Editing Sulfur dioxide (section)

==Uses==
The overarching, dominant use of sulfur dioxide is in the production of [[sulfuric acid]].<ref name = Ullmann/>

===Precursor to sulfuric acid===
Sulfur dioxide is an intermediate in the production of sulfuric acid, being converted to [[sulfur trioxide]], and then to [[oleum]], which is made into sulfuric acid. Sulfur dioxide for this purpose is made when sulfur combines with oxygen. The method of converting sulfur dioxide to sulfuric acid is called the [[contact process]]. Several million tons are produced annually for this purpose.

===Food preservative===
{{See also|Food preservation}}
Sulfur dioxide is sometimes used as a preservative for dried apricots, dried figs, and other dried fruits, owing to its [[antimicrobial]] properties and ability to prevent [[oxidation]],<ref>{{cite conference |last1=Zamboni |first1=Cibele B. |last2=Medeiros |first2=Ilca M. M. A. |last3=de Medeiros |first3=José A. G. |title=Analysis of Sulfur in Dried Fruits Using NAA |url=https://www.ipen.br/biblioteca/2011/inac/17204.pdf |conference=2011 International Nuclear Atlantic Conference – INAC 2011 |isbn=978-85-99141-03-8 |date=October 2011 |access-date=2020-06-04 |archive-date=2020-06-04 |archive-url=https://web.archive.org/web/20200604193519/https://www.ipen.br/biblioteca/2011/inac/17204.pdf |url-status=dead }}</ref> and is called [[E number|E]]220<ref>[http://www.food.gov.uk/safereating/chemsafe/additivesbranch/enumberlist#h_3 Current EU approved additives and their E Numbers], The Food Standards Agency website.</ref> when used in this way in Europe. As a preservative, it maintains the colorful appearance of the fruit and prevents [[Decomposition|rotting]]. Historically, [[molasses]] was "sulfured" as a preservative and also to lighten its color. Treatment of dried fruit was usually done outdoors, by igniting sublimed sulfur and burning in an enclosed space with the fruits.<ref name="University of Georgia">{{Citation |title=Preserving foods: Drying fruits and Vegetable |url=https://nchfp.uga.edu/publications/uga/uga_dry_fruit.pdf |publisher=University of Georgia cooperative extension service |access-date=2022-06-06 |archive-date=2022-09-27 |archive-url=https://web.archive.org/web/20220927163031/https://nchfp.uga.edu/publications/uga/uga_dry_fruit.pdf |url-status=dead }}</ref> Fruits may be sulfured by dipping them into [[sodium bisulfite]], [[sodium sulfite]] or [[sodium metabisulfite]].<ref name="University of Georgia" />

==== Winemaking ====
Sulfur dioxide was first used in [[winemaking]] by the Romans, when they discovered that burning sulfur candles inside empty wine vessels keeps them fresh and free from vinegar smell.<ref>{{cite web|url=http://www.practicalwinery.com/janfeb09/page1.htm|publisher=www.practicalwinery.com|date=February 1, 2009|title=Practical Winery & vineyard Journal Jan/Feb 2009|url-status=dead|archive-url=https://web.archive.org/web/20130928111625/http://www.practicalwinery.com/janfeb09/page1.htm|archive-date=2013-09-28}}</ref>

It is still an important compound in winemaking, and is measured in [[parts per million]] (''ppm'') in wine. It is present even in so-called unsulfurated wine at concentrations of up to 10&nbsp;mg/L.<ref>[http://www.morethanorganic.com/sulphur-in-the-bottle Sulphites in wine], MoreThanOrganic.com.</ref> It serves as an [[antibiotic]] and [[antioxidant]], protecting wine from spoilage by bacteria and oxidation – a phenomenon that leads to the browning of the wine and a loss of cultivar specific flavors.<ref name="Jackson">Jackson, R.S. (2008) Wine science: principles and applications, Amsterdam; Boston: Elsevier/Academic Press</ref><ref name="Guerrero">{{cite journal | doi = 10.1016/j.tifs.2014.11.004| title = Demonstrating the efficiency of sulphur dioxide replacements in wine: A parameter review| journal = Trends in Food Science & Technology| volume = 42| pages = 27–43| year = 2015| last1 = Guerrero| first1 = Raúl F| last2 = Cantos-Villar| first2 = Emma| issue = 1}}</ref> Its antimicrobial action also helps minimize volatile acidity. Wines containing sulfur dioxide are typically labeled with "containing [[sulfite]]s".

