View source for Chemistry:Sulfur dioxide

{{Chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 477313199
| ImageFile = Sulfur-dioxide-2D.svg
| ImageFile_Ref = {{chemboximage|correct|??}}
| ImageSize = 160
| ImageName = Skeletal formula sulfur dioxide with assorted dimensions
| ImageFile1 = Sulfur-dioxide-3D-vdW.png
| ImageFile1_Ref = {{chemboximage|correct|??}}
| ImageSize1 = 140
| ImageName1 = Spacefill model of sulfur dioxide
| ImageFile2 = Sulfur dioxide lewis structure.png
| ImageFile2_Ref = {{chemboximage|correct|??}}
| ImageSize2 = 160
| ImageName2 = The Lewis structure of sulfur dioxide (SO2), showing unshared electron pairs.
| IUPACName = Sulfur dioxide
| OtherNames = Sulfurous anhydride<br/>
Sulfur(IV) oxide
|Section1={{Chembox Identifiers
| CASNo = 7446-09-5
| CASNo_Ref = {{cascite|correct|CAS}}
| PubChem = 1119
| ChemSpiderID = 1087
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| UNII = 0UZA3422Q4
| UNII_Ref = {{fdacite|correct|FDA}}
| EINECS = 231-195-2
| UNNumber = 1079, 2037
| KEGG = D05961
| KEGG_Ref = {{keggcite|correct|kegg}}
| MeSHName = Sulfur+dioxide
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 18422
| ChEMBL = 1235997
| ChEMBL_Ref = {{ebicite|changed|EBI}}
| RTECS = WS4550000
| Beilstein = 3535237
| Gmelin = 1443
| SMILES = O=S=O
| StdInChI = 1S/O2S/c1-3-2
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| InChI = 1/O2S/c1-3-2
| StdInChIKey = RAHZWNYVWXNFOC-UHFFFAOYSA-N
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| InChIKey = RAHZWNYVWXNFOC-UHFFFAOYAT
}}
|Section2={{Chembox Properties
| Formula = {{Chem|SO|2}}
| MolarMass = 64.066 g mol<sup>−1</sup>
| Appearance = Colorless gas
| Density = 2.6288 kg m<sup>−3</sup>
| Solubility = 94 g/L<ref>{{RubberBible87th}}</ref><br/>forms [[Chemistry:Sulfurous acid|sulfurous acid]]
| MeltingPtK = 201
| BoilingPtC = −10
| VaporPressure = 237.2 kPa
| pKa = 1.81
| pKb = 12.19
| Viscosity = 0.403 cP (at 0&nbsp;°C)
| Odor = Pungent; similar to a just-struck match<ref>[http://toxtown.nlm.nih.gov/text_version/chemicals.php?id=29 Sulfur dioxide], U.S. National Library of Medicine</ref>
| MagSus = &minus;18.2·10<sup>−6</sup> cm<sup>3</sup>/mol
}}
|Section3={{Chembox Structure
| PointGroup = ''C''<sub>2''v''</sub>
| Coordination = Digonal
| MolShape = Dihedral
| Dipole = 1.62 D
}}
|Section4={{Chembox Thermochemistry
| DeltaHf = −296.81 kJ mol<sup>−1</sup>
| Entropy = 248.223 J K<sup>−1</sup> mol<sup>−1</sup>
}}
|Section5={{Chembox Hazards
| GHSPictograms = {{GHS corrosion}} {{GHS skull and crossbones}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|314|331}}<ref>https://echa.europa.eu/information-on-chemicals/cl-inventory-database/-/discli/notification-details/115657/1409763</ref>
| SPhrases = {{S1/2}}, {{S9}}, {{S26}}, {{S36/37/39}}, {{S45}}
| NFPA-H = 3
| NFPA-F = 0
| NFPA-R = 0
| IDLH = 100 ppm<ref name=PGCH>{{PGCH|0575}}</ref>
| REL = TWA 2 ppm (5 mg/m<sup>3</sup>) ST 5 ppm (13 mg/m<sup>3</sup>)<ref name=PGCH/>
| PEL = TWA 5 ppm (13 mg/m<sup>3</sup>)<ref name=PGCH/>
| LC50 = 3000 ppm (mouse, 30 min)<br/>2520 ppm (rat, 1 hr)<ref name=IDLH>{{IDLH|7446095|Sulfur dioxide}}</ref>
| LCLo = 993 ppm (rat, 20 min)<br/>611 ppm (rat, 5 hr)<br/>764 ppm (mouse, 20 min)<br/>1000 ppm (human, 10 min)<br/>3000 ppm (human, 5 min)<ref name=IDLH/>
}}
|Section8={{Chembox Related
| OtherFunction_label = [[Chemistry:Sulfur|sulfur]] [[Chemistry:Oxide|oxide]]s
| OtherFunction = [[Chemistry:Sulfur monoxide|Sulfur monoxide]]<br/>[[Chemistry:Sulfur trioxide|Sulfur trioxide]]
| OtherCompounds = [[Chemistry:Ozone|Ozone]]<br/>
[[Chemistry:Selenium dioxide|Selenium dioxide]]<br/>
[[Chemistry:Sulfurous acid|Sulfurous acid]]<br/>
[[Chemistry:Tellurium dioxide|Tellurium dioxide]]
}}
}}

