Iodine: occurrence, extraction and uses (updated)

Iodine is a halogen element present in seawater, brines and kelp. Historically recovered from burned seaweed and Chilean caliche, modern industry primarily recovers iodine from brines and mineral deposits using chemical oxidation/reduction and stripping steps . Iodine forms the basis of antiseptics (tincture, povidone-iodine), is essential for thyroid hormones (T4/T3), and is added to table salt in public-health programs to prevent deficiency. Elemental iodine sublimes to a purple vapor, dissolves poorly in water but forms triiodide in iodide solutions, and gives a characteristic blue-black starch test.

What iodine is

Iodine (symbol I, atomic number 53, standard atomic weight 126.90447) is a nonmetal in the halogen group. In its pure form it is a dark grey crystalline solid with a metallic sheen that sublimes to a characteristic purple vapor when heated. Solid iodine has a density of about 4.93 g/cm3 and melts at about 113.7 °C.

Where iodine occurs

Iodine occurs naturally as iodide in seawater and in brines, and it concentrates in certain brown seaweeds (kelps) such as Laminaria species. Historically, kelp ashes and a Chilean mineral mixture called caliche were primary raw materials for iodine production. Today industrial recovery focuses on brines and mineral deposits as well as some seaweed-derived sources .

How iodine is extracted (historical and modern notes)

Traditional kelp processing involved burning the seaweed, extracting the ashes with water and chemically oxidizing iodide to free iodine for collection by distillation. Caliche processing historically involved leaching the deposit, treating the liquor to separate nitrates, and using sulfur dioxide or other reducing/oxidizing steps to precipitate and then sublimate iodine.

Modern commercial recovery still uses chemical conversion between iodide and iodine (oxidation/reduction) plus thermal or air-stripping and absorption steps to isolate elemental iodine from brines and mineral concentrates. Specific industrial flows vary by deposit and producer ¹.

Chemical properties

Iodine is the least reactive common halogen but forms compounds with many elements. It reacts with hydrogen to form hydrogen iodide (HI) and forms salts (iodides and iodates) with metals. Iodine dissolves sparingly in water, more readily in ethanol, and especially in solutions of iodide (forming triiodide ions). In nonpolar solvents such as carbon disulfide and chloroform it gives deep purple solutions. Iodine forms a deep blue-black complex with starch, a sensitive qualitative test widely used in laboratory work.

Uses and biological role

Iodine and its compounds have broad uses. Antiseptics include tincture of iodine and iodine-polyvinylpyrrolidone complexes (povidone-iodine). Iodides (for example potassium iodide) have medical uses and dietary roles: iodine is an essential micronutrient required for synthesis of thyroid hormones (thyroxine, T4, and triiodothyronine, T3). Dietary iodine deficiency can cause goiter and other forms of hypothyroidism.

Public-health programs commonly add small amounts of iodine to table salt (iodized salt) to prevent deficiency. Silver iodide has specialized uses such as cloud seeding and certain photographic processes. Large-scale technical and chemical applications also consume iodine and its derivatives.

Safety and regulation

Elemental iodine and many iodine compounds can be hazardous in concentrated forms; they require appropriate handling and regulation. Public-health iodine fortification programs follow guidelines from national health authorities and international bodies such as the World Health Organization.

1. Confirm current major iodine-producing countries and their relative production shares (modern primary sources: brines, caliche, seaweed).
2. Verify details of modern industrial extraction processes and typical process steps used by current producers.