Iodine is a
chemical element
A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemical compounds, chemical elements canno ...
with the
symbol
A symbol is a mark, sign, or word that indicates, signifies, or is understood as representing an idea, object, or relationship. Symbols allow people to go beyond what is known or seen by creating linkages between otherwise very different conc ...
I and
atomic number
The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of ever ...
53. The heaviest of the stable
halogen
The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this grou ...
s, it exists as a semi-lustrous, non-metallic solid at
standard conditions that melts to form a deep violet liquid at , and boils to a violet gas at . The element was discovered by the French chemist
Bernard Courtois in 1811 and was named two years later by
Joseph Louis Gay-Lussac, after the
Ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic pe ...
'violet-coloured'.
Iodine occurs in many oxidation states, including
iodide (I
−),
iodate (), and the various
periodate anions. It is the least abundant of the stable
halogen
The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this grou ...
s, being the sixty-first most abundant element. As the heaviest essential
mineral nutrient, iodine is required for the synthesis of
thyroid hormone
File:Thyroid_system.svg, upright=1.5, The thyroid system of the thyroid hormones T3 and T4
rect 376 268 820 433 Thyroid-stimulating hormone
rect 411 200 849 266 Thyrotropin-releasing hormone
rect 297 168 502 200 Hypothalamus
rect 66 216 386 25 ...
s.
Iodine deficiency affects about two billion people and is the leading preventable cause of
intellectual disabilities
Intellectual disability (ID), also known as general learning disability in the United Kingdom and formerly mental retardation,Rosa's Law, Pub. L. 111-256124 Stat. 2643(2010). is a generalized neurodevelopmental disorder characterized by signifi ...
.
The dominant producers of iodine today are
Chile
Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the eas ...
and
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...
. Due to its high atomic number and ease of attachment to
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
s, it has also found favour as a non-toxic
radiocontrast material. Because of the specificity of its uptake by the human body, radioactive isotopes of iodine can also be used to treat
thyroid cancer
Thyroid cancer is cancer that develops from the tissues of the thyroid gland. It is a disease in which cells grow abnormally and have the potential to spread to other parts of the body. Symptoms can include swelling or a lump in the neck. Ca ...
. Iodine is also used as a
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
in the industrial production of
acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...
and some
polymer
A polymer (; Greek '' poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s.
It is on the
World Health Organization's List of Essential Medicines.
History
In 1811, iodine was discovered by French chemist
Bernard Courtois,
[ In French, seaweed that had been washed onto the shore was called "varec", "varech", or "vareck", whence the English word "wrack". Later, "varec" also referred to the ashes of such seaweed: the ashes were used as a source of iodine and salts of sodium and potassium.] who was born to a manufacturer of
saltpetre
Potassium nitrate is a chemical compound with the chemical formula . This alkali metal nitrate salt is also known as Indian saltpetre (large deposits of which were historically mined in India). It is an ionic salt of potassium ions K+ and nitra ...
(an essential component of
gunpowder
Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, carbon (in the form of charcoal) and potassium nitrate (saltpeter). T ...
). At the time of the
Napoleonic Wars
The Napoleonic Wars (1803–1815) were a series of major global conflicts pitting the French Empire and its allies, led by Napoleon I, against a fluctuating array of European states formed into various coalitions. It produced a period of Fre ...
, saltpetre was in great demand in
France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
. Saltpetre produced from French
nitre beds required
sodium carbonate
Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...
, which could be isolated from
seaweed
Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of '' Rhodophyta'' (red), ''Phaeophyta'' (brown) and '' Chlorophyta'' (green) macroalgae. Seaweed species such as ...
collected on the coasts of
Normandy
Normandy (; french: link=no, Normandie ; nrf, Normaundie, Nouormandie ; from Old French , plural of ''Normant'', originally from the word for "northman" in several Scandinavian languages) is a geographical and cultural region in Northwestern ...
and
Brittany
Brittany (; french: link=no, Bretagne ; br, Breizh, or ; Gallo: ''Bertaèyn'' ) is a peninsula, historical country and cultural area in the west of modern France, covering the western part of what was known as Armorica during the period ...
. To isolate the sodium carbonate, seaweed was burned and the ash washed with water. The remaining waste was destroyed by adding
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
. Courtois once added excessive sulfuric acid and a cloud of purple vapour rose. He noted that the vapour crystallised on cold surfaces, making dark crystals.
[Greenwood and Earnshaw, p. 794] Courtois suspected that this material was a new element but lacked funding to pursue it further.
Courtois gave samples to his friends,
Charles Bernard Desormes
Charles Bernard Desormes (; 3 June 1777 – 30 August 1862) was a French physicist and chemist. He determined the ratio of the specific heats of gases in 1819. He did this and almost all his scientific work in collaboration with his son-in-law N ...
(1777–1838) and
Nicolas Clément
Nicolas Clément (12 January 1779 – 21 November 1841) was a French physicist and chemist.
He was a colleague of Charles Desormes, with whom he conducted the Clément-Desormes experiment. The two chemists are also credited with determining a ...
(1779–1841), to continue research. He also gave some of the substance to
chemist
A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties. Chemists carefully describe th ...
Joseph Louis Gay-Lussac (1778–1850), and to
physicist
A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe.
Physicists generally are interested in the root or ultimate cau ...
André-Marie Ampère
André-Marie Ampère (, ; ; 20 January 177510 June 1836) was a French physicist and mathematician who was one of the founders of the science of classical electromagnetism, which he referred to as "electrodynamics". He is also the inventor of nu ...
(1775–1836). On 29 November 1813, Desormes and Clément made Courtois' discovery public. They described the substance to a meeting of the Imperial
Institute of France
The (; ) is a French learned society, grouping five , including the Académie Française. It was established in 1795 at the direction of the National Convention. Located on the Quai de Conti in the 6th arrondissement of Paris, the institute m ...
. On 6 December, Gay-Lussac announced that the new substance was either an element or a compound of
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
.
Gay-Lussac suggested the name ''"iode"'', from the
Ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic pe ...
(, "violet"), because of the colour of iodine vapor.
Ampère had given some of his sample to English chemist
Humphry Davy
Sir Humphry Davy, 1st Baronet, (17 December 177829 May 1829) was a British chemist and inventor who invented the Davy lamp and a very early form of arc lamp. He is also remembered for isolating, by using electricity, several elements for ...
(1778–1829), who experimented on the substance and noted its similarity to
chlorine
Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...
. Davy sent a letter dated 10 December to the
Royal Society of London
The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, r ...
stating that he had identified a new element. Arguments erupted between Davy and Gay-Lussac over who identified iodine first, but both scientists acknowledged Courtois as the first to isolate the element.
In 1873 the French medical researcher
Casimir Joseph Davaine (1812–1882) discovered the antiseptic action of iodine.
Antonio Grossich
Antonio Grossich (7 June 1849 – 1 October 1926) was an italian surgeon from Fiume (now Rijeka, Croatia), a politician and a writer.
Born in Draguć (Draguccio d'Istria), near Buzet (Pinguente), Istria, Grossich at first studied law in Graz ...
(1849–1926), an Istrian-born surgeon, was among the first to use
sterilisation of the operative field. In 1908, he introduced tincture of iodine as a way to rapidly sterilise the human skin in the surgical field.
In early
periodic table
The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ch ...
s, iodine was often given the symbol ''J'', for ''Jod'', its name in
German
German(s) may refer to:
* Germany (of or related to)
**Germania (historical use)
* Germans, citizens of Germany, people of German ancestry, or native speakers of the German language
** For citizens of Germany, see also German nationality law
**Ge ...
.
Properties
Iodine is the fourth
halogen
The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this grou ...
, being a member of group 17 in the periodic table, below
fluorine
Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reactiv ...
,
chlorine
Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...
, and
bromine
Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...
; it is the heaviest stable member of its group. (The fifth and sixth halogens, the radioactive
astatine
Astatine is a chemical element with the symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. All of astatine's isotopes are short-live ...
and
tennessine, are not well-studied due to their expense and inaccessibility in large quantities, but appear to show various unusual properties for the group due to
relativistic effects
Relativistic quantum chemistry combines relativistic mechanics with quantum chemistry to calculate elemental properties and structure, especially for the heavier elements of the periodic table. A prominent example is an explanation for the color of ...
.) Iodine has an electron configuration of
rd
105s
25p
5, with the seven electrons in the fifth and outermost shell being its
valence electrons. Like the other halogens, it is one electron short of a full octet and is hence an oxidising agent, reacting with many elements in order to complete its outer shell, although in keeping with
periodic trends, it is the weakest oxidising agent among the stable halogens: it has the lowest
electronegativity among them, just 2.66 on the Pauling scale (compare fluorine, chlorine, and bromine at 3.98, 3.16, and 2.96 respectively; astatine continues the trend with an electronegativity of 2.2). Elemental iodine hence forms
diatomic molecule
Diatomic molecules () are molecules composed of only two atoms, of the same or different chemical elements. If a diatomic molecule consists of two atoms of the same element, such as hydrogen () or oxygen (), then it is said to be homonuclear. O ...
s with chemical formula I
2, where two iodine atoms share a pair of electrons in order to each achieve a stable octet for themselves; at high temperatures, these diatomic molecules reversibly dissociate a pair of iodine atoms. Similarly, the iodide anion, I
−, is the strongest reducing agent among the stable halogens, being the most easily oxidised back to diatomic I
2.
[Greenwood and Earnshaw, pp. 800–4] (Astatine goes further, being indeed unstable as At
− and readily oxidised to At
0 or At
+.)
The halogens darken in colour as the group is descended: fluorine is a very pale yellow, chlorine is greenish-yellow, bromine is reddish-brown, and iodine is violet.
Elemental iodine is slightly soluble in water, with one gram dissolving in 3450 ml at 20 °C and 1280 ml at 50 °C;
potassium iodide may be added to increase solubility via formation of
triiodide ions, among other polyiodides.