'''Sulfur dioxide''' (also '''sulphur dioxide''' in British English) is the [[Chemistry:Chemical compound|chemical compound]] with the formula {{chem|[[Chemistry:Sulfur|S]][[Chemistry:Oxygen|O]]|2}}. It is a toxic [[Physics:Gas|gas]] with a burnt match smell. It is released naturally by volcanic activity and is produced as a by-product of the burning of fossil fuels contaminated with sulfur compounds and copper extraction.

== Structure and bonding ==

SO<sub>2</sub> is a bent molecule with ''C''<sub>2v</sub> [[Point groups in three dimensions|symmetry point group]].
A [[Chemistry:Valence bond theory|valence bond theory]] approach considering just ''s'' and ''p'' orbitals would describe the bonding in terms of [[Chemistry:Resonance|resonance]] between two resonance structures.
[[File:Sulfur-dioxide-resonance-2D.svg|center|upright=1.6|thumb|Two resonance structures of sulfur dioxide]]
The sulfur–oxygen bond has a [[Chemistry:Bond order|bond order]] of 1.5. There is support for this simple approach that does not invoke ''d'' orbital participation.<ref>{{cite journal
| title = Chemical bonding in oxofluorides of hypercoordinatesulfur
|author1=Cunningham, Terence P. |author2=Cooper, David L. |author3=Gerratt, Joseph |author4=Karadakov, Peter B. |author5=Raimondi, Mario  |lastauthoramp=yes | journal = Journal of the Chemical Society, Faraday Transactions
| year = 1997
| volume = 93
| issue = 13
| pages = 2247–2254
| doi = 10.1039/A700708F
}}</ref>
In terms of [[Chemistry:Electron counting|electron-counting]] formalism, the sulfur atom has an [[Chemistry:Oxidation state|oxidation state]] of +4 and a [[Physics:Formal charge|formal charge]] of +1.

==Occurrence==
[[File:Io Aurorae color.jpg|thumb|left|The blue auroral glows of Io's upper atmosphere are caused by volcanic sulfur dioxide.]]

It is found on Earth and exists in very small concentrations and in the atmosphere at about 1 ppm.<ref name="OwenPickering1997">{{cite book|author1=Owen, Lewis A. |author2=Pickering, Kevin T |title=An Introduction to Global Environmental Issues|url=https://books.google.com/books?id=6nVXMjonU38C&pg=PA33|year= 1997|publisher=Taylor & Francis|isbn=978-0-203-97400-1|pages=33–}}</ref><ref name="TaylorSimpson1987">{{cite journal|last1=Taylor|first1=J.A.|last2=Simpson|first2=R.W.|last3=Jakeman|first3=A.J.|title=A hybrid model for predicting the distribution of sulphur dioxide concentrations observed near elevated point sources|journal=Ecological Modelling|volume=36|issue=3–4|year=1987|pages=269–296|issn=0304-3800|doi=10.1016/0304-3800(87)90071-8}}</ref>{{clarify|reason=not clear where, when, how much variability in time or space|date=March 2016}}