Sulfur dioxide exists in wine in free and bound forms, and the combinations are referred to as total SO<sub>2</sub>. Binding, for instance to the carbonyl group of [[acetaldehyde]], varies with the wine in question. The free form exists in equilibrium between molecular SO<sub>2</sub> (as a dissolved gas) and bisulfite ion, which is in turn in equilibrium with sulfite ion. These equilibria depend on the pH of the wine. Lower pH shifts the equilibrium towards molecular (gaseous) SO<sub>2</sub>, which is the active form, while at higher pH more SO<sub>2</sub> is found in the inactive sulfite and bisulfite forms. The molecular SO<sub>2</sub> is active as an antimicrobial and antioxidant, and this is also the form which may be perceived as a pungent odor at high levels. Wines with total SO<sub>2</sub> concentrations below 10 ppm do not require "contains sulfites" on the label by US and EU laws. The upper limit of total SO<sub>2</sub> allowed in wine in the US is 350 ppm; in the EU it is 160 ppm for red wines and 210 ppm for white and rosé wines. In low concentrations, SO<sub>2</sub> is mostly undetectable in wine, but at free SO<sub>2</sub> concentrations over 50 ppm, SO<sub>2</sub> becomes evident in the smell and taste of wine.{{Citation needed|date=May 2009}}

SO<sub>2</sub> is also a very important compound in winery sanitation. Wineries and equipment must be kept clean, and because bleach cannot be used in a winery due to the risk of [[cork taint]],<ref>[http://www.extension.purdue.edu/extmedia/FS/FS-50-W.pdf Chlorine Use in the Winery]. Purdue University</ref> a mixture of SO<sub>2</sub>, water, and citric acid is commonly used to clean and sanitize equipment. [[Ozone]] (O<sub>3</sub>) is now used extensively for sanitizing in wineries due to its efficacy, and because it does not affect the wine or most equipment.<ref>[https://www.practicalwinery.com/janfeb00/ozone.htm Use of ozone for winery and environmental sanitation] {{Webarchive|url=https://web.archive.org/web/20170912102459/https://www.practicalwinery.com/janfeb00/ozone.htm |date=September 12, 2017 }}, Practical Winery & Vineyard Journal.</ref>

===As a reducing agent===
Sulfur dioxide is also a good [[Reducing agent|reductant]]. In the presence of water, sulfur dioxide is able to decolorize substances. Specifically, it is a useful reducing [[bleach]] for papers and delicate materials such as clothes. This bleaching effect normally does not last very long. Oxygen in the atmosphere reoxidizes the reduced dyes, restoring the color. In municipal wastewater treatment, sulfur dioxide is used to treat chlorinated wastewater prior to release. Sulfur dioxide reduces free and combined chlorine to [[chloride]].<ref>{{cite book |last=Tchobanoglous |first=George |title=Wastewater Engineering |edition=3rd |location=New York |publisher=McGraw Hill |year=1979 |isbn=0-07-041677-X }}</ref>

Sulfur dioxide is fairly soluble in water, and by both IR and Raman spectroscopy; the hypothetical [[sulfurous acid]], H<sub>2</sub>SO<sub>3</sub>, is not present to any extent. However, such solutions do show spectra of the hydrogen sulfite ion, HSO<sub>3</sub><sup>−</sup>, by reaction with water, and it is in fact the actual reducing agent present:
:SO<sub>2</sub> + H<sub>2</sub>O ⇌ HSO<sub>3</sub><sup>−</sup> + H<sup>+</sup>

