In 1811 the French chemist Bernard Courtois (Courtois, Bernard) obtained a violet vapour by heating seaweed ashes with sulfuric acid as a by-product of the manufacture of saltpetre. This vapour condensed to a black crystalline substance, which he called “substance X.” In 1813 British chemist Sir Humphry Davy (Davy, Sir Humphry, Baronet), who was passing through Paris on his way to Italy, recognized substance X as an element analogous to chlorine; he suggested the name iodine from the Greek word ioeides, “violet coloured.”

The only naturally occurring isotope of iodine is stable iodine-127. An exceptionally useful radioactive isotope is iodine-131, which has a half-life of eight days. It is employed in medicine to monitor thyroid gland functioning, to treat goitre and thyroid cancer, and to locate tumours (tumour) of the brain and of the liver. It is also used in investigations to trace the course of compounds in metabolism. Several iodine compounds are used as contrast mediums in diagnostic radiology. In aqueous solution even minute amounts of iodine in the presence of starch produce a blue-black colour.

Iodine is a nonmetallic, nearly black solid at room temperature and has a glittering crystalline appearance. The molecular lattice contains discrete diatomic molecules (molecule), which are also present in the molten and the gaseous states. Above 700 °C (1,300 °F), dissociation into iodine atoms (atom) becomes appreciable.

Iodine has a moderate vapour pressure at room temperature and in an open vessel slowly sublimes to a deep violet vapour that is irritating to the eyes, nose, and throat. (Highly concentrated iodine is poisonous and may cause serious damage to skin and tissues.) For this reason, iodine is best weighed in a stoppered bottle; for the preparation of an aqueous solution, the bottle may contain a solution of potassium iodide, which considerably decreases the vapour pressure of iodine; a brown complex (triiodide) is readily formed:

KI + I₂ {long right arrow} KI₃.

Molten iodine may be used as a nonaqueous solvent for iodides. The electrical conductivity of molten iodine has in part been ascribed to the following self-ionization equilibrium:

3I₂ ⇌ I₃⁺ + I₃₋.

The alkali iodides are soluble in molten iodine and give conducting solutions typical of weak electrolytes (electrolyte). Alkali iodides react with compounds containing iodine with the oxidation number +1, such as iodine bromide, as in the following equation:

In such reactions the alkali iodides may be regarded as bases.

The iodine molecule can act as a Lewis acid in that it combines with various Lewis bases. The interaction is weak, however, and few solid complex compounds have been isolated. The complexes are easily detected in solution and are referred to as charge-transfer complexes. Iodine, for example, is slightly soluble in water and gives a yellowish-brown solution. Brown solutions are also formed with alcohol, ether, ketones (ketone), and other compounds acting as Lewis bases through an oxygen atom, as in the following example:

in which the R groups represent various organic groups.

Iodine gives a red solution in benzene, which is regarded as the result of a different type of charge-transfer complex. In inert solvents, such as carbon tetrachloride or carbon disulfide, violet-coloured solutions that contain uncoordinated iodine molecules are obtained. Iodine reacts also with iodide ions, because the latter can act as Lewis bases, and for this reason the solubility of iodine in water is greatly enhanced in the presence of an iodide. When cesium iodide is added, crystalline cesium triiodide may be isolated from the reddish-brown aqueous solution. Iodine forms a blue complex with starch, and this colour test is used to detect small amounts of iodine.

The electron affinity of the iodine atom is not much different from those of the other halogen atoms. Iodine is a weaker oxidizing agent than bromine, chlorine, or fluorine. The following reaction—oxidation of arsenite, (AsO₃)³⁻—in aqueous solution proceeds only in the presence of sodium hydrogen carbonate, which acts as a buffer:

In acidic solution, arsenate (AsO₄)³⁻) is reduced to arsenite, whereas, in strongly alkaline solution, iodine is unstable, and the reverse reaction occurs.

The most familiar oxidation by iodine is that of the thiosulfate ion, which is oxidized quantitatively to tetrathionate, as shown:

This reaction is used to determine iodine volumetrically. The consumption of iodine at the end point is detected by the disappearance of the blue colour produced by iodine in the presence of a fresh starch solution.

The first ionization potential of the iodine atom is considerably smaller than that of the lighter halogen atoms, and this is in accord with the existence of numerous compounds containing iodine in the positive oxidation states +1 (iodides), +3, +5 (iodates), and +7 (periodates). Iodine combines directly with many elements. Iodine combines readily with most metals (metal) and some nonmetals to form iodides; for example, silver and aluminum are easily converted into their respective iodides, and white phosphorus unites readily with iodine. The iodide ion is a strong reducing agent; that is, it readily gives up one electron. Although the iodide ion is colourless, iodide solutions may acquire a brownish tint as a result of oxidation of iodide to free iodine by atmospheric oxygen. Molecules of elemental iodine, consisting of two atoms (I₂), combine with iodides to form polyiodides (typically I₂ + I⁻ → I⁻₃), accounting for the high solubility of iodine in solutions that contain soluble iodide. The aqueous solution of hydrogen iodide (HI), known as hydroiodic acid, is a strong acid that is used to prepare iodides by reaction with metals or their oxides (oxide), hydroxides (hydroxide), and carbonates (carbonate). Iodine exhibits a +5 oxidation state in the moderately strong iodic acid (HIO₃), which can be readily dehydrated to yield the white solid iodine pentoxide (I₂O₅). Periodates may take a form represented by, for example, potassium metaperiodate (KIO₄) or silver paraperiodate (Ag₅IO₆), because the large size of the central iodine atom allows a relatively large number of oxygen atoms to get close enough to form bonds.