On other planets, it can be found in various concentrations, the most significant being the [[Astronomy:Atmosphere of Venus|atmosphere of Venus]], where it is the third-most significant atmospheric gas at 150 ppm. There, it condenses to form clouds, and is a key component of chemical reactions in the planet's atmosphere and contributes to [[Earth:Global warming|global warming]].<ref name="MarcqBertaux2012">{{cite journal|last1=Marcq|first1=Emmanuel|last2=Bertaux|first2=Jean-Loup|last3=Montmessin|first3=Franck|last4=Belyaev|first4=Denis|title=Variations of sulphur dioxide at the cloud top of Venus's dynamic atmosphere|journal=Nature Geoscience|volume=6|pages=25|year=2012|issn=1752-0894|doi=10.1038/ngeo1650}}</ref> It has been implicated as a key agent in the warming of early [[Astronomy:Mars|Mars]], with estimates of concentrations in the lower atmosphere as high as 100 ppm,<ref name="HalevyZuber2007">{{cite journal|last1=Halevy|first1=I.|last2=Zuber|first2=M. T.|last3=Schrag|first3=D. P.|title=A Sulfur Dioxide Climate Feedback on Early Mars|journal=Science|volume=318|issue=5858|year=2007|pages=1903–1907|issn=0036-8075|doi=10.1126/science.1147039|pmid=18096802}}</ref> though it only exists in trace amounts. On both Venus and Mars, as on Earth, its primary source is thought to be volcanic. The [[Astronomy:Atmosphere of Io|atmosphere of Io]] is 90% sulfur dioxide<ref name="IobookChap10">{{cite book |last=Lellouch |first=E. |display-authors=0 |editor=Lopes, R. M. C. |editor2=Spencer, J. R. |title=Io after Galileo |date=2007 |publisher=Springer-Praxis |isbn=3-540-34681-3 |pages=231–264 |chapter=Io's atmosphere }}</ref> and trace amounts are thought to also exist in the [[Astronomy:Atmosphere of Jupiter|atmosphere of Jupiter]].

As an ice, it is thought to exist in abundance on the [[Astronomy:Galilean moons|Galilean moons]]—as sublimating ice or frost on the trailing hemisphere of [[Astronomy:Io (moon)|Io]], a natural satellite of [[Astronomy:Jupiter|Jupiter]]<ref name="CruikshankHowell1985">{{cite journal|last1=Cruikshank|first1=D. P.|last2=Howell|first2=R. R.|last3=Geballe|first3=T. R.|last4=Fanale|first4=F. P.|title=Sulfur Dioxide Ice on IO|year=1985|pages=805–815|doi=10.1007/978-94-009-5418-2_55|journal=Ices in the Solar System|isbn=978-94-010-8891-6}}</ref> and in the crust and mantle of [[Astronomy:Europa (moon)|Europa]], [[Astronomy:Ganymede (moon)|Ganymede]], and [[Astronomy:Callisto (moon)|Callisto]], possibly also in liquid form and readily reacting with water.<ref>[http://www.jpl.nasa.gov/news/news.php?release=2010-319 Europa's Hidden Ice Chemistry – NASA Jet Propulsion Laboratory]. Jpl.nasa.gov (2010-10-04). Retrieved on 2013-09-24.</ref>

==Production==
Sulfur dioxide is primarily produced for sulfuric acid manufacture (see [[Chemistry:Contact process|contact process]]). In the United States in 1979, 23.6 million tonnes (26,014,547 US short tons) of sulfur dioxide were used in this way, compared with 150 thousand tonnes (165,347 US short tons) used for other purposes. Most sulfur dioxide is produced by the combustion of elemental sulfur. Some sulfur dioxide is also produced by roasting [[Chemistry:Pyrite|pyrite]] and other sulfide ores in air.<ref name = Ullmann>{{Ullmann| author = Müller, Hermann | title = Sulfur Dioxide | doi = 10.1002/14356007.a25_569}}</ref>
[[File:03. Горење на сулфур во атмосфера од кислород.webm|thumb|left|upright=1.25|An experiment showing burning of sulfur in oxygen. A flow-chamber joined to a gas washing bottle (filled with a solution of [[Chemistry:Methyl orange|methyl orange]]) is being used. The product is sulfur dioxide (SO<sub>2</sub>) with some traces of sulfur trioxide (SO<sub>3</sub>). The "smoke" that exits the gas washing bottle is, in fact, a sulfuric acid fog generated in a reaction.]]