===As a fumigant===
In the beginning of the 20th century sulfur dioxide was used in [[Buenos Aires]] as a fumigant to kill rats that carried the ''[[Yersinia pestis]]'' bacterium, which causes bubonic plague. The application was successful, and the application of this method was extended to other areas in South America. In Buenos Aires, where these apparatuses were known as [[Sulfurozador]], but later also in Rio de Janeiro, New Orleans and San Francisco, the sulfur dioxide treatment machines were brought into the streets to enable extensive disinfection campaigns, with effective results.<ref>{{cite journal |last1=Engelmann |first1=Lukas |title=Fumigating the Hygienic Model City: Bubonic Plague and the Sulfurozador in Early-Twentieth-Century Buenos Aires |journal=Medical History |date=July 2018 |volume=62 |issue=3 |pages=360–382 |doi=10.1017/mdh.2018.37 |pmid=29886876 |pmc=6113751 }}</ref>

===Biochemical and biomedical roles===
Sulfur dioxide or its conjugate base bisulfite is produced biologically as an intermediate in both sulfate-reducing organisms and in sulfur-oxidizing bacteria, as well. The role of sulfur dioxide in mammalian biology is not yet well understood.<ref>{{cite journal |last1=Liu |first1=D. |last2=Jin |first2=H. |last3=Tang |first3=C. |last4=Du |first4=J. |title=Sulfur Dioxide: a Novel Gaseous Signal in the Regulation of Cardiovascular Functions |journal=Mini-Reviews in Medicinal Chemistry |year=2010 |volume=10 |issue=11 |pages=1039–1045 |doi=10.2174/1389557511009011039 |pmid=20540708 }}</ref>  Sulfur dioxide blocks nerve signals from the [[pulmonary stretch receptors]] and abolishes the [[Hering–Breuer reflex|Hering–Breuer inflation reflex]].

It is considered that endogenous sulfur dioxide plays a significant physiological role in regulating [[cardiac]] and [[blood vessel]] function, and aberrant or deficient sulfur dioxide metabolism can contribute to several different cardiovascular diseases, such as [[arterial hypertension]], [[atherosclerosis]], [[pulmonary arterial hypertension]], and [[stenocardia]].<ref>{{cite journal |last1=Tian |first1=Hong |title=Advances in the study on endogenous sulfur dioxide in the cardiovascular system |journal=Chinese Medical Journal |date=November 5, 2014 |volume=127 |issue=21 |pages=3803–3807 |doi=10.3760/cma.j.issn.0366-6999.20133031 |pmid=25382339 |s2cid=11924999 |doi-access=free }}</ref>

It was shown that in children with pulmonary arterial hypertension due to congenital heart diseases the level of [[homocysteine]] is higher and the level of endogenous sulfur dioxide is lower than in normal control children. Moreover, these biochemical parameters strongly correlated to the severity of pulmonary arterial hypertension. Authors considered homocysteine to be one of useful biochemical markers of disease severity and sulfur dioxide metabolism to be one of potential therapeutic targets in those patients.<ref>{{cite journal|vauthors=Yang R, Yang Y, Dong X, Wu X, Wei Y |title=Correlation between endogenous sulfur dioxide and homocysteine in children with pulmonary arterial hypertension associated with congenital heart disease|language=zh|journal=Zhonghua Er Ke Za Zhi|date=Aug 2014|volume=52|issue=8|pages=625–629|pmid=25224243}}</ref>

Endogenous sulfur dioxide also has been shown to lower the [[Cell proliferation|proliferation]] rate of endothelial [[smooth muscle]] cells in blood vessels, via lowering the [[MAPK]] activity and activating [[adenylyl cyclase]] and [[protein kinase A]].<ref>{{cite journal|vauthors=Liu D, Huang Y, Bu D, Liu AD, Holmberg L, Jia Y, Tang C, Du J, Jin H |title=Sulfur dioxide inhibits vascular smooth muscle cell proliferation via suppressing the Erk/MAP kinase pathway mediated by cAMP/PKA signaling|journal=Cell Death Dis.|date=May 2014|volume=5|issue=5|pages=e1251|doi=10.1038/cddis.2014.229|pmid=24853429|pmc=4047873}}</ref> Smooth muscle cell proliferation is one of important mechanisms of hypertensive remodeling of blood vessels and their [[stenosis]], so it is an important pathogenetic mechanism in arterial hypertension and atherosclerosis.