[Greenwood and Earnshaw, pp. 804–9] Nonpolar solvents such as
hexane
Hexane () is an organic compound, a straight-chain alkane with six carbon atoms and has the molecular formula C6H14.
It is a colorless liquid, odorless when pure, and with boiling points approximately . It is widely used as a cheap, relative ...
and
carbon tetrachloride
Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVACR) is an organic compound with the chemi ...
provide a higher solubility. Polar solutions, such as aqueous solutions, are brown, reflecting the role of these solvents as
Lewis base
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
s; on the other hand, nonpolar solutions are violet, the color of iodine vapour.
Charge-transfer complex
In chemistry, a charge-transfer (CT) complex or electron-donor-acceptor complex describes a type of supramolecular assembly of two or more molecules or ions. The assembly consists of two molecules that self-attract through electrostatic forc ...
es form when iodine is dissolved in polar solvents, hence changing the colour. Iodine is violet when dissolved in carbon tetrachloride and saturated hydrocarbons but deep brown in
alcohol
Alcohol most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
s and
amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent ...
s, solvents that form charge-transfer adducts.
The melting and boiling points of iodine are the highest among the halogens, conforming to the increasing trend down the group, since iodine has the largest electron cloud among them that is the most easily polarised, resulting in its molecules having the strongest
van der Waals interactions among the halogens. Similarly, iodine is the least volatile of the halogens, though the solid still can be observed to give off purple vapor.
Due to this property Iodine is commonly used to demonstrate
sublimation directly from
solid
Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structur ...
to
gas, which gives rise to a misconception that it does not
melt in
atmospheric pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
. Because it has the largest
atomic radius
The atomic radius of a chemical element is a measure of the size of its atom, usually the mean or typical distance from the center of the nucleus to the outermost isolated electron. Since the boundary is not a well-defined physical entity, there ...
among the halogens, iodine has the lowest first
ionisation energy, lowest
electron affinity, lowest
electronegativity and lowest reactivity of the halogens.
The interhalogen bond in diiodine is the weakest of all the halogens. As such, 1% of a sample of gaseous iodine at atmospheric pressure is dissociated into iodine atoms at 575 °C. Temperatures greater than 750 °C are required for fluorine, chlorine, and bromine to dissociate to a similar extent. Most bonds to iodine are weaker than the analogous bonds to the lighter halogens.
Gaseous iodine is composed of I
2 molecules with an I–I bond length of 266.6 pm. The I–I bond is one of the longest single bonds known. It is even longer (271.5 pm) in solid
orthorhombic
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with ...
crystalline iodine, which has the same crystal structure as chlorine and bromine. (The record is held by iodine's neighbour
xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
: the Xe–Xe bond length is 308.71 pm.) As such, within the iodine molecule, significant electronic interactions occur with the two next-nearest neighbours of each atom, and these interactions give rise, in bulk iodine, to a shiny appearance and
semiconducting properties.
Iodine is a two-dimensional semiconductor with a
band gap of 1.3 eV (125 kJ/mol): it is a semiconductor in the plane of its crystalline layers and an insulator in the perpendicular direction.
Isotopes
Of the thirty-seven known
isotopes of iodine, only one occurs in nature, iodine-127. The others are radioactive and have half-lives too short to be
primordial
Primordial may refer to:
* Primordial era, an era after the Big Bang. See Chronology of the universe
* Primordial sea (a.k.a. primordial ocean, ooze or soup). See Abiogenesis
* Primordial nuclide, nuclides, a few radioactive, that formed before t ...
. As such, iodine is both
monoisotopic and
mononuclidic and its atomic weight is known to great precision, as it is a constant of nature.
The longest-lived of the radioactive isotopes of iodine is
iodine-129, which has a half-life of 15.7 million years, decaying via
beta decay
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
to stable
xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
-129.
Some iodine-129 was formed along with iodine-127 before the formation of the Solar System, but it has by now completely decayed away, making it an
extinct radionuclide that is nevertheless still useful in dating the history of the early Solar System or very old groundwaters, due to its mobility in the environment. Its former presence may be determined from an excess of its
daughter
A daughter is a female offspring; a girl or a woman in relation to her parents. Daughterhood is the state of being someone's daughter. The male counterpart is a son. Analogously the name is used in several areas to show relations between groups ...
xenon-129. Traces of iodine-129 still exist today, as it is also a
cosmogenic nuclide, formed from
cosmic ray spallation of atmospheric xenon: these traces make up 10
−14 to 10
−10 of all terrestrial iodine. It also occurs from open-air nuclear testing, and is not hazardous because of its very long half-life, the longest of all fission products. At the peak of thermonuclear testing in the 1960s and 1970s, iodine-129 still made up only about 10
−7 of all terrestrial iodine.
[SCOPE 50 - Radioecology after Chernobyl]
, the Scientific Committee on Problems of the Environment (SCOPE), 1993. See table 1.9 in Section 1.4.5.2. Excited states of iodine-127 and iodine-129 are often used in
Mössbauer spectroscopy.
The other iodine radioisotopes have much shorter half-lives, no longer than days.
Some of them have medical applications involving the
thyroid gland
The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The thy ...
, where the iodine that enters the body is stored and concentrated.
Iodine-123 has a half-life of thirteen hours and decays by
electron capture
Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. ...
to
tellurium-123
There are 39 known isotopes and 17 nuclear isomers of tellurium (52Te), with atomic masses that range from 104 to 142. These are listed in the table below.
Naturally-occurring tellurium on Earth consists of eight isotopes. Two of these have been ...
, emitting
gamma radiation
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically s ...
; it is used in
nuclear medicine
Nuclear medicine or nucleology is a medical specialty involving the application of radioactive substances in the diagnosis and treatment of disease. Nuclear imaging, in a sense, is " radiology done inside out" because it records radiation emi ...
imaging, including
single photon emission computed tomography (SPECT) and
X-ray computed tomography
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30 ...
(X-Ray CT) scans.
Iodine-125
Iodine-125 (125I) is a radioisotope of iodine which has uses in biological assays, nuclear medicine imaging and in radiation therapy as brachytherapy to treat a number of conditions, including prostate cancer, uveal melanomas, and brain tumor ...
has a half-life of fifty-nine days, decaying by electron capture to
tellurium-125
There are 39 known isotopes and 17 nuclear isomers of tellurium (52Te), with atomic masses that range from 104 to 142. These are listed in the table below.
Naturally-occurring tellurium on Earth consists of eight isotopes. Two of these have been ...
and emitting low-energy gamma radiation; the second-longest-lived iodine radioisotope, it has uses in
biological assay
An assay is an investigative (analytic) procedure in laboratory medicine, mining, pharmacology, environmental biology and molecular biology for qualitatively assessing or quantitatively measuring the presence, amount, or functional activity of a ...
s,
nuclear medicine
Nuclear medicine or nucleology is a medical specialty involving the application of radioactive substances in the diagnosis and treatment of disease. Nuclear imaging, in a sense, is " radiology done inside out" because it records radiation emi ...
imaging and in
radiation therapy
Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Rad ...
as
brachytherapy
Brachytherapy is a form of radiation therapy where a sealed radiation source is placed inside or next to the area requiring treatment. ''Brachy'' is Greek for short. Brachytherapy is commonly used as an effective treatment for cervical, pro ...
to treat a number of conditions, including
prostate cancer
Prostate cancer is cancer of the prostate. Prostate cancer is the second most common cancerous tumor worldwide and is the fifth leading cause of cancer-related mortality among men. The prostate is a gland in the male reproductive system that su ...
,
uveal melanoma
Uveal melanoma is a type of eye cancer in the uvea of the eye. It is traditionally classed as originating in the iris, choroid, and ciliary body, but can also be divided into class I (low metastatic risk) and class II (high metastatic risk). S ...
s, and
brain tumour
A brain tumor occurs when abnormal cells form within the brain. There are two main types of tumors: malignant tumors and benign (non-cancerous) tumors. These can be further classified as primary tumors, which start within the brain, and second ...
s. Finally,
iodine-131
Iodine-131 (131I, I-131) is an important radioisotope of iodine discovered by Glenn Seaborg and John Livingood in 1938 at the University of California, Berkeley. It has a radioactive decay half-life of about eight days. It is associated with n ...
, with a half-life of eight days, beta decays to an excited state of stable
xenon-131 that then converts to the ground state by emitting gamma radiation. It is a common
fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
and thus is present in high levels in radioactive
fallout
Nuclear fallout is the residual radioactive material propelled into the upper atmosphere following a nuclear blast, so called because it "falls out" of the sky after the explosion and the shock wave has passed. It commonly refers to the radioac ...
. It may then be absorbed through contaminated food, and will also accumulate in the thyroid. As it decays, it may cause damage to the thyroid. The primary risk from exposure to high levels of iodine-131 is the chance occurrence of
radiogenic thyroid cancer
Thyroid cancer is cancer that develops from the tissues of the thyroid gland. It is a disease in which cells grow abnormally and have the potential to spread to other parts of the body. Symptoms can include swelling or a lump in the neck. Ca ...
in later life. Other risks include the possibility of non-cancerous growths and
thyroiditis.
The usual means of protection against the negative effects of iodine-131 is by saturating the thyroid gland with stable iodine-127 in the form of
potassium iodide tablets, taken daily for optimal prophylaxis. However, iodine-131 may also be used for medicinal purposes in
radiation therapy
Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Rad ...
for this very reason, when tissue destruction is desired after iodine uptake by the tissue. Iodine-131 is also used as a
radioactive tracer
A radioactive tracer, radiotracer, or radioactive label is a chemical compound in which one or more atoms have been replaced by a radionuclide so by virtue of its radioactive decay it can be used to explore the mechanism of chemical reactions by ...
.
Chemistry and compounds
Iodine is quite reactive, but it is much less reactive than the other halogens. For example, while chlorine gas will halogenate
carbon monoxide
Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...
,
nitric oxide
Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its ...
, and
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic a ...
