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Name , symbol , atomic number Iodine, I, 53
Element category Halogens
Group , period , block 17 , 5 , p
Appearance gaseous: dark purple,
as solid: gray-black, shiny
CAS number 7553-56-2
EC number 231-442-4
ECHA InfoCard 100.028.585
ATC code
Mass fraction of the earth's envelope 0.06 ppm
Atomic mass 126.90447 (3) and
Atomic radius (calculated) 140 (115) pm
Covalent radius 139 pm
Van der Waals radius 198 pm
Electron configuration [ Kr ] 4 d 10 5 s 2 5 p 5
1. Ionization energy 10.451 260 (25) eV 1 008.39 kJ / mol
2. Ionization energy 19th.13126 (12) eV1 845.89 kJ / mol
3. Ionization energy 29.570 (25) eV2 853.1 kJ / mol
4. Ionization energy 40.357 (25) eV3 893.9 kJ / mol
5. Ionization energy 51.52 (4) eV4 971 kJ / mol
6. Ionization energy 74.4 (4) eV7 179 kJ / mol
7. Ionization energy 87.61 (6) eV8 453 kJ / mol
Physical state firmly
Crystal structure orthorhombic
density 4.94 g cm −3
magnetism diamagnetic ( Χ m = −4.3 10 −5 )
Melting point 386.85 K (113.70 ° C)
boiling point 457.2 K (184 ° C)
Molar volume 25.72 · 10 −6 m 3 · mol −1
Heat of evaporation 41.6 kJ / mol
Heat of fusion 7.76 kJ mol −1
Vapor pressure 35 Pa at 298 K
Thermal conductivity 0.449 W m −1 K −1
Oxidation states ± 1, 3, 5, 7
Normal potential 0.536 V (I 2 + 2 e - → 2 I - )
Electronegativity 2.66 ( Pauling scale )
isotope NH t 1/2 ZA ZE (M eV ) ZP
123 I. {syn.} 13.2235 h ε , γ 1.2 + 0.16 123 te
124 I. {syn.} 4.1760 d ε 124 te
β + 124 te
125 I. {syn.} 59.408 d ε 0.186 125 te
126 I. {syn.} 13.11 d ε 2.155 126 te
β - 1.258 126 Xe
127 I. 100  % Stable
128 I. {syn.} 24.99 min β - 2.118 128 Xe
ε 1.251 128 te
129 I. {syn.} 1.57 · 10 7 a β - 0.194 129 Xe
130 I. {syn.} 12.365 h β - 2.949 130 Xe
131 I. {syn.} 8.02070 d β - 0.971 131 Xe
For other isotopes see list of isotopes
safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary
08 - Dangerous to health 07 - Warning 09 - Dangerous for the environment


H and P phrases H: 312 + 332-315-319-335-372-400
P: 273-302 + 352-305 + 351 + 338-314

Switzerland: 0.1 ml m −3 or 1 mg m −3

Toxicological data
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

Iodine (standard language, but technically outdated iodine ) is a chemical element with the element symbol I (before the international element symbol introduction it was J ) and the ordinal number 53. In the periodic table it is in the 7th main group or the 17th  IUPAC group and belongs thus to the halogens . The name is derived from the ancient Greek word "ioeides" (ιο-ειδής) for "violet-colored, violet". Vapors produced when heated are characteristically purple.

Older names are iodine, iodina and iodine.

At room temperature, iodine is a solid that is poorly soluble in water, but readily soluble in aqueous potassium iodide solution and very soluble in ethanol and other organic solvents .

In everyday language , iodine is the common spelling, this spelling is also used in the older chemical and predominantly in current medical specialist literature. The spelling iodine is listed in the Duden with a reference to iodine ; reference is made to the technical language. The element symbol I has been used in the IUPAC publication in the specialist chemical literature since 1970, and the element designation J can be found throughout the older German-language literature .