===Combustion routes===
Sulfur dioxide is the product of the burning of [[Chemistry:Sulfur|sulfur]] or of burning materials that contain sulfur:
:S + O<sub>2</sub> → SO<sub>2</sub>, ΔH = −297 kJ/mol

To aid combustion, liquefied sulfur (140–150&nbsp;°C, 284-302&nbsp;°F) is sprayed through an atomizing nozzle to generate fine drops of sulfur with a large surface area. The reaction is exothermic, and the combustion produces temperatures of 1000–1600&nbsp;°C, 1832-2912&nbsp;°F). The significant amount of heat produced is recovered by steam generation that can subsequently be converted to electricity.<ref name = Ullmann/>

The combustion of [[Chemistry:Hydrogen sulfide|hydrogen sulfide]] and organosulfur compounds proceeds similarly. For example:
:2 H<sub>2</sub>S + 3 O<sub>2</sub> → 2 H<sub>2</sub>O + 2 SO<sub>2</sub>

The [[Engineering:Roasting (metallurgy)|roasting]] of sulfide ores such as [[Chemistry:Pyrite|pyrite]], [[Chemistry:Sphalerite|sphalerite]], and [[Chemistry:Cinnabar|cinnabar]] (mercury sulfide) also releases SO<sub>2</sub>:<ref>Shriver, Atkins. Inorganic Chemistry, Fifth Edition. W. H. Freeman and Company; New York, 2010; p. 414.</ref>
:4 FeS<sub>2</sub>  +  11 O<sub>2</sub>   →   2 Fe<sub>2</sub>O<sub>3</sub>  +  8 SO<sub>2</sub>
:2 ZnS  +  3 O<sub>2</sub> →   2 ZnO  +  2 SO<sub>2</sub>
:HgS  + O<sub>2</sub>   →   Hg  +  SO<sub>2</sub>
:4 FeS + 7O<sub>2</sub> → 2 Fe<sub>2</sub>O<sub>3</sub> + 4 SO<sub>2</sub>

A combination of these reactions is responsible for the largest source of sulfur dioxide, volcanic eruptions.  These events can release millions of tonnes of SO<sub>2</sub>.

===Reduction of higher oxides===
Sulfur dioxide can also be a byproduct in the manufacture of [[Chemistry:Calcium silicate|calcium silicate]] cement; [[Chemistry:Calcium sulfate|CaSO<sub>4</sub>]] is heated with [[Chemistry:Coke (fuel)|coke]] and sand in this process:
:2 CaSO<sub>4</sub> + 2 SiO<sub>2</sub> + C → 2 CaSiO<sub>3</sub> + 2 SO<sub>2</sub> + CO<sub>2</sub>

Until the 1970s, commercial quantities of sulfuric acid and cement were produced by this process in Whitehaven, England. Upon being mixed with [[Chemistry:Shale|shale]] or [[Earth:Marl|marl]], and roasted, the sulfate liberated sulfur dioxide gas, used in sulfuric acid production, the reaction also produced calcium silicate, a precursor in cement production.<ref>[http://www.lakestay.co.uk/whitehavenmininghistory.html WHITEHAVEN COAST ARCHAEOLOGICAL SURVEY]. lakestay.co.uk (2007)</ref>

On a laboratory scale, the action of hot concentrated sulfuric acid on copper [[Engineering:Swarf|turnings]] produces sulfur dioxide.
:Cu  +  2 H<sub>2</sub>SO<sub>4</sub> → CuSO<sub>4</sub>  +  SO<sub>2</sub>  +  2 H<sub>2</sub>O

===From sulfite===
Sulfite results from the reaction of aqueous base and sulfur dioxide.  The reverse reaction involves acidification of [[Chemistry:Sodium metabisulfite|sodium metabisulfite]]:
:H<sub>2</sub>SO<sub>4</sub>  +  Na<sub>2</sub>S<sub>2</sub>O<sub>5</sub>  →  2 SO<sub>2</sub>  +  Na<sub>2</sub>SO<sub>4</sub>  + H<sub>2</sub>O