(to
phosgene
Phosgene is the organic chemical compound with the formula COCl2. It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. Phosgene is a valued and important industrial building block, esp ...
,
nitrosyl chloride
Nitrosyl chloride is the chemical compound with the formula NOCl. It is a yellow gas that is commonly encountered as a component of aqua regia, a mixture of 3 parts concentrated hydrochloric acid and 1 part of concentrated nitric acid. It is a ...
, and
sulfuryl chloride respectively), iodine will not do so. Furthermore, iodination of metals tends to result in lower oxidation states than chlorination or bromination; for example,
rhenium metal reacts with chlorine to form
rhenium hexachloride
Rhenium(VI) chloride is the inorganic compound with the formula ReCl6. It is a black paramagnetic solid. The molecules adopt an octahedral structure as seen in tungsten(VI) chloride.
Preparation and reactions
Rhenium(VI) chloride was first gener ...
, but with bromine it forms only rhenium pentabromide and iodine can achieve only rhenium tetraiodide.
By the same token, however, since iodine has the lowest ionisation energy among the halogens and is the most easily oxidised of them, it has a more significant cationic chemistry and its higher oxidation states are rather more stable than those of bromine and chlorine, for example in
iodine heptafluoride
Iodine heptafluoride, also known as iodine(VII) fluoride or iodine fluoride, is an interhalogen compound with the chemical formula I F7. It has an unusual pentagonal bipyramidal structure, as predicted by VSEPR theory. The molecule can undergo ...
.
Charge-transfer complexes
The iodine molecule, I
2, dissolves in CCl
4 and aliphatic hydrocarbons to give bright violet solutions. In these solvents the absorption band maximum occurs in the 520 – 540 nm region and is assigned to a
* to ''σ''
* transition. When I
2 reacts with Lewis bases in these solvents a blue shift in I
2 peak is seen and the new peak (230 – 330 nm) arises that is due to the formation of adducts, which are referred to as charge-transfer complexes.
[Greenwood and Earnshaw, pp. 806-7]
Hydrogen iodide
The simplest compound of iodine is
hydrogen iodide
Hydrogen iodide () is a diatomic molecule and hydrogen halide. Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid, a strong acid. Hydrogen iodide and hydroiodic acid are, however, different in that the former is a gas under ...
, HI. It is a colourless gas that reacts with oxygen to give water and iodine. Although it is useful in
iodination
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers ...
reactions in the laboratory, it does not have large-scale industrial uses, unlike the other hydrogen halides. Commercially, it is usually made by reacting iodine with
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The under ...
or
hydrazine
Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, hydrazine ...
:
[Greenwood and Earnshaw, pp. 809–12]
:2 I
2 + N
2H
4 4 HI + N
2
At room temperature, it is a colourless gas, like all of the hydrogen halides except
hydrogen fluoride
Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock ...
, since hydrogen cannot form strong
hydrogen bond
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...
s to the large and only mildly electronegative iodine atom. It melts at −51.0 °C and boils at −35.1 °C. It is an
endothermic
In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. ...
compound that can exothermically dissociate at room temperature, although the process is very slow unless a
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
is present: the reaction between hydrogen and iodine at room temperature to give hydrogen iodide does not proceed to completion. The H–I
bond dissociation energy is likewise the smallest of the hydrogen halides, at 295 kJ/mol.
[Greenwood and Earnshaw, pp. 812–9]
Aqueous hydrogen iodide is known as
hydroiodic acid, which is a strong acid. Hydrogen iodide is exceptionally soluble in water: one litre of water will dissolve 425 litres of hydrogen iodide, and the saturated solution has only four water molecules per molecule of hydrogen iodide. Commercial so-called "concentrated" hydroiodic acid usually contains 48–57% HI by mass; the solution forms an
azeotrope
An azeotrope () or a constant heating point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation.Moore, Walter J. ''Physical Chemistry'', 3rd e Prentice-Hall 1962, pp. 140–142 This ...
with boiling point 126.7 °C at 56.7 g HI per 100 g solution. Hence hydroiodic acid cannot be concentrated past this point by evaporation of water.
Unlike
hydrogen fluoride
Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock ...
, anhydrous liquid hydrogen iodide is difficult to work with as a solvent, because its boiling point is low, it has a small liquid range, its
dielectric constant
The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insula ...
is low and it does not dissociate appreciably into H
2I
+ and ions – the latter, in any case, are much less stable than the
bifluoride ions () due to the very weak hydrogen bonding between hydrogen and iodine, though its salts with very large and weakly polarising cations such as
Cs+ and
(R =
Me,
Et,
Bu''n'') may still be isolated. Anhydrous hydrogen iodide is a poor solvent, able to dissolve only small molecular compounds such as
nitrosyl chloride
Nitrosyl chloride is the chemical compound with the formula NOCl. It is a yellow gas that is commonly encountered as a component of aqua regia, a mixture of 3 parts concentrated hydrochloric acid and 1 part of concentrated nitric acid. It is a ...
and
phenol
Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it r ...
, or salts with very low
lattice energies such as tetraalkylammonium halides.
Other binary iodides
Nearly all elements in the periodic table form binary iodides. The exceptions are decidedly in the minority and stem in each case from one of three causes: extreme inertness and reluctance to participate in chemical reactions (the
noble gas
The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low ch ...
es); extreme nuclear instability hampering chemical investigation before decay and transmutation (many of the heaviest elements beyond
bismuth
Bismuth is a chemical element with the symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs ...
); and having an electronegativity higher than iodine's (
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
,
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
, and the first three halogens), so that the resultant binary compounds are formally not iodides but rather oxides, nitrides, or halides of iodine. (Nonetheless,
nitrogen triiodide
Nitrogen triiodide is an inorganic compound with the formula N I3. It is an extremely sensitive contact explosive: small quantities explode with a loud, sharp snap when touched even lightly, releasing a purple cloud of iodine vapor; it can even ...
is named as an iodide as it is analogous to the other nitrogen trihalides.)
Given the large size of the iodide anion and iodine's weak oxidising power, high oxidation states are difficult to achieve in binary iodides, the maximum known being in the pentaiodides of
niobium
Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has s ...
,
tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that ...
, and
protactinium. Iodides can be made by reaction of an element or its oxide, hydroxide, or carbonate with hydroiodic acid, and then dehydrated by mildly high temperatures combined with either low pressure or anhydrous hydrogen iodide gas. These methods work best when the iodide product is stable to hydrolysis; otherwise, the possibilities include high-temperature oxidative iodination of the element with iodine or hydrogen iodide, high-temperature iodination of a metal oxide or other halide by iodine, a volatile metal halide,
carbon tetraiodide, or an organic iodide. For example,
molybdenum(IV) oxide
Molybdenum dioxide is the chemical compound with the formula MoO. It is a violet-colored solid and is a metallic conductor. The mineralogical form of this compound is called tugarinovite, and is only very rarely found.
Structure
It crystallizes ...
reacts with
aluminium(III) iodide at 230 °C to give
molybdenum(II) iodide. An example involving halogen exchange is given below, involving the reaction of
tantalum(V) chloride
Tantalum(V) chloride, also known as tantalum pentachloride, is an inorganic compound with the formula TaCl5. It takes the form of a white powder and is commonly used as a starting material in tantalum chemistry. It readily hydrolyzes to form tantal ...
with excess aluminium(III) iodide at 400 °C to give
tantalum(V) iodide
Tantalum(V) iodide is the inorganic compound with the formula Ta2I10. Its name comes from the compound's empirical formula, TaI5. It is a diamagnetic, black solid that hydrolyses readily. The compound adopts an edge-shared bioctahedral structure ...
:
[Greenwood and Earnshaw, pp. 821–4]
:
3TaCl5 + \underset -> 3TaI5 + 5AlCl3
Lower iodides may be produced either through thermal decomposition or disproportionation, or by reducing the higher iodide with hydrogen or a metal, for example:
:
TaI5 + Ta -> text ceTa6I14
Most metal iodides with the metal in low oxidation states (+1 to +3) are ionic. Nonmetals tend to form covalent molecular iodides, as do metals in high oxidation states from +3 and above. Both ionic and covalent iodides are known for metals in oxidation state +3 (e.g.
scandium iodide is mostly ionic, but
aluminium iodide is not). Ionic iodides MI
''n'' tend to have the lowest melting and boiling points among the halides MX
''n'' of the same element, because the electrostatic forces of attraction between the cations and anions are weakest for the large iodide anion. In contrast, covalent iodides tend to instead have the highest melting and boiling points among the halides of the same element, since iodine is the most polarisable of the halogens and, having the most electrons among them, can contribute the most to van der Waals forces. Naturally, exceptions abound in intermediate iodides where one trend gives way to the other. Similarly, solubilities in water of predominantly ionic iodides (e.g.
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...
and
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
) are the greatest among ionic halides of that element, while those of covalent iodides (e.g.
silver
Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...
) are the lowest of that element. In particular,
silver iodide
Silver iodide is an inorganic compound with the formula Ag I. The compound is a bright yellow solid, but samples almost always contain impurities of metallic silver that give a gray coloration. The silver contamination arises because AgI is hi ...
is very insoluble in water and its formation is often used as a qualitative test for iodine.
Iodine halides
The halogens form many binary,
diamagnetic interhalogen
In chemistry, an interhalogen compound is a molecule which contains two or more different halogen atoms (fluorine, chlorine, bromine, iodine, or astatine) and no atoms of elements from any other group.
Most interhalogen compounds known are binar ...
compounds with stoichiometries XY, XY
3, XY
5, and XY
7 (where X is heavier than Y), and iodine is no exception. Iodine forms all three possible diatomic interhalogens, a trifluoride and trichloride, as well as a pentafluoride and, exceptionally among the halogens, a heptafluoride. Numerous cationic and anionic derivatives are also characterised, such as the wine-red or bright orange compounds of and the dark brown or purplish black compounds of I
2Cl
+. Apart from these, some
pseudohalides are also known, such as
cyanogen iodide (ICN), iodine
thiocyanate (ISCN), and iodine
azide
In chemistry, azide is a linear, polyatomic anion with the formula and structure . It is the conjugate base of hydrazoic acid . Organic azides are organic compounds with the formula , containing the azide functional group. The dominant appli ...