Iodine is produced commercially from iodine-containing brines (brine). Natural brines, or brines extracted from oil wells containing up to 150 mg per litre (0.02 ounce per gallon) of iodine, are found in Java, California, and northern Italy. Impurities, such as clay, sand, and oil, are removed by filtration, and the solution is passed through a stream of sulfur dioxide and then through a number of containers holding bundles of copper wire. The copper iodide that forms is removed by filtration, washed with water, dried, and finely ground. The product is heated with potassium carbonate to give potassium iodide, which is then oxidized to the free element with dichromate and sulfuric acid:

In an alternate process, chlorine is used as the oxidizing agent:

For a long time, iodine has been recovered on a commercial scale from seaweed. This is dried and burned; the ash is leached with water; sodium sulfate and sodium chloride are removed by crystallization; and the remaining solution is concentrated by evaporation of water. The final solution, which contains 30–100 grams per litre (4–13 ounces per gallon) of iodine, is treated with sulfuric acid in order to decompose any sulfite, and sulfide and manganese dioxide are added to release iodine, which is vaporized and purified by sublimation. Alternatively, addition of cupric sulfate gives cuprous iodide. Another formerly important source of iodine for commercial preparation was the saltpetre or nitrate deposits of Chile, in which iodine is present as solid iodates, especially calcium iodate, Ca(IO₃)₂.

Iodine is widely used as a disinfectant and antiseptic, frequently in a solution of alcohol and water containing potassium iodide. Several compounds of iodine, such as iodoform (CHI₃), also serve as antiseptics.

Because iodine is converted to thyroxine in the thyroid gland, a small amount of iodine is essential for the body, which contains an average of 14 mg (0.00049 ounce) of the element. Thyroxine is a hormone that is necessary for maintaining normal metabolism in all the body's cells (cell). In many places, drinking water contains sufficient iodine for this purpose. In the absence of iodine in the water supply, however, goitre and myxedema are prevalent, and a small quantity of iodine in the form of potassium iodide (KI) is frequently added to table salt in order to ensure against iodine deficiency.

Iodine and its compounds are used extensively in analytical chemistry. Many analytical procedures are based on the release or uptake of iodine and its subsequent titration with sodium thiosulfate (iodometry). Unsaturation of fats (fat) (that is, the number of double or triple bonds between carbon atoms (atom)) is determined by addition of free iodine (iodine number). Iodine compounds are also employed as catalysts in certain classes of organic reactions. Iodine, silver iodide, and potassium iodide are used in photography (photography, technology of). Silver iodide is also used to seed clouds to induce rain. Iodine has been introduced into metallurgical processes for the production of certain transition metals in a high state of purity, among them titanium, zirconium, thorium, chromium, and cobalt. Electronic equipment, such as scintillation counters (scintillation counter) or neutron detectors, contains single-crystal prisms (prism) consisting of alkali metal iodides. Iodine is also used in the production of dyes (dye).

Free iodine is detected (1) by the violet colour of the vapour or of its solution in carbon tetrachloride or carbon disulfide or (2) by the bright-blue colour produced in the presence of fresh starch solution in water, a very sensitive test.

Iodide ions (ion) may be detected in water (1) by the yellow precipitate of silver iodide, insoluble in water and ammonia solution, which is produced by addition of silver nitrate in the presence of dilute nitric acid, (2) by the formation of iodine on addition of chlorine or bromine water, (3) by the formation of iodine in the presence of other oxidizing agents, such as hydrogen peroxide or potassium dichromate, or (4) by the scarlet precipitate of mercuric iodide formed on addition of mercury dichloride, as in the equation below:

This precipitate is dissolved by excess of iodide ions because of the formation of a complex ion:

Iodate (【IO₃】⁻) or periodate (【IO₄】⁻) is reduced by sulfurous acid to iodide and may be detected as such.

For the quantitative determination of iodine, one of the following methods may be recommended: (1) gravimetrically, by precipitation as silver iodide; (2) volumetrically, by titrating iodine with a standardized solution of sodium thiosulfate (using starch as an indicator); or (3) potentiometric titration with silver nitrate, which is applicable in the presence of both chloride and bromide. The second method is applied in the determination of many oxidizing substances. Dichromate, for example, reacts with excess potassium iodide in the presence of sulfuric acid, as shown:

The iodine liberated is treated with standard thiosulfate solution.