==Reactions==

===Industrial reactions===
Treatment of basic solutions with sulfur dioxide affords sulfite salts (e.g. [[Chemistry:Sodium sulfite|sodium sulfite]]):
:SO<sub>2</sub> + 2 NaOH → Na<sub>2</sub>SO<sub>3</sub> + H<sub>2</sub>O

Featuring sulfur in the +4 oxidation state, sulfur dioxide is a [[Chemistry:Reducing agent|reducing agent]].  It is oxidized by halogens to give the sulfuryl halides, such as [[Chemistry:Sulfuryl chloride|sulfuryl chloride]]:

:SO<sub>2</sub> + Cl<sub>2</sub> → SO<sub>2</sub>Cl<sub>2</sub>

Sulfur dioxide is the oxidising agent in the [[Chemistry:Claus process|Claus process]], which is conducted on a large scale in oil refineries.  Here, sulfur dioxide is reduced by hydrogen sulfide to give elemental sulfur:
:SO<sub>2</sub> + 2 H<sub>2</sub>S → 3 S + 2 H<sub>2</sub>O

The sequential oxidation of sulfur dioxide followed by its hydration is used in the production of sulfuric acid.
: 2 SO<sub>2</sub>  +  2 H<sub>2</sub>O  +  O<sub>2</sub>   →   2 H<sub>2</sub>SO<sub>4</sub>

===Laboratory reactions===
Sulfur dioxide is one of the few common acidic yet reducing gases. It turns moist litmus pink (being acidic), then white (due to its bleaching effect). It may be identified by bubbling it through a [[Chemistry:Dichromate|dichromate]] solution, turning the solution from orange to green (Cr<sup>3+</sup> (aq)). It can also reduce ferric ions to ferrous. {{Citation needed|Formula looks wrong. Seems like that would go directly to sulfuric acid: SO2(2-) + 4H(+) => H2SO4 + 2H(+)|date=March 2014}}

Sulfur dioxide can react with certain 1,3-dienes in a [[Chemistry:Cheletropic reaction|cheletropic reaction]] to form cyclic [[Chemistry:Sulfone|sulfone]]s. This reaction is exploited on an industrial scale for the synthesis of [[Chemistry:Sulfolane|sulfolane]], which is an important solvent in the petrochemical industry.

:[[File:Cheletropic reaction of butadiene with SO2.svg|frameless|upright=0.7]]

Sulfur dioxide can bind to metal ions as a [[Chemistry:Ligand|ligand]] to form [[Chemistry:Metal sulfur dioxide complex|metal sulfur dioxide complex]]es, typically where the transition metal is in oxidation state 0 or +1.  Many different bonding modes (geometries) are recognized, but in most cases, the ligand is monodentate, attached to the metal through sulfur, which can be either planar and pyramidal [[Chemistry:Hapticity|η]]<sup>1</sup>.<ref name=Greenwood>{{Greenwood&Earnshaw2nd}}</ref>

==Uses==

===Precursor to sulfuric acid===
Sulfur dioxide is an intermediate in the production of sulfuric acid, being converted to [[Chemistry:Sulfur trioxide|sulfur trioxide]], and then to [[Chemistry:Oleum|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 [[Chemistry:Contact process|contact process]]. Several billion kilograms are produced annually for this purpose.

===As a preservative===
Sulfur dioxide is sometimes used as a [[Chemistry:Preservative|preservative]] for dried [[Biology:Apricot|apricot]]s, dried [[Biology:Common fig|figs]], and other dried fruits, owing to its [[Biology:Antimicrobial|antimicrobial]] properties, and is called [[Chemistry: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 [[Medicine:Decomposition|rotting]]. It is also added to sulfured [[Chemistry:Molasses|molasses]].