(IN
3).
[Greenwood and Earnshaw, pp. 824–8]
Iodine monofluoride
Iodine monofluoride is an interhalogen compound of iodine and fluorine with formula IF. It is a chocolate-brown solid that decomposes at 0 °C, disproportionating to elemental iodine and iodine pentafluoride:
:5 IF → 2 I2 + IF5
However, ...
(IF) is unstable at room temperature and disproportionates very readily and irreversibly to iodine and
iodine pentafluoride, and thus cannot be obtained pure. It can be synthesised from the reaction of iodine with fluorine gas in
trichlorofluoromethane at −45 °C, with
iodine trifluoride
Iodine trifluoride is an interhalogen compound with the chemical formula IF3. It is a yellow solid which decomposes above −28 °C. It can be synthesised from the elements, but care must be taken to avoid the formation of IF5.
Reactions
F2 r ...
in trichlorofluoromethane at −78 °C, or with
silver(I) fluoride at 0 °C.
Iodine monochloride (ICl) and
iodine monobromide
Iodine monobromide is an interhalogen compound with the chemical symbol IBr. It is a dark red solid that melts near room temperature. Like iodine monochloride, IBr is used in some types of iodometry. It serves as a source of I+. Its Lewis acid pr ...
(IBr), on the other hand, are moderately stable. The former, a volatile red-brown compound, was discovered independently by
Joseph Louis Gay-Lussac and
Humphry Davy
Sir Humphry Davy, 1st Baronet, (17 December 177829 May 1829) was a British chemist and inventor who invented the Davy lamp and a very early form of arc lamp. He is also remembered for isolating, by using electricity, several elements for ...
in 1813–1814 not long after the discoveries of chlorine and iodine, and it mimics the intermediate halogen bromine so well that
Justus von Liebig
Justus Freiherr von Liebig (12 May 1803 – 20 April 1873) was a German scientist who made major contributions to agricultural and biological chemistry, and is considered one of the principal founders of organic chemistry. As a professor at th ...
was misled into mistaking bromine (which he had found) for iodine monochloride. Iodine monochloride and iodine monobromide may be prepared simply by reacting iodine with chlorine or bromine at room temperature and purified by
fractional crystallisation. Both are quite reactive and attack even
platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Pla ...
and
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...
, though not
boron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the '' boron group'' it has t ...
,
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
,
cadmium
Cadmium is a chemical element with the Symbol (chemistry), symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Li ...
,
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...
,
zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...
,
niobium
Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has s ...
,
molybdenum
Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lead ...
, and
tungsten
Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...
. Their reaction with organic compounds depends on conditions. Iodine chloride vapour tends to chlorinate
phenol
Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it r ...
and
salicyclic acid
Salicylic acid is an organic compound with the formula HOC6H4CO2H. A colorless, bitter-tasting solid, it is a precursor to and a metabolite of aspirin (acetylsalicylic acid). It is a plant hormone, and has been listed by the EPA Toxic Substance ...
, since when iodine chloride undergoes homolytic dissociation, chlorine and iodine are produced and the former is more reactive. However, iodine chloride in tetrachloromethane solution results in iodination being the main reaction, since now heterolytic fission of the I–Cl bond occurs and I
+ attacks phenol as an electrophile. However, iodine monobromide tends to brominate phenol even in tetrachloromethane solution because it tends to dissociate into its elements in solution, and bromine is more reactive than iodine.
When liquid, iodine monochloride and iodine monobromide dissociate into and anions (X = Cl, Br); thus they are significant conductors of electricity and can be used as ionising solvents.
Iodine trifluoride
Iodine trifluoride is an interhalogen compound with the chemical formula IF3. It is a yellow solid which decomposes above −28 °C. It can be synthesised from the elements, but care must be taken to avoid the formation of IF5.
Reactions
F2 r ...
(IF
3) is an unstable yellow solid that decomposes above −28 °C. It is thus little-known. It is difficult to produce because fluorine gas would tend to oxidise iodine all the way to the pentafluoride; reaction at low temperature with
xenon difluoride
Xenon difluoride is a powerful fluorinating agent with the chemical formula , and one of the most stable xenon compounds. Like most covalent inorganic fluorides it is moisture-sensitive. It decomposes on contact with water vapor, but is otherwis ...
is necessary.
Iodine trichloride, which exists in the solid state as the planar dimer I
2Cl
6, is a bright yellow solid, synthesised by reacting iodine with liquid chlorine at −80 °C; caution is necessary during purification because it easily dissociates to iodine monochloride and chlorine and hence can act as a strong chlorinating agent. Liquid iodine trichloride conducts electricity, possibly indicating dissociation to and ions.
[Greenwood and Earnshaw, pp. 828–831]
Iodine pentafluoride (IF
5), a colourless, volatile liquid, is the most thermodynamically stable iodine fluoride, and can be made by reacting iodine with fluorine gas at room temperature. It is a fluorinating agent, but is mild enough to store in glass apparatus. Again, slight electrical conductivity is present in the liquid state because of dissociation to and . The
pentagonal bipyramidal iodine heptafluoride
Iodine heptafluoride, also known as iodine(VII) fluoride or iodine fluoride, is an interhalogen compound with the chemical formula I F7. It has an unusual pentagonal bipyramidal structure, as predicted by VSEPR theory. The molecule can undergo ...
(IF
7) is an extremely powerful fluorinating agent, behind only
chlorine trifluoride,
chlorine pentafluoride, and
bromine pentafluoride among the interhalogens: it reacts with almost all the elements even at low temperatures, fluorinates
Pyrex glass to form iodine(VII) oxyfluoride (IOF
5), and sets carbon monoxide on fire.
[Greenwood and Earnshaw, pp. 832–835]
Iodine oxides and oxoacids
Iodine oxide
Iodine oxides are chemical compounds of oxygen and iodine. Iodine has only two stable oxides which are isolatable in bulk, iodine tetroxide and iodine pentoxide, but a number of other oxides are formed in trace quantities or have been hypothesized ...
s are the most stable of all the halogen oxides, because of the strong I–O bonds resulting from the large electronegativity difference between iodine and oxygen, and they have been known for the longest time.
The stable, white,
hygroscopic
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance' ...
iodine pentoxide (I
2O
5) has been known since its formation in 1813 by Gay-Lussac and Davy. It is most easily made by the dehydration of
iodic acid (HIO
3), of which it is the anhydride. It will quickly oxidise carbon monoxide completely to
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
at room temperature, and is thus a useful reagent in determining carbon monoxide concentration. It also oxidises nitrogen oxide,
ethylene
Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds).
Ethylene ...
, and
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The under ...
. It reacts with
sulfur trioxide
Sulfur trioxide (alternative spelling sulphur trioxide, also known as ''nisso sulfan'') is the chemical compound with the formula SO3. It has been described as "unquestionably the most important economically" sulfur oxide. It is prepared on an ind ...
and
peroxydisulfuryl difluoride (S
2O
6F
2) to form salts of the iodyl cation,
2">O2sup>+, and is reduced by concentrated
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
s to iodosyl salts involving
Osup>+. It may be fluorinated by fluorine, bromine trifluoride,
sulfur tetrafluoride, or
chloryl fluoride, resulting iodine pentafluoride, which also reacts with iodine pentoxide, giving iodine(V) oxyfluoride, IOF
3. A few other less stable oxides are known, notably I
4O
9 and I
2O
4; their structures have not been determined, but reasonable guesses are I
III(I
VO
3)
3 and
Osup>+
3">O3sup>− respectively.
[Greenwood and Earnshaw, pp. 851–3]
More important are the four oxoacids:
hypoiodous acid
Hypoiodous acid is the inorganic compound with the chemical formula HIO. It forms when an aqueous solution of iodine is treated with mercuric or silver salts. It rapidly decomposes by disproportionation:
: 5 HIO → HIO3 + 2 I2 ...
(HIO),
iodous acid (HIO
2),
iodic acid (HIO
3), and
periodic acid (HIO
4 or H
5IO
6). When iodine dissolves in aqueous solution, the following reactions occur:
[Greenwood and Earnshaw, pp. 853–9]
:
Hypoiodous acid is unstable to disproportionation. The hypoiodite ions thus formed disproportionate immediately to give iodide and iodate:
:
Iodous acid and iodite are even less stable and exist only as a fleeting intermediate in the oxidation of iodide to iodate, if at all.
Iodates are by far the most important of these compounds, which can be made by oxidising
alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
iodides with oxygen at 600 °C and high pressure, or by oxidising iodine with
chlorate
The chlorate anion has the formula ClO3-. In this case, the chlorine atom is in the +5 oxidation state. "Chlorate" can also refer to chemical compounds containing this anion; chlorates are the salts of chloric acid. "Chlorate", when followed by ...
s. Unlike chlorates, which disproportionate very slowly to form chloride and perchlorate, iodates are stable to disproportionation in both acidic and alkaline solutions. From these, salts of most metals can be obtained. Iodic acid is most easily made by oxidation of an aqueous iodine suspension by
electrolysis
In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from n ...
or fuming
nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
. Iodate has the weakest oxidising power of the halates, but reacts the quickest.
