Instead, it exists primarily as iodide ions dissolved in seawater and concentrated by certain organisms and geological processes.

• Seawater: Contains approximately 0.05 parts per million iodine as iodide ions, with approximately 76 billion pounds total dissolved in the world's oceans.
• Seaweed: Dried seaweeds, particularly those of the Laminaria family, concentrate iodine from seawater and can contain as much as 0.45 percent iodine by dry weight. Seaweed ash was a major source before 1959.
• Underground Brines: Naturally occurring brines associated with natural gas and oil deposits contain concentrated iodide. These brines are the primary commercial source in Japan and the United States.
• Caliche Ore: Natural deposits of sodium nitrate found in northern Chile contain sodium iodate at concentrations of approximately 0.04 percent or 400 parts per million. This is currently the most economical source of iodine.

Iodine is produced primarily in Chile and Japan, which together account for the majority of world production. The United States produces iodine from deep well brines in northern Oklahoma. Russia also produces significant quantities. Historically, seaweed harvested off the coast of China was an important source, and iodine is still obtained as a byproduct during sodium alginate processing from seaweed.

Is iodine expensive?

Iodine commands moderate prices reflecting its relative scarcity on land despite oceanic abundance. While more expensive than common industrial chemicals, iodine remains affordable for essential health applications. Only two ounces of potassium iodate, costing approximately one dollar, are required to iodize one ton of salt, making public health fortification highly cost-effective. Prices fluctuate based on production from Chilean caliche deposits and Japanese brine operations, as well as global demand from pharmaceutical and industrial sectors.

Does iodine have a biological role?

Yes, iodine is an essential trace element for human health and all vertebrate life. As the heaviest essential mineral nutrient, iodine is required for the synthesis of thyroid hormones thyroxine and triiodothyronine, which regulate metabolism, growth, development, and body temperature. The human body contains 10 to 20 milligrams of iodine, concentrated mainly in the thyroid gland. The presence of iodine in the thyroid was discovered in 1895 when concentrated nitric acid spilled on thyroid tissue produced purple fumes.

Iodine deficiency is the leading preventable cause of intellectual disability worldwide, affecting approximately two billion people. During pregnancy and early childhood, adequate iodine is critical for proper brain development. Even mild deficiency can reduce IQ and impair cognitive function. The introduction of iodized salt in 1924 largely eliminated widespread goiter in the United States and many other countries, representing one of the greatest public health achievements of the 20th century. Adults need approximately 150 micrograms daily, while pregnant and lactating women require 220 to 290 micrograms to support fetal and infant development.

What is pure iodine used for?

• Iodized Salt: Potassium iodide is added to table salt to prevent iodine deficiency and goiter. This simple fortification has prevented intellectual disability in millions of people worldwide since its introduction in 1924. One-quarter teaspoon of iodized salt provides about 100 micrograms of iodine.
• Disinfectants and Antiseptics: Tincture of iodine, a solution of iodine in alcohol, rapidly sterilizes human skin for surgery and disinfects wounds. Povidone-iodine solutions are used for surgical preparation and wound care. Iodine's antiseptic action was discovered in 1873.
• Medical Diagnostics: X-ray contrast media containing iodine compounds allow visualization of blood vessels, organs, and soft tissues during CT scans and angiography. Radioactive iodine-131 is used to diagnose and treat thyroid conditions, including thyroid cancer.
• Photography: Silver iodide was essential in early photography for creating light-sensitive daguerreotypes, invented in 1839. It remains important in modern photographic film and cloud seeding for weather modification.
• Animal Feed Supplements: Iodine is added to livestock feed to ensure proper thyroid function and optimal growth in farm animals.
• Industrial Catalysts: Iodine compounds serve as catalysts in various chemical synthesis reactions, including production of certain polymers and specialty chemicals.
• LCD Displays: Iodine is used to manufacture polarizing filters essential for liquid crystal displays in televisions, computer monitors, and mobile devices.
• Printing Inks and Dyes: Iodine compounds produce distinctive colors in printing inks, textile dyes, and colorants.

What are the main compounds with iodine?