===In 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=1 Feb 2009|title=Practical Winery & vineyard Journal Jan/Feb 2009|deadurl=yes|archiveurl=https://web.archive.org/web/20130928111625/http://www.practicalwinery.com/janfeb09/page1.htm|archivedate=2013-09-28|df=}}</ref>

It is still an important compound in winemaking, and is measured in [[Parts per million|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 [[Chemistry:Antibiotic|antibiotic]] and [[Biology:Antioxidant|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| year = 2015| last1 = Guerrero| first1 = Raúl F| last2 = Cantos-Villar| first2 = Emma}}</ref> Its antimicrobial action also helps minimize volatile acidity. Wines containing sulfur dioxide are typically labeled with "containing [[Chemistry:Sulfite|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 [[Chemistry:Acetaldehyde|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 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. [[Chemistry:Ozone|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], Practical Winery & Vineyard Journal.</ref>

===As a reducing agent===
Sulfur dioxide is also a good [[Chemistry:Reducing agent|reductant]]. In the presence of water, sulfur dioxide is able to decolorize substances.  Specifically, it is a useful reducing [[Chemistry:Bleach|bleach]] for [[Chemistry:Paper|paper]]s and delicate materials such as clothes.  This bleaching effect normally does not last very long. [[Chemistry:Oxygen|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 [[Chemistry:Chloride|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>

===Biochemical and biomedical roles===
Sulfur dioxide is toxic in large amounts.  It 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 |last=Liu |first=D. |url=http://benthamscience.com/mrmc/contabs/mrmc10-11.html#4 |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 |pmid=20540708 |last2=Jin |first2=H |last3=Tang |first3=C |last4=Du |first4=J |doi=10.2174/1389557511009011039 |deadurl=yes |archiveurl=https://web.archive.org/web/20130426082343/http://benthamscience.com/mrmc/contabs/mrmc10-11.html#4 |archivedate=2013-04-26 |df= }}</ref>  Sulfur dioxide blocks nerve signals from the [[Biology:Pulmonary stretch receptors|pulmonary stretch receptors]] and abolishes the [[Biology:Hering–Breuer reflex|Hering–Breuer inflation reflex]].

It was shown that endogenous sulfur dioxide plays a role in diminishing an experimental [[Medicine:Lung|lung]] damage caused by [[Chemistry:Oleic acid|oleic acid]]. Endogenous sulfur dioxide lowered lipid peroxidation, free radical formation, oxidative stress and inflammation during an experimental lung damage. Conversely, a successful lung damage caused a significant lowering of endogenous sulfur dioxide production, and an increase in lipid peroxidation, free radical formation, oxidative stress and inflammation. Moreover, blockade of an [[Biology:Enzyme|enzyme]] that produces endogenous SO<sub>2</sub> significantly increased the amount of lung tissue damage in the experiment. Conversely, adding [[Chemistry:Acetylcysteine|acetylcysteine]] or [[Chemistry:Glutathione|glutathione]] to the rat diet increased the amount of endogenous SO<sub>2</sub> produced and decreased the lung damage, the free radical formation, oxidative stress, inflammation and apoptosis.<ref>{{cite journal|vauthors=Chen S, Zheng S, Liu Z, Tang C, Zhao B, Du J, Jin H |title=Endogenous sulfur dioxide protects against oleic acid-induced acute lung injury in association with inhibition of oxidative stress in rats.|journal=Lab. Invest.|date=Feb 2015|volume=95|issue=2|pages=142–156|doi=10.1038/labinvest.2014.147|pmid=25581610}}</ref>

It is considered that endogenous sulfur dioxide plays a significant physiological role in regulating cardiac and [[Biology:Blood vessel|blood vessel]] function, and aberrant or deficient sulfur dioxide metabolism can contribute to several different cardiovascular diseases, such as arterial hypertension, [[Medicine:Atherosclerosis|atherosclerosis]], pulmonary arterial hypertension, stenocardia.<ref>{{cite journal|author=Tian H.|title=Advances in the study on endogenous sulfur dioxide in the cardiovascular system.|journal=Chin Med J|date=Nov 2014|volume=127|issue=21|pages=3803–3807|pmid=25382339}}</ref>

It was shown that in children with pulmonary arterial hypertension due to congenital heart diseases the level of [[Chemistry:Homocysteine|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=chinese|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 [[Biology:Cell proliferation|proliferation]] rate of endothelial [[Biology:Smooth muscle|smooth muscle]] cells in blood vessels, via lowering the MAPK activity and activating [[Biology:Adenylyl cyclase|adenylyl cyclase]] and [[Biology:Protein kinase A|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 [[Medicine:Stenosis|stenosis]], so it is an important pathogenetic mechanism in arterial hypertension and atherosclerosis.