[Greenwood and Earnshaw, pp. 863–4]
Many periodates are known, including not only the expected tetrahedral , but also square-pyramidal , octahedral orthoperiodate ,
3(OH)3">O3(OH)3sup>2−,
2O8(OH2)">2O8(OH2)sup>4−, and . They are usually made by oxidising alkaline
sodium iodate electrochemically (with
lead(IV) oxide
Lead(IV) oxide is the inorganic compound with the formula PbO2. It is an oxide where lead is in an oxidation state of +4. It is a dark-brown solid which is insoluble in water. It exists in two crystalline forms. It has several important applicatio ...
as the anode) or by chlorine gas:
[Greenwood and Earnshaw, pp. 872–5]
: + 6 OH
− → + 3 H
2O + 2 e
−
: + 6 OH
− + Cl
2 → + 2 Cl
− + 3 H
2O
They are thermodymically and kinetically powerful oxidising agents, quickly oxidising Mn
2+ to
, and cleaving
glycols, α-
diketone
In organic chemistry, a dicarbonyl is a molecule containing two carbonyl () groups. Although this term could refer to any organic compound containing two carbonyl groups, it is used more specifically to describe molecules in which both carbony ...
s, α-
ketols, α-
aminoalcohols, and α-
diamine
A diamine is an amine with exactly two amino groups. Diamines are used as monomers to prepare polyamides, polyimides, and polyureas. The term ''diamine'' refers mostly to primary diamines, as those are the most reactive.
In terms of quantities ...
s.
Orthoperiodate especially stabilises high oxidation states among metals because of its very high negative charge of −5.
Orthoperiodic acid, H
5IO
6, is stable, and dehydrates at 100 °C in a vacuum to
metaperiodic acid, HIO
4. Attempting to go further does not result in the nonexistent iodine heptoxide (I
2O
7), but rather iodine pentoxide and oxygen. Periodic acid may be protonated by
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
to give the cation, isoelectronic to Te(OH)
6 and , and giving salts with bisulfate and sulfate.
Polyiodine compounds
When iodine dissolves in strong acids, such as fuming sulfuric acid, a bright blue
paramagnetic solution including cations is formed. A solid salt of the diiodine cation may be obtained by oxidising iodine with
antimony pentafluoride
Antimony pentafluoride is the inorganic compound with the formula Sb F5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, formed when mixing liquid HF with liquid SbF5 in a 2:1 ratio ...
:
:2 I
2 + 5 SbF
5 2 I
2Sb
2F
11 + SbF
3
The salt I
2Sb
2F
11 is dark blue, and the blue
tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that ...
analogue I
2Ta
2F
11 is also known. Whereas the I–I bond length in I
2 is 267 pm, that in is only 256 pm as the missing electron in the latter has been removed from an antibonding orbital, making the bond stronger and hence shorter. In
fluorosulfuric acid
Fluorosulfuric acid (IUPAC name: sulfurofluoridic acid) is the inorganic compound with the chemical formula HSO3F. It is one of the strongest acids commercially available. It is a tetrahedral molecule and is closely related to sulfuric acid, H2SO ...
solution, deep-blue reversibly dimerises below −60 °C, forming red rectangular diamagnetic . Other polyiodine cations are not as well-characterised, including bent dark-brown or black and centrosymmetric ''C''
2''h'' green or black , known in the and salts among others.
[Greenwood and Earnshaw, pp. 842–4]
The only important polyiodide anion in aqueous solution is linear
triiodide, . Its formation explains why the solubility of iodine in water may be increased by the addition of potassium iodide solution:
:I
2 + I
− (''K''
eq = ~700 at 20 °C)
Many other polyiodides may be found when solutions containing iodine and iodide crystallise, such as , , , and , whose salts with large, weakly polarising cations such as
Cs+ may be isolated.
[Greenwood and Earnshaw, pp. 835–9]
Organoiodine compounds
Organoiodine compounds have been fundamental in the development of organic synthesis, such as in the
Hofmann elimination
Hofmann elimination is an elimination reaction of an amine to form alkenes. The least stable alkene (the one with the least number of substituents on the carbons of the double bond), called the Hofmann product, is formed. This tendency, known as ...
of
amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent ...
s, the
Williamson ether synthesis, the
Wurtz coupling reaction, and in
Grignard reagent
A Grignard reagent or Grignard compound is a chemical compound with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromide . ...
s.
The
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
–iodine bond is a common functional group that forms part of core
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J ...
; formally, these compounds may be thought of as organic derivatives of the iodide anion. The simplest organoiodine compounds,
alkyl iodides, may be synthesised by the reaction of
alcohol
Alcohol most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
s with
phosphorus triiodide; these may then be used in
nucleophilic substitution reactions, or for preparing
Grignard reagent
A Grignard reagent or Grignard compound is a chemical compound with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromide . ...
s. The C–I bond is the weakest of all the carbon–halogen bonds due to the minuscule difference in electronegativity between carbon (2.55) and iodine (2.66). As such, iodide is the best
leaving group among the halogens, to such an extent that many organoiodine compounds turn yellow when stored over time due to decomposition into elemental iodine; as such, they are commonly used in
organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
, because of the easy formation and cleavage of the C–I bond. They are also significantly denser than the other organohalogen compounds thanks to the high atomic weight of iodine.
A few organic oxidising agents like the
iodanes contain iodine in a higher oxidation state than −1, such as
2-iodoxybenzoic acid, a common reagent for the oxidation of alcohols to
aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl gro ...
s, and
iodobenzene dichloride
Iodobenzene dichloride (PhICl2) is a complex of iodobenzene with chlorine. As a reagent for organic chemistry, it is used as an oxidant and chlorinating agent.
Chemical structure
Single-crystal X-ray crystallography has been used to determine i ...
(PhICl
2), used for the selective chlorination of
alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic ...
s and
alkyne
\ce
\ce
Acetylene
\ce
\ce
\ce
Propyne
\ce
\ce
\ce
\ce
1-Butyne
In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...
s. One of the more well-known uses of organoiodine compounds is the so-called
iodoform test, where
iodoform (CHI
3) is produced by the exhaustive iodination of a
methyl ketone (or another compound capable of being oxidised to a methyl ketone), as follows:
:
Some drawbacks of using organoiodine compounds as compared to organochlorine or organobromine compounds is the greater expense and toxicity of the iodine derivatives, since iodine is expensive and organoiodine compounds are stronger alkylating agents. For example,
iodoacetamide and
iodoacetic acid
Iodoacetic acid is a derivative of acetic acid. It is a toxic compound, because, like many alkyl halides, it is an alkylating agent.
It reacts with cysteine residues in proteins. It is often used to modify SH-groups to prevent the re-formation ...
denature proteins by irreversibly alkylating
cysteine
Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile.
When present as a deprotonated catalytic residue, some ...
residues and preventing the reformation of
disulfide
In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...
linkages.
Halogen exchange to produce iodoalkanes by the
Finkelstein reaction is slightly complicated by the fact that iodide is a better leaving group than chloride or bromide. The difference is nevertheless small enough that the reaction can be driven to completion by exploiting the differential solubility of halide salts, or by using a large excess of the halide salt.
In the classic Finkelstein reaction, an
alkyl chloride
An organochloride, organochlorine compound, chlorocarbon, or chlorinated hydrocarbon is an organic compound containing at least one covalently bonded atom of chlorine. The chloroalkane class (alkanes with one or more hydrogens substituted by chlor ...
or an
alkyl bromide
Organobromine compounds, also called organobromides, are organic compounds that contain carbon bonded to bromine. The most pervasive is the naturally produced bromomethane.
One prominent application of synthetic organobromine compounds is the ...
is converted to an
alkyl iodide by treatment with a solution of
sodium iodide
Sodium iodide (chemical formula NaI) is an ionic compound formed from the chemical reaction of sodium metal and iodine. Under standard conditions, it is a white, water-soluble solid comprising a 1:1 mix of sodium cations (Na+) and iodide anions (I ...
in
acetone
Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour.
Acetone is miscibl ...
. Sodium iodide is soluble in acetone and
sodium chloride
Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35. ...
and
sodium bromide are not. The reaction is driven toward products by
mass action due to the precipitation of the insoluble salt.
Occurrence and production
Iodine is the least abundant of the stable halogens, comprising only 0.46
parts per million
In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, th ...
of Earth's crustal rocks (compare: fluorine 544 ppm, chlorine 126 ppm, bromine 2.5 ppm).
[Greenwood and Earnshaw, pp. 795–796.] Among the 84
elements which occur in significant quantities (elements 1–42, 44–60, 62–83, 90 and 92), it ranks 61st in abundance. Iodide minerals are rare, and most deposits that are concentrated enough for economical extraction are
iodate minerals instead. Examples include
lautarite, Ca(IO
3)
2, and
dietzeite, 7Ca(IO
3)
2·8CaCrO
4.
These are the minerals that occur as trace impurities in the
caliche, found in
Chile
Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the eas ...
, whose main product is
sodium nitrate
Sodium nitrate is the chemical compound with the formula . This alkali metal nitrate salt is also known as Chile saltpeter (large deposits of which were historically mined in Chile) to distinguish it from ordinary saltpeter, potassium nitrate. ...
. In total, they can contain at least 0.02% and at most 1% iodine by mass.
Sodium iodate is extracted from the caliche and reduced to iodide by
sodium bisulfite
Sodium bisulfite (or sodium bisulphite, sodium hydrogen sulfite) is a chemical mixture with the approximate chemical formula NaHSO3. Sodium bisulfite in fact is not a real compound, but a mixture of salts that dissolve in water to give solutio ...
. This solution is then reacted with freshly extracted iodate, resulting in comproportionation to iodine, which may be filtered off.
The caliche was the main source of iodine in the 19th century and continues to be important today, replacing
kelp
Kelps are large brown algae seaweeds that make up the order Laminariales. There are about 30 different genera. Despite its appearance, kelp is not a plant - it is a heterokont, a completely unrelated group of organisms.
Kelp grows in "underwa ...