Iodine is an indispensable component of the animal and human organism and is consumed with food. The concentration in humans is highest in the thyroid gland , where it is used in the hormones thyroxine and triiodothyronine and as diiodotyrosine . Iodine deficiency in drinking water and food is usually responsible for the occurrence of goiter . As a preventive measure, we recommend consuming sea fish once or several times a week and using so-called iodized salt (table salt mixed with sodium or potassium iodate ). This individual iodine prophylaxis and the iodination of animal feed partially compensate for the lack of iodine in the soil in Germany. It was temporarily achieved that the overall iodine supply of the population just adhered to the WHO guidelines . According to the Federal Institute for Risk Assessment , the iodine supply as of 2020 shows a downward trend and is not optimal.


The pharmacological importance of preparations containing iodine was known in ancient times . For example, 1500 years before our era, goiter sufferers were prescribed the iodine-containing thyroid glands of sheep or the ashes of sea sponges .

Iodine was first extracted in 1811 by the Parisian saltpeter and soda maker Bernard Courtois in the manufacture of gunpowder from the ashes of seaweed . The elementary character (which, for example, Humphry Davy made public early on and who gave the iodine its name) was not researched until 1813 by the French scientists Nicolas Clément and Joseph Louis Gay-Lussac , who gave it its current name a year later.


Apart from astatine, iodine is much rarer than the other halogens. It is widespread in nature, but only in the form of its compounds , for example enriched (0.02–1%) in Chile's nitrate , mainly in the form of sodium iodate (NaIO 3 ), but also sodium periodate (NaIO 4 ) and Lautarit (Ca (IO 3 ) 2 ). It can be detected in small traces in soils and rocks . On average, 1000 grams of anhydrous fine soil from Germany contains  around 2.5  milligrams of iodine. The iodine content of the soil is essential for supplying the population with natural iodine. As hydrogen iodide , it occurs in very small quantities in volcanic gases.

Soluble iodine compounds such as alkali and alkaline earth iodides are released during the weathering of rocks by rainwater or disintegrate at higher temperatures. This is how they get into the groundwater and eventually into the oceans . Some mineral waters contain iodine. The mineral spring at Woodhall Spa in Lincolnshire, England produces water that is colored brown by iodine. The amount of iodine in seawater is 0.05 milligrams per liter . It occurs there in the form of iodide (I - ) and iodate (IO 3 - ) in a concentration of around 500 nmol / L. The distribution in surface water generally varies from 0–200 nmol I - / L.

In the earth's atmosphere , iodine can be found in the form of organic compounds or inorganically in the form of iodine oxide (IO), iodine nitrate or higher oxides. Little information is available for the stratosphere and an upper limit of 0.1 ppt for inorganic iodine. High concentrations of more than 10 ppt IO were detected in algae fields on coasts and the iodine oxide radical was also detected on the tropical Atlantic.

Organic iodine compounds can be isolated from marine algae (19 grams of iodine per kilogram of dry matter ), seaweed and sponges (up to 14 grams of iodine per kilogram of dry matter). The thyroid is an important store for organically bound iodine .


So far, 36 isotopes and 10 nuclear isomers are known of iodine . Of these, only one isotope is stable, so that naturally occurring iodine consists of 100% of the only stable isotope 127 I. Iodine is therefore a pure element ( anisotopic ). Of the unstable isotopes, the beta emitter 129 I has a very long half-life of 15,700,000 years. There are also four isotopes with a half-life of more than one day: 124 I (4.2 days), 125 I (59 days), 126 I (13 days) and 131 I (8.0 days). Unstable iodine isotopes arise z. B. in nuclear fission and, if released into the air, represent a health hazard because they can accumulate in the thyroid gland.

Extraction and presentation

It used to be won iodine in the form of iodides and iodates by the stranded by the tide on the beach Tange gathered up and burned. The ash obtained contained about 0.1-0.5% iodine. Today this iodine production is only of local importance and accounts for around 2% of the world’s annual production.

The technical production of iodine is closely linked to the production of saltpetre . The iodate contained in the mother liquors is converted into elemental iodine by reduction . In the first reaction step, sulphurous acid is used to reduce iodic acid (iodate is the anion of this acid) to hydrogen iodide :

Iodic acid and sulphurous acid react to form hydrogen iodide and sulfuric acid

In a second step, the hydrogen iodide is in turn oxidized to iodine by the iodic acid present in the solution . In this case, one speaks of a comproportionation , as iodine becomes elemental iodine with the oxidation state 0 in two different oxidation states (−1 in hydrogen iodide and +5 in iodic acid).

