Editing Iodine (section)

==Applications==
About half of all produced iodine goes into various [[Organoiodine chemistry|organoiodine compounds]], another 15% remains as the pure element, another 15% is used to form [[potassium iodide]], and another 15% for other [[Iodine compounds|inorganic iodine compounds]].<ref name="Greenwood800" /> Among the major uses of iodine compounds are [[Catalysis|catalysts]], animal feed supplements, stabilisers, dyes, colourants and pigments, pharmaceutical, sanitation (from [[tincture of iodine]]), and photography; minor uses include [[Smog tower|smog inhibition]], [[cloud seeding]], and various uses in [[analytical chemistry]].<ref name="Greenwood800" />

===X-ray imaging===
As an element with high [[electron density]] and atomic number, iodine efficiently absorbs X-rays.  X-ray [[Radiocontrast agent|radiocontrast]] agents is the top application for iodine.<ref name="Ullmann"/>  In this application, Organoiodine compounds are injected intravenously. This application is often in conjunction with advanced X-ray techniques such as [[angiography]] and [[CT scan]]ning. At present, all water-soluble radiocontrast agents rely on [[Iodinated contrast|iodine-containing compounds]].

Iodine absorbs X-rays with energies less than 33.3&nbsp;keV due to the [[photoelectric effect]] of the innermost electrons.<ref>{{cite book | vauthors = Lancaster JL | chapter-url = http://ric.uthscsa.edu/personalpages/lancaster/DI-II_Chapters/DI_chap4.pdf | chapter = Chapter 4: Physical Determinants of Contrast | archive-url =https://web.archive.org/web/20151010172937/http://ric.uthscsa.edu/personalpages/lancaster/DI-II_Chapters/DI_chap4.pdf | archive-date=10 October 2015 | title = Physics of Medical X-Ray Imaging | publisher = The University of Texas Health Science Center }}</ref> 

===Biocide===
{{Main|Iodine (medical use)}}
[[File:Diatrizoic acid.svg|thumb|right|[[Diatrizoate|Diatrizoic acid]], an iodine-containing [[radiocontrast agent]]]]

Use of iodine as a biocide represents a major application of the element, ranked 2nd by weight.<ref name="Ullmann"/> Elemental iodine (I<sup>2</sup>) is used as an [[antiseptic]] in medicine.<ref name="WHO2008">{{cite book | title = WHO Model Formulary 2008 | year = 2009 | isbn = 978-92-4-154765-9 | vauthors = ((World Health Organization)) | veditors = Stuart MC, Kouimtzi M, Hill SR | hdl = 10665/44053 | author-link = World Health Organization | publisher = World Health Organization | hdl-access=free | page=499 }}</ref> A number of water-soluble compounds, from [[triiodide]] (I<sub>3</sub><sup>−</sup>, generated ''in situ'' by adding [[iodide]] to poorly water-soluble elemental iodine) to various [[iodophor]]s, slowly decompose to release I<sup>2</sup> when applied.<ref>{{cite book | vauthors = Block SS |title=Disinfection, sterilization, and preservation |publisher=Lippincott Williams & Wilkins |location=Hagerstwon, MD |date=2001 |page=159 |isbn=978-0-683-30740-5}}</ref>

===Optical polarising films===
[[TFT LCD|Thin-film-transistor liquid crystal displays]] rely on [[polarization (waves)| polarisation]]. The liquid crystal transistor is sandwiched between two polarising films and illuminated from behind. The two films prevent light transmission unless the transistor in the middle of the sandwich rotates the light.<ref>{{Cite journal |last=Ma |first=Ji |last2=Ye |first2=Xin |last3=Jin |first3=Bo |date=1 April 2011 |title=Structure and application of polarizer film for thin-film-transistor liquid crystal displays |url=https://linkinghub.elsevier.com/retrieve/pii/S0141938211000126 |journal=Displays |volume=32 |issue=2 |pages=49–57 |doi=10.1016/j.displa.2010.12.006 |issn=0141-9382}}</ref> Iodine-impregnated polymer films are used in  [[polarization (waves)| polarising]] optical components with the highest transmission and degree of polarisation.<ref>{{cite book |doi=10.1002/9781118909911.ch26 |chapter=Polarizing Films |title=Iodine Chemistry and Applications |date=2014 |last1=Kahr |first1=Bart |last2=Knowles |first2=Kevin M. |pages=479–488 |isbn=978-1-118-46629-2 }}</ref>

===Co-catalyst===
Another significant use of iodine is as a cocatalyst for the production of [[acetic acid]] by the [[Monsanto process|Monsanto]] and [[Cativa process]]es. In these technologies, [[hydroiodic acid]] converts the [[methanol]] feedstock into methyl iodide, which undergoes [[carbonylation]]. Hydrolysis of the resulting acetyl iodide regenerates hydroiodic acid and gives acetic acid. The majority of acetic acid is produced by these approaches.<ref name=UllmannAA>{{Ullmann | vauthors = Le Berre C, Serp P, Kalck, P, Torrence GP | title = Acetic Acid | doi = 10.1002/14356007.a01_045.pub3|year=2013|publisher=Wiley-VCH|location=Weinheim}}</ref><ref>{{Cite journal |last=Sunley |first=Glenn J |last2=Watson |first2=Derrick J |date=26 May 2000 |title=High productivity methanol carbonylation catalysis using iridium: The Cativa™ process for the manufacture of acetic acid |url=https://linkinghub.elsevier.com/retrieve/pii/S0920586100002637 |journal=Catalysis Today |volume=58 |issue=4 |pages=293–307 |doi=10.1016/S0920-5861(00)00263-7 |issn=0920-5861}}</ref>