(which is no longer an economically viable source), but in the late 20th century
brine
Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for ...
s emerged as a comparable source. The Japanese
Minami Kanto gas field east of
Tokyo
Tokyo (; ja, 東京, , ), officially the Tokyo Metropolis ( ja, 東京都, label=none, ), is the capital and largest city of Japan. Formerly known as Edo, its metropolitan area () is the most populous in the world, with an estimated 37.46 ...
and the American
Anadarko Basin gas field in northwest
Oklahoma
Oklahoma (; Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a state in the South Central region of the United States, bordered by Texas on the south and west, Kansas on the north, Missouri on the northeast, Arkansas on the east, New ...
are the two largest such sources. The brine is hotter than 60 °C from the depth of the source. The
brine
Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for ...
is first
purified and acidified using
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
, then the iodide present is oxidised to iodine with
chlorine
Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...
. An iodine solution is produced, but is dilute and must be concentrated. Air is blown into the solution to
evaporate
Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humi ...
the iodine, which is passed into an absorbing tower, where
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic a ...
reduces the iodine. The
hydrogen iodide
Hydrogen iodide () is a diatomic molecule and hydrogen halide. Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid, a strong acid. Hydrogen iodide and hydroiodic acid are, however, different in that the former is a gas under ...
(HI) is reacted with chlorine to precipitate the iodine. After filtering and purification the iodine is packed.
: 2 HI + Cl
2 → I
2↑ + 2 HCl
: I
2 + 2 H
2O + SO
2 → 2 HI + H
2SO
4
: 2 HI + Cl
2 → I
2↓ + 2 HCl
These sources ensure that Chile and Japan are the largest producers of iodine today.
Alternatively, the brine may be treated with
silver nitrate
Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...
to precipitate out iodine as
silver iodide
Silver iodide is an inorganic compound with the formula Ag I. The compound is a bright yellow solid, but samples almost always contain impurities of metallic silver that give a gray coloration. The silver contamination arises because AgI is hi ...
, which is then decomposed by reaction with iron to form metallic silver and a solution of
. The iodine may then be liberated by displacement with
chlorine
Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...
.
[Greenwood and Earnshaw, p. 799.]
Applications
About half of all produced iodine goes into various organoiodine compounds, another 15% remains as the pure element, another 15% is used to form
potassium iodide, and another 15% for other inorganic iodine compounds.
Among the major uses of iodine compounds are
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
s, animal feed supplements, stabilisers, dyes, colourants and pigments, pharmaceutical, sanitation (from
tincture of iodine
Tincture of iodine, iodine tincture, or weak iodine solution is an antiseptic. It is usually 2 to 7% elemental iodine, along with potassium iodide or sodium iodide, dissolved in a mixture of ethanol and water. Tincture solutions are characterized ...
), and photography; minor uses include smog inhibition,
cloud seeding
Cloud seeding is a type of weather modification that aims to change the amount or type of precipitation that falls from clouds by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysica ...
, and various uses in analytical chemistry.
Chemical analysis
The iodide and iodate anions are often used for quantitative volumetric analysis, for example in
iodometry. Iodine and starch form a blue complex, and this reaction is often used to test for either starch or iodine and as an
indicator
Indicator may refer to:
Biology
* Environmental indicator of environmental health (pressures, conditions and responses)
* Ecological indicator of ecosystem health (ecological processes)
* Health indicator, which is used to describe the health o ...
in iodometry. The iodine test for starch is still used to detect
counterfeit
To counterfeit means to imitate something authentic, with the intent to steal, destroy, or replace the original, for use in illegal transactions, or otherwise to deceive individuals into believing that the fake is of equal or greater value tha ...
banknotes printed on starch-containing paper.
The
iodine value is the mass of iodine in grams that is consumed by 100 grams of a
chemical substance
A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wit ...
typically fats or oils. Iodine numbers are often used to determine the amount of unsaturation in
fatty acid
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...
s. This unsaturation is in the form of
double bond
In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...
s, which react with iodine compounds.
Potassium tetraiodomercurate(II)
Potassium tetraiodomercurate(II) is an inorganic compound consisting of potassium cations and the tetraiodomercurate(II) anion. It is mainly used as Nessler's reagent, a 0.09 mol/L solution of potassium tetraiodomercurate(II) (K2 gI4 in 2.5&nb ...
, K
2HgI
4, is also known as Nessler's reagent. It is often used as a sensitive spot test for
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
. Similarly,
Mayer's reagent
Mayer's reagent is an alkaloidal precipitating reagent used for the detection of alkaloids in natural products. Mayer's reagent is freshly prepared by dissolving a mixture of mercuric chloride (1.36 g) and of potassium iodide (5.00 g) in water (100 ...
(potassium tetraiodomercurate(II) solution) is used as a precipitating reagent to test for
alkaloid
Alkaloids are a class of basic, naturally occurring organic compounds that contain at least one nitrogen atom. This group also includes some related compounds with neutral and even weakly acidic properties. Some synthetic compounds of simila ...
s. Aqueous alkaline iodine solution is used in the iodoform test for methyl ketones.
Spectroscopy
The spectrum of the iodine molecule, I
2, consists of (not exclusively) tens of thousands of sharp spectral lines in the wavelength range 500–700 nm. It is therefore a commonly used wavelength reference (secondary standard). By measuring with a
spectroscopic Doppler-free technique while focusing on one of these lines, the
hyperfine
In atomic physics, hyperfine structure is defined by small shifts in otherwise degenerate energy levels and the resulting splittings in those energy levels of atoms, molecules, and ions, due to electromagnetic multipole interaction between the ...
structure of the iodine molecule reveals itself. A line is now resolved such that either 15 components (from even rotational quantum numbers, ''J''
even), or 21 components (from odd rotational quantum numbers, ''J''
odd) are measurable.
Cesium iodide and thallium-doped sodium iodide are used in crystal
scintillator
A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbe ...
s for the detection of gamma rays. The efficiency is high and energy dispersive spectroscopy is possible, but the resolution is rather poor.
Spacecraft propulsion
Propulsion systems employing iodine as the
propellant
A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or other motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the ...
can be built more compactly, with less mass (and cost), and operate more efficiently than the
gridded ion thrusters that were utilized to propel previous spacecraft, such as Japan's
Hayabusa probes, ESA's
GOCE satellite, or NASA's
DART
Dart or DART may refer to:
* Dart, the equipment in the game of darts
Arts, entertainment and media
* Dart (comics), an Image Comics superhero
* Dart, a character from ''G.I. Joe''
* Dart, a ''Thomas & Friends'' railway engine character
* Da ...
mission, all of which used xenon as the
reaction mass. Iodine's
atomic weight
Relative atomic mass (symbol: ''A''; sometimes abbreviated RAM or r.a.m.), also known by the deprecated synonym atomic weight, is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a giv ...
is only 3.3% less than that of xenon, while its first two
ionisation energies average 12% less; together, these make iodine ions a promising substitute.
Use of iodine should allow more widespread application of ion-thrust technology, particularly with smaller-scale space vehicles.
According to the
European Space Agency
, owners =
, headquarters = Paris, Île-de-France, France
, coordinates =
, spaceport = Guiana Space Centre
, seal = File:ESA emblem seal.png
, seal_size = 130px
, image = Views in the Main Control Room (120 ...
, "This small but potentially disruptive innovation could help to clear the skies of
space junk, by enabling tiny satellites to self-destruct cheaply and easily at the end of their missions, by steering themselves into the atmosphere where they would burn up."
In early 2021, the French group
ThrustMe performed an in-orbit demonstration of an electric-powered
ion thruster
An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity.
An ion thruster ionizes a neutral gas by extracting some electrons out of ...
for spacecraft, where iodine was used in lieu of
xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
as the source of
plasma, in order to generate
thrust
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...
by accelerating
ions with an
electrostatic
Electrostatics is a branch of physics that studies electric charges at rest ( static electricity).
Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for ...
field.
Medicine
Elemental iodine
Elemental iodine is used as an
antiseptic
An antiseptic (from Greek ἀντί ''anti'', "against" and σηπτικός ''sēptikos'', "putrefactive") is an antimicrobial substance or compound that is applied to living tissue/skin to reduce the possibility of infection, sepsis, or putre ...
either as the element, or as the water-soluble
triiodide anion I
3− generated ''in situ'' by adding
iodide to poorly water-soluble elemental iodine (the reverse chemical reaction makes some free elemental iodine available for antisepsis). Elemental iodine may also be used to treat
iodine deficiency.
In the alternative, iodine may be produced from
iodophors, which contain iodine complexed with a solubilizing agent (the iodide ion may be thought of loosely as the iodophor in triiodide water solutions). Examples of such preparations include:
*
Tincture of iodine
Tincture of iodine, iodine tincture, or weak iodine solution is an antiseptic. It is usually 2 to 7% elemental iodine, along with potassium iodide or sodium iodide, dissolved in a mixture of ethanol and water. Tincture solutions are characterized ...
: iodine in ethanol, or iodine and
sodium iodide
Sodium iodide (chemical formula NaI) is an ionic compound formed from the chemical reaction of sodium metal and iodine. Under standard conditions, it is a white, water-soluble solid comprising a 1:1 mix of sodium cations (Na+) and iodide anions (I ...
in a mixture of ethanol and water.
*
Lugol's iodine
Lugol's iodine, also known as aqueous iodine and strong iodine solution, is a solution of potassium iodide with iodine in water. It is a medication and disinfectant used for a number of purposes. Taken by mouth it is used to treat thyrotoxicosi ...
: iodine and iodide in water alone, forming mostly triiodide. Unlike tincture of iodine, Lugol's iodine has a minimised amount of the free iodine (I
2) component.
*
Povidone iodine (an
iodophor).
* Iodine-V: iodine (I
2) and
fulvic acid Humic substances (HS) are organic compounds that are important components of humus, the major organic fraction of soil, peat, and coal (and also a constituent of many upland streams, dystrophic lakes, and ocean water). For a long era in the 19th an ...
form a
clathrate compound
A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. The word ''clathrate'' is derived from the Latin (), meaning ‘with bars, latticed’. Most clathrate compounds are polymeric and completely envel ...
(iodine molecules are "caged" by fulvic acid in this
host-guest complex). A water-soluble, solid, stable, crystalline complex. Unlike other iodophors, Iodine-V only contains iodine in molecular (I
2) form.