Iodic acid and hydrogen iodide comproportion to water and iodine

Alternatively, sulfur dioxide (SO 2 ) can also be added to the final liquors, from which the saltpeter has already crystallized , for reduction .

During the production of oil and natural gas , significant amounts of brine are produced , which have an iodine content of 30 to over 100 ppm . The iodine in the form of sodium iodide is released from the brine through oxidation with chlorine :

Sodium iodide and chlorine react to form sodium chloride and iodine

The iodine obtained is further purified by blowing it out with air, then reducing it again with sulfur dioxide in a sulfuric acid solution and finally oxidizing it back to iodine with gaseous chlorine.

Iodine is reduced to hydrogen iodide by sulfur dioxide
Hydrogen iodide and chlorine react to form hydrogen chloride and iodine

Chromatographically iodine can by means of adsorption of polyiodide to anion exchangers are enriched. Potassium iodide and halogen-free copper sulfate are used for the purest production .

On a laboratory scale, iodine can be produced by the action of sulfuric acid and manganese (IV) oxide on potassium iodide . It can also be extracted from the iodine-containing ash of seaweed by treatment with chlorine.


Physical Properties

Under normal conditions, iodine is a solid that forms gray-black, shiny metallic flakes with a density of 4.94 g · cm −3 . When it melts ( melting point 113.70 ° C), iodine turns into a brown, electrically conductive liquid. It boils at 184.2 ° C with the formation of a purple vapor that consists of I 2 molecules. Iodine sublimes even at room temperature, so that melting is only possible with a rapid and strong increase in temperature.

Crystal and molecular structure

Iodine has the properties of a semiconductor . These properties are due to the presence of a layer lattice in which individual levels consist of I 2 molecules (bond length 271.5  pm ). The distance between the levels in an orthorhombic layer crystal is 441.2 pm and thus corresponds to the van der Waals distance between two iodine atoms (430 pm). The result of the measurement of the shortest distance between two iodine molecules is significantly lower at 349.6 pm.

Chemical properties

Iodine reacts much less violently with other elements such as phosphorus , aluminum , iron and mercury than chlorine and bromine . With hydrogen iodine reacts to hydrogen iodide , but which decomposes upon slight heating quickly in the elements.

Iodine and hydrogen are in equilibrium with hydrogen iodide. If the temperature increases, this is shifted to the left.

An explosive reaction occurs with ammonia due to the associated increase in volume.

Three moles of iodine and two moles of ammonia react to form six moles of hydrogen iodide and one mole of nitrogen .

With ammonia solution , iodine forms triiodine nitrogen (NI 3 ).

An interesting property of iodine is that it forms polyiodide compounds. The dissolved I 2 molecules combine with an iodide anion to form the single negatively charged I 3 - anion. A property of this polyiodide compound is that it is embedded in starch helices . These intercalation compounds cause an intense blue color even in low concentrations (sensitive and specific iodine starch detection ).

Iodine cations

Various cations of iodine are known. This is how the blue diiodine cation I 2 + is formed when iodine is oxidized by sulfur trioxide (65% oleum : H 2 SO 4 2 SO 3 ) dissolved in sulfuric acid :

Antimony (V) fluoride or tantalum pentafluoride in liquid sulfur dioxide can also be used for oxidation :

However, it has not managed to connect to the un solvated , that is solvent-free monoiodo cation I + to produce. Even in connection with an extremely weak Lewis base such as the perchlorate anion ClO 4 - one finds covalently bound iodine. In the gas phase, however, the I + ion can be detected in the absence of a counterion.