• Potassium Iodide (KI) : The most important iodine compound, added to table salt for iodine supplementation. Also used as a dietary supplement, in nuclear emergency preparedness to protect the thyroid from radioactive iodine, and in photography. Approximately 15 percent of worldwide iodine production is used to manufacture potassium iodide.
• Silver Iodide (AgI) : A light-sensitive compound used in photographic film and paper. Also employed in cloud seeding operations to induce rainfall by serving as nucleation sites for ice crystal formation.
• Iodoform (CHI₃) : A yellow crystalline antiseptic compound with a distinctive penetrating odor. Historically used as a wound dressing and disinfectant, though less common today due to more effective alternatives.
• Povidone-Iodine : A stable complex of iodine with the polymer povidone, used widely as a surgical scrub, antiseptic, and disinfectant. Sold under brand names including Betadine, it releases iodine slowly for sustained antimicrobial action.
• Thyroxine (T4) : The primary thyroid hormone containing four iodine atoms, with the chemical name 3,5,3',5'-tetraiodothyronine. Essential for regulating metabolism, growth, and development throughout life.
• Triiodothyronine (T3) : The more active thyroid hormone containing three iodine atoms, with the chemical name 3,3',5-triiodothyronine. Produced from thyroxine and regulates metabolic rate at the cellular level.
• Sodium Iodide (NaI) : Used in scintillation detectors for gamma ray detection, in medical imaging, and as a dietary supplement. When activated with thallium, it produces light pulses in response to radiation.

Who discovered iodine?

Iodine was discovered accidentally in 1811 by French chemist Bernard Courtois during the Napoleonic Wars. Courtois was helping his father manufacture saltpeter, an essential ingredient for gunpowder that was in heavy demand at the time. Initially, he used wood ash as the source of potassium nitrate needed for saltpeter production. However, due to wood ash shortages, he began using seaweed ash instead as his source of potassium.

One day in 1811, Courtois added sulfuric acid to saltpeter derived from seaweed ash and was surprised to observe purple fumes rising from the mixture. When he repeated the reaction in a retort, the purple fumes condensed to form beautiful black crystals with a metallic luster. Courtois suspected this was a new element but lacked resources for full investigation, so he sent samples to two prominent chemists for analysis.

In 1813, British chemist Sir Humphry Davy, while passing through Paris on his way to Italy, examined Courtois's substance and recognized it as an element analogous to chlorine. Davy suggested the name iodine, derived from the Greek word ioeides meaning "violet-colored." French chemist Joseph Louis Gay-Lussac independently confirmed it was a new element and officially named it iodine in 1813, referring to the striking violet color of its vapor. Arguments erupted between Davy and Gay-Lussac over who identified iodine first, but both scientists acknowledged that Courtois was the first to isolate the element.

Is iodine dangerous?

Yes, elemental iodine and concentrated iodine solutions are toxic and must be handled carefully. Pure elemental iodine is toxic if taken orally undiluted, with a lethal dose for adult humans of approximately 30 milligrams per kilogram of body weight, which equals about 2.1 to 2.4 grams for a person weighing 70 to 80 kilograms. The toxicity derives from iodine's oxidizing properties, which denature proteins including essential enzymes.

Elemental iodine is also a skin irritant, and direct contact can cause lesions and chemical burns. Iodine vapor is intensely irritating to the eyes and mucous membranes. The maximum allowable concentration of iodine in air should not exceed 1 milligram per cubic meter for an 8-hour time-weighted average. Solutions with high elemental iodine concentration, such as tincture of iodine and Lugol's solution, can cause tissue damage if used for prolonged cleaning or antisepsis.

However, iodine compounds used in proper medical and dietary applications are safe. Excess dietary iodine can cause thyroid problems in some individuals, but taking 0.5 milligrams or less per day is unlikely to cause harm. The tolerable upper intake level for adults is 1,100 micrograms daily. Workers handling iodine should use proper ventilation, protective equipment, and follow safety protocols to prevent exposure.

Fun facts about iodine

• Iodine is the only halogen that exists as a solid at room temperature. All other halogens are either gases like fluorine and chlorine, or liquids like bromine at standard conditions.
• Despite having a higher atomic number than tellurium, iodine has a lower atomic mass at 126.904 atomic mass units compared to tellurium's 127.60. This atomic mass inversion initially puzzled scientists before atomic number was understood as the true organizing principle.
• Only one naturally occurring isotope exists: stable iodine-127. All 36 other known isotopes of iodine are radioactive. This makes iodine a mononuclidic element with constant atomic weight determined by nature.
• The distinctive starch-iodine test is one of the most sensitive chemical tests known. Even minute amounts of iodine in the presence of starch produce an intense blue-black color, making it useful for detecting iodine deficiency and testing for the presence of starch in various substances.
• In early periodic tables, iodine was often given the symbol J for Jod, its name in German. In German scientific texts, J is still frequently used instead of I.
• Iodine forms the longest single bond among all the halogens. The I-I bond length is 266.6 picometers in gaseous iodine molecules and even longer at 271.5 picometers in solid crystalline iodine.
• Seaweed accumulates iodine from seawater possibly as protection from UV damage. Iodide acts as an antioxidant, and in the process converts to elemental iodine, which explains why I₂ is detectable in the atmosphere of coastal regions where seaweed grows.

Scientific data verified from RSC, Britannica, and the Minerals Education Coalition.