The antimicrobial action of iodine is quick and works at low concentrations, and thus it is used in operating theatres. Its specific mode of action is unknown. It penetrates into microorganisms and attacks particular amino acids (such as
cysteine
Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile.
When present as a deprotonated catalytic residue, some ...
and
methionine
Methionine (symbol Met or M) () is an essential amino acid in humans. As the precursor of other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical ...
),
nucleotides, and
fatty acid
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...
s, ultimately resulting in
cell death. It also has an
antiviral
Antiviral drugs are a class of medication used for treating viral infections. Most antivirals target specific viruses, while a broad-spectrum antiviral is effective against a wide range of viruses. Unlike most antibiotics, antiviral drugs do n ...
action, but nonlipid viruses and
parvoviruses are less sensitive than lipid enveloped viruses. Iodine probably attacks surface proteins of
enveloped viruses, and it may also destabilise membrane fatty acids by reacting with
unsaturated carbon bonds.
Other formulations
Before the advent of organic
chelating agent
Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands a ...
s, salts of iodide were given orally in the treatment of
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...
or
mercury poisoning
Mercury poisoning is a type of metal poisoning due to exposure to mercury. Symptoms depend upon the type, dose, method, and duration of exposure. They may include muscle weakness, poor coordination, numbness in the hands and feet, skin rashes ...
, such as heavily popularized by
Louis Melsens
Louis-Henri-Frédéric Melsens (July 11, 1814 in Leuven – April 20, 1886 in Brussels) was a Belgian physicist and chemist. In 1846, he became professor of chemistry at the Royal Veterinary School of Cureghem in Anderlecht, Brussels. Melsen ...
and many nineteenth and early twentieth century doctors.
["On the Employment of Iodide of Potassium as a Remedy for the Affections Caused by Lead and Mercury"]
in ''Br Foreign Med Chir Rev.'' 1853 Jan; 11(21): 201–224.
In medicine, a saturated solution of
potassium iodide is used to treat acute
thyrotoxicosis. It is also used to block uptake of
iodine-131
Iodine-131 (131I, I-131) is an important radioisotope of iodine discovered by Glenn Seaborg and John Livingood in 1938 at the University of California, Berkeley. It has a radioactive decay half-life of about eight days. It is associated with n ...
in the thyroid gland (see isotopes section above), when this isotope is used as part of radiopharmaceuticals (such as
iobenguane) that are not targeted to the thyroid or thyroid-type tissues.
Iodine-131 (usually as iodide) is a component of
nuclear fallout, and is particularly dangerous owing to the thyroid gland's propensity to concentrate ingested iodine and retain it for periods longer than this isotope's radiological half-life of eight days. For this reason, people at risk of exposure to environmental radioactive iodine (iodine-131) in fallout may be instructed to take non-radioactive potassium iodide tablets. The typical adult dose is one 130 mg tablet per 24 hours, supplying 100 mg (100,000
micrograms
In the metric system, a microgram or microgramme is a unit of mass equal to one millionth () of a gram. The unit symbol is μg according to the International System of Units (SI); the recommended symbol in the United States and United Kingdom ...
) of ionic iodine. (The typical daily dose of iodine for normal health is of order 100 micrograms; see "Dietary Intake" below.) Ingestion of this large dose of non-radioactive iodine minimises the uptake of radioactive iodine by the thyroid gland.
As an element with high
electron density and atomic number, iodine absorbs X-rays weaker than 33.3 keV due to the
photoelectric effect
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...
of the innermost electrons. Organoiodine compounds are used with intravenous injection as X-ray
radiocontrast agents. This application is often in conjunction with advanced X-ray techniques such as
angiography
Angiography or arteriography is a medical imaging technique used to visualize the inside, or lumen, of blood vessels and organs of the body, with particular interest in the arteries, veins, and the heart chambers. Modern angiography is perfor ...
and
CT scanning. At present, all water-soluble radiocontrast agents rely on iodine.
Others
The production of
ethylenediamine dihydroiodide, provided as a
nutritional supplement for livestock, consumes a large portion of available iodine. Another significant use is a catalyst for the production of
acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...
by the
Monsanto and
Cativa process
The Cativa process is a method for the production of acetic acid by the carbonylation of methanol. The technology, which is similar to the Monsanto process, was developed by BP Chemicals and is under license by BP Plc. The process is based on an ...
es. In these technologies, which support the world's demand for acetic acid,
hydroiodic acid converts the
methanol
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is ...
feedstock into methyl iodide, which undergoes
carbonylation
Carbonylation refers to reactions that introduce carbon monoxide into organic and inorganic substrates. Carbon monoxide is abundantly available and conveniently reactive, so it is widely used as a reactant in industrial chemistry. The term carbon ...
. Hydrolysis of the resulting acetyl iodide regenerates hydroiodic acid and gives acetic acid.
Inorganic iodides find specialised uses. Titanium, zirconium, hafnium, and
thorium
Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high ...
are purified by the
van Arkel–de Boer process
The van Arkel–de Boer process, also known as the iodide process or crystal-bar process, was the first industrial process for the commercial production of pure ductile titanium, zirconium and some other metals. It was developed by Anton Eduard v ...
, which involves the reversible formation of the tetraiodides of these elements. Silver iodide is a major ingredient to traditional photographic film. Thousands of kilograms of silver iodide are used annually for
cloud seeding
Cloud seeding is a type of weather modification that aims to change the amount or type of precipitation that falls from clouds by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysica ...
to induce rain.
[
The organoiodine compound erythrosine is an important food coloring agent. Perfluoroalkyl iodides are precursors to important surfactants, such as perfluorooctanesulfonic acid.][
The ]iodine clock reaction
The iodine clock reaction is a classical chemical clock demonstration experiment to display chemical kinetics in action; it was discovered by Hans Heinrich Landolt in 1886. The iodine clock reaction exists in several variations, which each involve ...
(in which iodine also serves as a test for starch, forming a dark blue complex), is a popular educational demonstration experiment and example of a seemingly oscillating reaction (it is only the concentration of an intermediate product that oscillates).
Although iodine has widespread roles in many species, agents containing it can exert a ''differential'' effect upon different species in an agricultural system. The growth of all strains of '' Fusarium verticillioides'' is significantly inhibited by an iodine-containing fungistatic (AJ1629-34EC) at concentrations that do not harm the crop. This might be a less toxic anti-fungal agricultural treatment due to its relatively natural chemistry.
I is used as the radiolabel in investigating which ligand
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elect ...
s go to which plant pattern recognition receptors (PRRs).
Biological role
Iodine is an essential element
In the context of nutrition, a mineral is a chemical element required as an essential nutrient by organisms to perform functions necessary for life. However, the four major structural elements in the human body by weight ( oxygen, hydrogen, ca ...
for life and, at atomic number ''Z'' = 53, is the heaviest element commonly needed by living organisms. (Lanthanum
Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between l ...
and the other lanthanide
The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yt ...
s, as well as tungsten
Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...
with ''Z'' = 74 and uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
with ''Z'' = 92, are used by a few microorganisms.) It is required for the synthesis of the growth-regulating thyroid hormones thyroxine and triiodothyronine
Triiodothyronine, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate.
Production of T3 and its prohormone thyrox ...
(T4 and T3 respectively, named after their number of iodine atoms). A deficiency of iodine leads to decreased production of T3 and T4 and a concomitant enlargement of the thyroid tissue in an attempt to obtain more iodine, causing the disease known as simple goitre. The major form of thyroid hormone in the blood is thyroxine (T4), which has a longer half-life than T3. In humans, the ratio of T4 to T3 released into the blood is between 14:1 and 20:1. T4 is converted to the active T3 (three to four times more potent than T4) within cells
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
by deiodinases (5'-iodinase). These are further processed by decarboxylation
Decarboxylation is a chemical reaction that removes a carboxyl group and releases carbon dioxide (CO2). Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is ...
and deiodination to produce iodothyronamine (T1a) and thyronamine (T0a'). All three isoforms of the deiodinases are selenium
Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
-containing enzymes; thus dietary selenium is essential for T3 production.
Iodine accounts for 65% of the molecular weight of T4 and 59% of T3. Fifteen to 20 mg of iodine is concentrated in thyroid tissue and hormones, but 70% of all iodine in the body is found in other tissues, including mammary glands, eyes, gastric mucosa, fetal thymus, cerebro-spinal fluid and choroid plexus, arterial walls, the cervix, and salivary glands. In the cells of those tissues, iodide enters directly by sodium-iodide symporter
The sodium/iodide cotransporter, also known as the sodium/iodide symporter (NIS), is a protein that in humans is encoded by the ''SLC5A5'' gene. It is a transmembrane glycoprotein with a molecular weight of 87 k Da and 13 transmembrane domains ...
(NIS). The action of iodine in mammary tissue is related to fetal and neonatal development, but in the other tissues, it is (at least) partially unknown.
Dietary intake
The daily levels of intake recommended by the United States National Academy of Medicine are between 110 and 130 µg for infants up to 12 months, 90 µg for children up to eight years, 130 µg for children up to 13 years, 150 µg for adults, 220 µg for pregnant women and 290 µg for lactation. The Tolerable Upper Intake Level (UL) for adults is 1,100 μg/day. This upper limit was assessed by analyzing the effect of supplementation on thyroid-stimulating hormone
Thyroid-stimulating hormone (also known as thyrotropin, thyrotropic hormone, or abbreviated TSH) is a pituitary hormone that stimulates the thyroid gland to produce thyroxine (T4), and then triiodothyronine (T3) which stimulates the metabolism ...
.
The thyroid gland needs no more than 70 μg/day to synthesise the requisite daily amounts of T4 and T3. The higher recommended daily allowance levels of iodine seem necessary for optimal function of a number of body systems, including lactation
Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals. The proces ...