Maximum iodine levels in feed
Animal group Amount of iodine
[mg / kg]
Odd-toed ungulates ( horses ,
donkeys, mules)
Dairy cows 5
Laying hens 5
Pigs 10
Broiler poultry 10
fishes 20th

The mineral iodine (mostly in the form of iodide) is considered a nutrient in the diet of humans and many animals, and an essential trace element . Iodized salt in particular is used as an iodized food (enriched with iodates) to prevent iodine deficiency. In Regulation (EC) no. 1925/2006 (enrichment Regulation) the iodine content was set in food to him health-related according to Regulation (EC) no. 1924/2006 (Health Claims) in the European Union (declared in accordance with national food labeling regulations in Germany, Austria and Switzerland). It is often added to this, usually as calcium iodide. Iodine is also added to ready-made animal feed, such as most cat and dog foods, but bird feed (e.g. for canaries) can also be iodinated. For the addition of iodine to slaughtered animals, maximum levels apply in the EU on the recommendation or on questioning of the European Food Safety Authority (EFSA) . (see adjacent table)

In Germany, iodine compounds are still found in too small amounts in the soil. The iodine prophylaxis, which essentially consists of the iodination of table salt ("iodized salt") and the iodination of animal feed in agriculture, has improved the iodine supply in Germany to such an extent that ioduria in children is the most reliable criterion for assessing the iodine supply applies - in a large sample between 2003 and 2006, a median of 117 µg / l was achieved. It was thus in the lower range of the range of 100 to 200 µg / l recommended by the World Health Organization (WHO). According to WHO criteria, Germany is therefore considered to be adequately supplied with iodine. Nevertheless, 36% of the population in Germany have a mild and 21% moderate to severe iodine deficiency .

Medical applications

Treating a wound with an iodine antiseptic solution.

Iodine tincture and iodoform contain iodine in elemental or bound form and serve as an antimycotic and antiseptic . It is assumed that the disinfecting effect is based on the separation of oxygen from water. Shortly after its release ( in statu nascendi ), this oxygen is particularly reactive:

This mechanism is also discussed for the other halogens. For this reason, iodine is also used in individual cases to disinfect water in bathing establishments. In this context, it is advantageous that iodine is less aggressive than chlorine. However, this water treatment is not able to kill algae, so that an algaecide must also be added. However, intensive use of iodine can lead to discoloration of the skin. There is also a risk of allergies . Both of these can be avoided by using so-called iodophores , carrier materials that can bind iodine.

Iodates are added to table salt in small amounts in the form of sodium iodate or potassium iodate in order to prevent iodine deficiency diseases. It is offered in the trade as iodized salt .

The radioactive iodine isotopes 131 I (8.02 days half-life) and 123 I (13.22 hours half-life) are used as radiopharmaceuticals in nuclear medicine diagnostics and therapy, mainly for thyroid diseases ( compare radioiodine therapy ), with 131 I being used more and more today . To determine the bone mineral content , 125 I ( gamma radiation of γ = 35 keV, 59.4 days half-life) is used.

Iodine is a widely used catalyst in chemical reactions . It is used for stereospecific polymerizations of 1,3-butadiene . The sulfurization of aromatic compounds and the alkylation and condensation of aromatic amines are further fields of application.

Aromatic iodine compounds are used as X-ray contrast media in diagnostics .

Sodium iodide is used as a scintillator in scintillation counters .

Radiation protection

Radioactive iodine isotopes are found in the fallout from nuclear explosions and in the reactor of nuclear power plants (NPP). In the event of a nuclear accident with the release of radioactive iodine isotopes, the federal and state governments stockpile a total of 137 million potassium iodide tablets with a high content of the stable iodine isotope 127 I (mostly referred to as "iodine tablets") in the vicinity of the German nuclear power plants Iodine blockade is intended to prevent the uptake of radioactive iodine isotopes in the thyroid. The stock was last renewed in 2004. In Switzerland, potassium iodide tablets are given to everyone who lives within 50 kilometers of a nuclear power plant (around 4.6 million) and to the companies located there.

Iodine is considered to be a very volatile radionuclide which, as a result of nuclear fission, is present in high concentrations in the gap, the gap between the fuel pellets and the cladding tube of the NPP fuel rods. If the containment of the nuclear power plant should leak in the event of core damage or even a core meltdown after the occurrence of an accident, it will cause major damage from the point of view of radiation protection, since it is practically the first nuclide, next to the radioactive isotopes 85 Kr and 135 Xe of the noble gases krypton and Xenon , is released in larger concentrations and enters the biological cycle.