Endogenous sulfur dioxide in low concentrations causes endothelium-dependent [[vasodilation]]. In higher concentrations it causes endothelium-independent vasodilation and has a negative inotropic effect on cardiac output function, thus effectively lowering blood pressure and myocardial oxygen consumption. The vasodilating and bronchodilating effects of sulfur dioxide are mediated via ATP-dependent [[calcium channel]]s and L-type ("dihydropyridine") calcium channels. Endogenous sulfur dioxide is also a potent antiinflammatory, antioxidant and cytoprotective agent. It lowers blood pressure and slows hypertensive remodeling of blood vessels, especially thickening of their intima. It also regulates lipid metabolism.<ref>{{cite journal|vauthors=Wang XB, Jin HF, Tang CS, Du JB |title=The biological effect of endogenous sulfur dioxide in the cardiovascular system.|journal=Eur J Pharmacol|date=November 16, 2011|volume=670|issue=1|doi=10.1016/j.ejphar.2011.08.031|pmid=21925165|pages=1–6}}</ref>

Endogenous sulfur dioxide also diminishes myocardial damage, caused by [[isoproterenol]] [[adrenergic]] hyperstimulation, and strengthens the myocardial antioxidant defense reserve.<ref>{{cite journal|vauthors=Liang Y, Liu D, Ochs T, Tang C, Chen S, Zhang S, Geng B, Jin H, Du J |title=Endogenous sulfur dioxide protects against isoproterenol-induced myocardial injury and increases myocardial antioxidant capacity in rats.|journal=Lab. Invest.|date=Jan 2011|volume=91|issue=1|pages=12–23|doi=10.1038/labinvest.2010.156|pmid=20733562|doi-access=free}}</ref>

===As a reagent and solvent in the laboratory===
Sulfur dioxide is a versatile inert solvent widely used for dissolving highly oxidizing salts. It is also used occasionally as a source of the sulfonyl group in [[organic synthesis]]. Treatment of aryl [[diazonium salt]]s with sulfur dioxide and [[cuprous chloride]] yields the corresponding aryl sulfonyl chloride, for example:<ref>{{OrgSynth | author = Hoffman, R. V. | title = m-Trifluoromethylbenzenesulfonyl Chloride | collvol = 7 | collvolpages = 508 | year = 1990| prep = CV7P0508}}</ref>

:[[File:Preparation of m-trifluoromethylbenzenesulfonyl chloride.svg|frameless|upright=2]]

As a result of its very low [[Lewis basicity]], it is often used as a low-temperature solvent/diluent for superacids like [[magic acid]] (FSO<sub>3</sub>H/SbF<sub>5</sub>), allowing for highly reactive species like ''tert''-butyl cation to be observed spectroscopically at low temperature (though tertiary carbocations do react with SO<sub>2</sub> above about −30&nbsp;°C, and even less reactive solvents like [[Sulfuryl chloride fluoride|SO<sub>2</sub>ClF]] must be used at these higher temperatures).<ref>{{Cite journal|last1=Olah|first1=George A.|last2=Lukas|first2=Joachim.|date=August 1, 1967|title=Stable carbonium ions. XLVII. Alkylcarbonium ion formation from alkanes via hydride (alkide) ion abstraction in fluorosulfonic acid-antimony pentafluoride-sulfuryl chlorofluoride solution|journal=Journal of the American Chemical Society|volume=89|issue=18|pages=4739–4744|doi=10.1021/ja00994a030|issn=0002-7863}}</ref>

===As a refrigerant===
Being easily condensed and possessing a high [[heat of evaporation]], sulfur dioxide is a candidate material for refrigerants. Before the development of [[chlorofluorocarbon]]s, sulfur dioxide was used as a [[refrigerant]] in [[refrigeration#Home and consumer use|home refrigerators]].

===As an indicator of volcanic activity===
Sulfur dioxide content in naturally-released geothermal gasses is measured by the [[Icelandic Meteorological Office]] as an indicator of possible volcanic activity.<ref>{{Cite web |date=n.d. |title=Volcanic gases |url=https://en.vedur.is/volcanoes/volcanic-hazards/volcanic-gases/ |website=Iceland Met Office}}</ref>