, gastric mucosa
The gastric mucosa is the mucous membrane layer of the stomach, which contains the glands and the gastric pits. In humans, it is about 1 mm thick, and its surface is smooth, soft, and velvety. It consists of simple columnar epithelium, lamin ...
, salivary gland
The salivary glands in mammals are exocrine glands that produce saliva through a system of ducts. Humans have three paired major salivary glands ( parotid, submandibular, and sublingual), as well as hundreds of minor salivary glands. Salivary ...
s, brain cells, choroid plexus, thymus
The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, thymus cell lymphocytes or '' T cells'' mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders ...
, and arterial walls.
Natural sources of dietary iodine include seafood
Seafood is any form of sea life regarded as food by humans, prominently including fish and shellfish. Shellfish include various species of molluscs (e.g. bivalve molluscs such as clams, oysters and mussels, and cephalopods such as octopus an ...
, such as fish, seaweeds (such as kelp
Kelps are large brown algae seaweeds that make up the order Laminariales. There are about 30 different genera. Despite its appearance, kelp is not a plant - it is a heterokont, a completely unrelated group of organisms.
Kelp grows in "underwa ...
) and shellfish
Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environ ...
, dairy products and eggs so long as the animals received enough iodine, and plants grown on iodine-rich soil.[ Iodised salt is fortified with iodine in the form of ]sodium iodide
Sodium iodide (chemical formula NaI) is an ionic compound formed from the chemical reaction of sodium metal and iodine. Under standard conditions, it is a white, water-soluble solid comprising a 1:1 mix of sodium cations (Na+) and iodide anions (I ...
.
As of 2000, the median intake of iodine from food in the United States was 240 to 300 μg/day for men and 190 to 210 μg/day for women. The general US population has adequate iodine nutrition, with women of childbearing age and pregnant women having a possible mild risk of deficiency. In Japan, consumption was considered much higher, ranging between 5,280 μg/day to 13,800 μg/day from dietary seaweed or kombu kelp, often in the form of kombu umami extracts for soup stock and potato chips. However, new studies suggest that Japan's consumption is closer to 1,000–3,000 μg/day. The adult UL in Japan was last revised to 3,000 µg/day in 2015.
After iodine fortification programs such as iodisation of salt have been implemented, some cases of iodine-induced hyperthyroidism
Hyperthyroidism is the condition that occurs due to excessive production of thyroid hormones by the thyroid gland. Thyrotoxicosis is the condition that occurs due to excessive thyroid hormone of any cause and therefore includes hyperthyroidis ...
have been observed (so-called Jod-Basedow phenomenon
The Jod-Basedow effect (also Jod-Basedow syndrome and Jod-Basedow phenomenon) is hyperthyroidism following administration of iodine or iodide, either as a dietary supplement, iodinated contrast medical imaging, or as a medication (mainly Amiodaron ...
). The condition seems to occur mainly in people over forty, and the risk appears higher when iodine deficiency is severe and the initial rise in iodine intake is high.
Deficiency
In areas where there is little iodine in the diet, typically remote inland areas and semi-arid equatorial climates where no marine foods are eaten, iodine deficiency gives rise to hypothyroidism
Hypothyroidism (also called ''underactive thyroid'', ''low thyroid'' or ''hypothyreosis'') is a disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormone. It can cause a number of symptoms, such as ...
, symptoms of which are extreme fatigue, goitre, mental slowing, depression, weight gain, and low basal body temperatures. Iodine deficiency is the leading cause of preventable intellectual disability
Intellectual disability (ID), also known as general learning disability in the United Kingdom and formerly mental retardation, Rosa's Law, Pub. L. 111-256124 Stat. 2643(2010). is a generalized neurodevelopmental disorder characterized by signif ...
, a result that occurs primarily when babies or small children are rendered hypothyroid
Hypothyroidism (also called ''underactive thyroid'', ''low thyroid'' or ''hypothyreosis'') is a disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormone. It can cause a number of symptoms, such as po ...
ic by a lack of the element. The addition of iodine to table salt has largely eliminated this problem in wealthier nations, but iodine deficiency remains a serious public health problem in the developing world today. Iodine deficiency is also a problem in certain areas of Europe. Information processing, fine motor skills, and visual problem solving are improved by iodine repletion in moderately iodine-deficient children.
Precautions
Toxicity
Elemental iodine (I2) is toxic
Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a sub ...
if taken orally undiluted. The lethal dose for an adult human is 30 mg/kg, which is about 2.1–2.4 grams for a human weighing 70 to 80 kg (even if experiments on rats demonstrated that these animals could survive after eating a 14000 mg/kg dose). Excess iodine can be more cytotoxic
Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are an immune cell or some types of venom, e.g. from the puff adder (''Bitis arietans'') or brown recluse spider (''Loxosceles reclusa'').
Cell physiology
Treating c ...
in the presence of selenium deficiency. Iodine supplementation in selenium-deficient populations is, in theory, problematic, partly for this reason. The toxicity derives from its oxidizing properties, through which it denaturates proteins (including enzymes).
Elemental iodine is also a skin irritant. Direct contact with skin can cause damage, and solid iodine crystals should be handled with care. Solutions with high elemental iodine concentration, such as tincture of iodine
Tincture of iodine, iodine tincture, or weak iodine solution is an antiseptic. It is usually 2 to 7% elemental iodine, along with potassium iodide or sodium iodide, dissolved in a mixture of ethanol and water. Tincture solutions are characterized ...
and Lugol's solution
Lugol's iodine, also known as aqueous iodine and strong iodine solution, is a solution of potassium iodide with iodine in water. It is a medication and disinfectant used for a number of purposes. Taken by mouth it is used to treat thyrotoxicosi ...
, are capable of causing tissue damage if used in prolonged cleaning or antisepsis; similarly, liquid Povidone-iodine
Povidone-iodine (PVP-I), also known as iodopovidone, is an antiseptic used for skin disinfection before and after surgery. It may be used both to disinfect the hands of healthcare providers and the skin of the person they are caring for. It may ...
(Betadine) trapped against the skin resulted in chemical burns in some reported cases.
Occupational exposure
People can be exposed to iodine in the workplace by inhalation, ingestion, skin contact, and eye contact. The Occupational Safety and Health Administration
The Occupational Safety and Health Administration'' (OSHA ) is a large regulatory agency of the United States Department of Labor that originally had federal visitorial powers to inspect and examine workplaces. Congress established the agen ...
(OSHA) has set the legal limit (Permissible exposure limit
The permissible exposure limit (PEL or OSHA PEL) is a legal limit in the United States for exposure of an employee to a chemical substance or physical agent such as high level noise. Permissible exposure limits are established by the Occupationa ...
) for iodine exposure in the workplace at 0.1 ppm (1 mg/m3) during an 8-hour workday. The National Institute for Occupational Safety and Health
The National Institute for Occupational Safety and Health (NIOSH, ) is the United States federal agency responsible for conducting research and making recommendations for the prevention of work-related injury and illness. NIOSH is part of the C ...
(NIOSH) has set a Recommended exposure limit (REL) of 0.1 ppm (1 mg/m3) during an 8-hour workday. At levels of 2 ppm, iodine is immediately dangerous to life and health.
Allergic reactions
Some people develop a hypersensitivity
Hypersensitivity (also called hypersensitivity reaction or intolerance) refers to undesirable reactions produced by the normal immune system, including allergies and autoimmunity. They are usually referred to as an over-reaction of the immune ...
to products and foods containing iodine. Applications of tincture of iodine or Betadine can cause rashes, sometimes severe.[DermNet New Zealand Trust]
Iodine
/ref> Parenteral
A route of administration in pharmacology and toxicology is the way by which a drug, fluid, poison, or other substance is taken into the body.
Routes of administration are generally classified by the location at which the substance is applied. ...
use of iodine-based contrast agents (see above) can cause reactions ranging from a mild rash to fatal anaphylaxis
Anaphylaxis is a serious, potentially fatal allergic reaction and medical emergency that is rapid in onset and requires immediate medical attention regardless of use of emergency medication on site. It typically causes more than one of the follo ...
. Such reactions have led to the misconception (widely held, even among physicians) that some people are allergic to iodine itself; even allergies to iodine-rich seafood have been so construed. In fact, there has never been a confirmed report of a true iodine allergy, and an allergy to elemental iodine or simple iodide salts is theoretically impossible. Hypersensitivity reactions to products and foods containing iodine are apparently related to their other molecular components;[UCSF Department of Radiology & Biomedical Imaging]
Iodine Allergy and Contrast Administration
/ref> thus, a person who has demonstrated an allergy to one food or product containing iodine may not have an allergic reaction to another. Patients with various food allergies (shellfish, egg, milk, etc.) do not have an increased risk for a contrast medium hypersensitivity. As with all medications, the patient's allergy history should be questioned and consulted before any containing iodine are administered.
US DEA List I status
Phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...
can reduce elemental iodine to hydroiodic acid, which is a reagent effective for reducing ephedrine or pseudoephedrine
Pseudoephedrine (PSE) is a sympathomimetic drug of the phenethylamine and amphetamine chemical classes. It may be used as a nasal/sinus decongestant, as a stimulant, or as a wakefulness-promoting agent in higher doses.
It was first charact ...
to methamphetamine
Methamphetamine (contracted from ) is a potent central nervous system (CNS) stimulant that is mainly used as a recreational drug and less commonly as a second-line treatment for attention deficit hyperactivity disorder and obesity. Methamp ...
. For this reason, iodine was designated by the United States Drug Enforcement Administration
The Drug Enforcement Administration (DEA; ) is a United States federal law enforcement agency under the U.S. Department of Justice tasked with combating drug trafficking and distribution within the U.S. It is the lead agency for domestic en ...
as a List I precursor chemical under 21 CFR 1310.02.
References
Bibliography
*
{{Authority control
Chemical elements
Halogens
Reactive nonmetals
Diatomic nonmetals
Dietary minerals
Oxidizing agents
Gases with color
Chemical elements with primitive orthorhombic structure