Biological importance

The thyroid hormone thyroxine

After the ingestion of larger doses of iodine compounds in the milligram range, irritative effects on the skin and mucous membranes occur. This can lead to "iodism", accompanied by the symptoms of runny nose ("iodine runny nose"), conjunctivitis , bronchitis and rashes . The effect on the bronchial mucosa has led to iodine salts being used as expectorants in the past . On the other hand, high-dose iodide can be used for suppression in hyperthyroidism , since it inhibits the release and synthesis of thyroid hormones.

Thyroid hormones

Iodine mainly plays a role in the production of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which contain four or three iodine atoms. The iodine supply in the human body is estimated at 10 to 30 milligrams. Iodine deficiency initially only leads to euthyroid goiter formation . Only a pronounced iodine deficiency also leads to an underactive thyroid ( hypothyroidism ), which is characterized by a reduced production of T4 and T3. Since the thyroid hormones take on essential functions in the regulation of metabolic processes in almost every cell in the body, severe metabolic and developmental disorders result from an underactive thyroid .

For the role of iodine supply in thyroid diseases see iodine prophylaxis , iodine intolerance , hyperthyroidism and Hashimoto's thyroiditis .


There is evidence that iodine and its metabolic products also influence the development and growth of neoplasms in the mammary gland .

safety instructions

Iodine is classified by the EU as a hazardous substance whose release into the environment must be avoided. Appropriate protective measures must be observed when handling the element. Residues of iodine must be treated with sodium thiosulphate solution (→ reduction to iodide). Before disposal in the wastewater, the pH value of the reaction mixture must be neutralized with sodium hydrogen carbonate . Iodine must never be brought into contact with ammonia, otherwise explosive iodine nitrogen can form.


Classic qualitative analysis of iodine

The starch- iodine reaction is the best known method for detecting elemental iodine. To prove this, a little aqueous starch solution is added to the sample to be examined. If iodine is present, a colored complex is formed.

Iodide ions can be detected by specific precipitation reactions . The reaction with silver nitrate , which leads to the precipitation of yellow silver iodide , is very well known :

Mercury (I) nitrate falls when it is used in excess, iodide ions in the form of yellow-green mercury (I) iodide :

If, on the other hand, mercury (II) nitrate is used, a red precipitate of mercury (II) iodide is obtained :

The redox reaction of elemental chlorine with iodide is the basis of a further qualitative detection reaction. Hydrogen chloride can iodide to elemental iodine oxidize while chlorine is reduced to chloride. The iodine formed becomes visible as a brownish color. It can be extracted with organic solvents (e.g. chloroform or carbon tetrachloride ) and used for photometric quantitative analysis.

Instrumental quantitative analysis of iodine

Atomic Absorption Spectrometry (AAS)

Iodine is practically not routinely detectable with the aid of AAS, since the resonance line in the vacuum UV is at 183.0 nm. Only a few analytical working groups report indirect methods, which, however, are fraught with great uncertainties.

Atomic Emission Spectrometry (AES)

Iodide can be detected using ICP-AES (ICP, inductively coupled plasma). One method is the oxidative generation of iodine vapor. The iodide ions produced reductively from iodate are atomized in the plasma and excited to emit at 178.3 nm. The detection limit achieved with this method is 0.4 µg / l.

Mass spectrometry (MS)

Only the isotope 127 I occurs in nature . It is the only one stable. With the help of ICP-Quadrupole-MS (ICP, inductively coupled plasma) it can be determined in digested rock samples with a detection limit of about 0.01 ng / ml. The coupling of ion chromatography and ICP-MS is available as a specific and sensitive method for the analysis of iodine in serum and urine samples .

Ion chromatography (IC)

A good method to separate iodides from a sample solution is anion exchange chromatography. Iodide is separated from other ions on an ion exchange column. The detection takes place z. B. with the help of electrochemical detectors that either measure the conductivity of the eluate or determine the amount of current that has flowed by means of pulsed amperometry . With the help of pulsed amperometry, a detection limit of 0.5 µg / l iodide in sea water could be achieved. For a short time now, ion chromatography has also been coupled with mass spectrometry. The IC-Tandem-MS technique achieves a detection limit of 0.33 µg / l urine.