===Nutrition===
Salts of iodide and iodate are used extensively in human and animal nutrition. This application reflects the status of iodide as an [[Mineral (nutrient)|essential element]], being required for two hormones.  The production of [[ethylenediamine dihydroiodide]], provided as a [[nutrition|nutritional supplement]] for livestock, consumes a large portion of available iodine.<ref name="Ullmann">{{cite book | vauthors = Lyday PA, Kaiho T | chapter = Iodine and Iodine Compounds | title = Ullmann's Encyclopedia of Industrial Chemistry | date = 2015 | publisher = Wiley-VCH | location = Weinheim | doi = 10.1002/14356007.a14_381.pub2 | volume = A14 | pages = 382–390 | isbn = 978-3-527-30673-2 }}</ref> Iodine is a component of [[iodised salt]]. 

A saturated solution of [[potassium iodide]] is used to treat acute [[Hyperthyroidism|thyrotoxicosis]]. It is also used to block uptake of [[iodine-131]] 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.<ref>{{cite web |url=http://hazard.com/msds/mf/baker/baker/files/p5906.htm |title=Solubility of KI in water |publisher=Hazard.com |date=21 April 1998 |access-date=21 January 2013 |archive-date=23 April 2012 |archive-url=https://web.archive.org/web/20120423195709/http://hazard.com/msds/mf/baker/baker/files/p5906.htm |url-status=live }}</ref><ref>{{cite web|url=http://www.eanm.org/scientific_info/guidelines/gl_radio_ther_benzyl.pdf|archive-url=https://web.archive.org/web/20090617073253/http://www.eanm.org/scientific_info/guidelines/gl_radio_ther_benzyl.pdf | title=EANM procedure guidelines for 131I-meta-iodobenzylguanidine (131I-mIBG) therapy|url-status=dead|archive-date=17 June 2009|date=17 June 2009}}</ref>

===Others===
Inorganic iodides find specialised uses. [[Titanium]], [[zirconium]], [[hafnium]], and [[thorium]] are purified by the [[Van Arkel–de Boer process]], 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]] to induce rain.<ref name = Ullmann/>

The organoiodine compound [[erythrosine]] is an important food colouring agent. Perfluoroalkyl iodides are precursors to important surfactants, such as [[perfluorooctanesulfonic acid]].<ref name = Ullmann/>

{{sup|125}}I is used as the [[Radioactive tracer|radiolabel]] in investigating which [[ligand (biochemistry)|ligand]]s go to which [[Pattern recognition receptor|plant pattern recognition receptors]] (PRRs).<ref name="Boutrot-Zipfel-2017">{{cite journal | vauthors = Boutrot F, Zipfel C | title = Function, Discovery, and Exploitation of Plant Pattern Recognition Receptors for Broad-Spectrum Disease Resistance | journal = Annual Review of Phytopathology | volume = 55 | issue = 1 | pages = 257–286 | date = August 2017 | pmid = 28617654 | doi = 10.1146/annurev-phyto-080614-120106 | publisher = [[Annual Reviews (publisher)|Annual Reviews]] | doi-access = free }}</ref>

An iodine based thermochemical cycle has been evaluated for hydrogen production using energy from nuclear power.<ref name="Corgnale-Review-2020"/> The cycle has  three steps. At {{convert|120|°C}}, iodine reacts with [[sulfur dioxide]] and water to give hydrogen iodide and [[sulfuric acid]]:
:{{chem2 | I2 + SO2 + 2 H2O ->  2 HI + H2SO4 }}
After a separation stage, at {{convert|830|–|850|°C}} sulfuric acid splits in sulfur dioxide and oxygen:
:{{chem2 | 2 H2SO4 ->  SO2 + 2 H2O + O2 }}
Hydrogen iodide, at {{convert|300|–|320|°C}}, gives hydrogen and the initial element, iodine:
:{{chem2 | 2 HI -> I2 + H2 }}
The yield of the cycle (ratio between lower heating value of the produced hydrogen and the consumed energy for its production, is approximately 38%. {{as of|2020}}, the cycle is not a competitive means of producing hydrogen.<ref name="Corgnale-Review-2020">{{Cite journal |last1=Corgnale |first1=Claudio |last2=Gorensek |first2=Maximilian B. |last3=Summers |first3=William A. |date=November 2020 |title=Review of Sulfuric Acid Decomposition Processes for Sulfur-Based Thermochemical Hydrogen Production Cycles |journal=Processes |language=en |volume=8 |issue=11 |pages=1383 |doi=10.3390/pr8111383 |doi-access=free |issn=2227-9717}}</ref>