The best known method for the photometric detection of iodide is the kinetic method according to Sandell and Kolthoff. The catalytic effect of iodide on the reduction of cerium (IV) by As (III) is proportional to the iodide concentration and can be determined from the rate at which the yellow cerium (III) solution decolorizes. This process was further developed by Moxon and Dixon and applied to the oxidation of thiocyanate by nitrite. The kinetic measurement of the absorbance is carried out at 450 nm. A detection limit of 0.01 µg / g for the total iodine content in food was reached.


Cathodic stripping voltammetry is ideal for the electrochemical determination of iodide . The actual voltammetric determination is preceded by an oxidative enrichment period on a carbon paste electrode at 700 mV. The enriched iodine is reduced again when a potential window of 700 mV to −400 mV is scanned. The reduction current is proportional to the iodide concentration. A detection limit of 0.25 µmol / l was achieved.


Structural formula of Iopamidol

→ Category: Iodine compound

Iodine forms compounds in various oxidation states from −1 to +7. The most stable and most frequent oxidation state is −1, the higher ones are only formed in compounds with the more electronegative elements oxygen , fluorine , chlorine and bromine . The odd oxidation states +1, +3, +5 and +7 are more stable than the even ones.

Hydrogen iodide and iodides

Inorganic compounds in which iodine is in the −1 oxidation state and thus as an anion are called iodides . These are derived from the gaseous hydrogen compound hydrogen iodide  (HI). An aqueous solution of this is called hydriodic acid . In aqueous solution it gives off the proton (pK s –10) very easily and is therefore more acidic than hydrogen bromide (pK s –8.9) or hydrogen chloride (pK s –6.2) in water.

The iodides of the alkali metals , especially sodium iodide and potassium iodide , are particularly well known . Iodides are usually well soluble in water, exceptions are many heavy metal iodides such as B. silver iodide , mercury (I) iodide , mercury (II) iodide and lead (II) iodide .

Iodides act as powerful reducing agents. Exposure to air causes iodides to gradually turn brown, copper (II) salt solutions cause the precipitation of Cu (I) iodide and the release of iodine. Silver iodide is only stable when light is excluded; when exposed to light, Ag (I) oxidizes the iodide to iodine and silver crystal nuclei ( black coloration in photo negatives ).

Iodine oxides

A large number of compounds of iodine and oxygen are known. These are based on the general formulas IO x (x = 1–4) and I 2 O x (x = 1–7). The compounds IO, IO 2 , I 2 O 4 , I 4 O 9 , I 2 O 5 and I 2 O 6 of the Iodoxids were detected, of which diiodopentoxide  (I 2 O 5 ) is the most stable compound.

Iodoic acids

In addition to the iodoxides, iodine and oxygen also form several oxygen acids in which an iodine atom is surrounded by one to four oxygen atoms, as well as the associated salts: hypoiodous acid  (HIO) and hypoiodite , iodic acid  (HIO 2 ) and the corresponding iodite , iodic acid  (HIO 3 ) and iodates as well as periodic acid  (H 5 IO 6 ) and the associated periodates .

Interhalogen compounds

Iodine forms a number of interhalogen compounds with the other halogens . These are iodine fluoride  (IF), iodine trifluoride  (IF 3 ), iodine pentafluoride  (IF 5 ), iodine heptafluoride  (IF 7 ), iodine chloride  (ICl), iodine dichloride  ((ICl 2 ) 2 ), iodine trichloride  (ICl 3 ) and iodobromide  (IBr). Iodine is always the more electropositive element in these compounds.

Organic iodine compounds

A large number of organic iodine compounds (also organoiodine compounds ) are produced synthetically, e.g. B. iodoalkanes and iodoaromatics . They are used, among other things, in medicine as contrast media . Iodine compounds are also produced in nature by, for example, macroalgae. When exposed to sunlight, the production of CH 3 I and other compounds on the sea surface is also possible.

See also


Web links

Commons : iodine  - collection of pictures, videos and audio files
Wiktionary: Jod  - explanations of meanings, word origins, synonyms, translations

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