Endogenous sulfur dioxide in low concentrations causes endothelium-dependent [[Medicine:Vasodilation|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 [[Biology:Calcium channel|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=16 Nov 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 [[Biology:Adrenergic|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}}</ref>

===As a refrigerant===
Being easily condensed and possessing a high heat of evaporation, sulfur dioxide is a candidate material for refrigerants.  Prior to the development of [[Earth:Chlorofluorocarbon|chlorofluorocarbon]]s, sulfur dioxide was used as a [[Chemistry:Refrigerant|refrigerant]] in [[Physics:Refrigeration#Home and consumer use|home refrigerators]].

===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 [[Chemistry:Organic synthesis|organic synthesis]].  Treatment of aryl diazonium salts 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=2.25]]

=== Proposed use in climate engineering ===
Injections of sulfur dioxide in the stratosphere has been proposed in [[Earth:Climate engineering|climate engineering]]. The cooling effect would be similar to what has been observed after the large explosive volcano eruption of [[Earth:Mount Pinatubo#Global environmental effects|Mount Pinatubo]] in 1991. However this form of geoengineering would have uncertain regional consequences on rainfall patterns, for example in [[Earth:Monsoon|monsoon]] regions.<ref>Clarke L., K. Jiang, K. Akimoto, M. Babiker, G. Blanford, K. Fisher-Vanden, J.-C. Hourcade, V. Krey, E. Kriegler, A. Löschel, D. McCollum, S. Paltsev, S. Rose, P. R. Shukla, M. Tavoni, B. C. C. van der Zwaan, and D.P. van Vuuren, 2014: Assessing Transformation Pathways. In: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Edenhofer, O., R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.</ref>

==As an air pollutant==
[[File:Sulfur dioxide emissions from the Halema`uma`u vent, glows at night.jpg|thumb|A sulfur dioxide plume from the Halemaʻumaʻu vent, glows at night]]
Sulfur dioxide is a noticeable component in the atmosphere, especially following volcanic eruptions.<ref>[http://volcanoes.usgs.gov/hazards/gas/index.php Volcanic Gases and Their Effects]. Volcanoes.usgs.gov. Retrieved on 2011-10-31.</ref>  According to the United States Environmental Protection Agency,<ref>[http://www.epa.gov/air/airtrends/sulfur.html National Trends in Sulfur Dioxide Levels], United States Environmental Protection Agency.</ref> the amount of sulfur dioxide released in the U.S. per year was:

[[File:Estimates of past and future SO2 global anthropogenic emissions.png|thumb|A collection of estimates of past and future anthropogenic global sulphur dioxide emissions. The Cofala et al. estimates are for sensitivity studies on SO<sub>2</sub> emission policies, CLE: Current Legislation, MFR: Maximum Feasible Reductions. RCPs (Representative Concentration Pathways) are used in CMIP5  simulations for latest (2013–2014) IPCC 5th assessment report.]]

{| class="wikitable"
|-
! Year
! SO<sub>2</sub>
|-
| 1970
| {{convert|31,161,000|ST|Mt|sigfig=3}}
|-
| 1980
| {{convert|25,905,000|ST|Mt|sigfig=3}}
|-
| 1990
|{{convert|23,678,000|ST|Mt|sigfig=3}}
|-
| 1996
|{{convert|18,859,000|ST|Mt|sigfig=3}}
|-
| 1997
|{{convert|19,363,000|ST|Mt|sigfig=3}}
|-
| 1998
|{{convert|19,491,000|ST|Mt|sigfig=3}}
|-
| 1999
|{{convert|18,867,000|ST|Mt|sigfig=3}}
|}

Sulfur dioxide is a major air pollutant and has significant impacts upon human health.<ref name="EPA">[http://www.epa.gov/airquality/sulfurdioxide/ Sulfur Dioxide]. United States Environmental Protection Agency</ref>  In addition, the concentration of sulfur dioxide in the atmosphere can influence the habitat suitability for plant communities, as well as animal life.<ref>Hogan, C. Michael (2010). [http://www.eoearth.org/article/Abiotic_factor?topic=49461 "Abiotic factor"] in ''Encyclopedia of Earth''. Emily Monosson and C. Cleveland (eds.). National Council for Science and the Environment. Washington DC</ref> Sulfur dioxide emissions are a precursor to [[Earth:Acid rain|acid rain]] and atmospheric particulates. Due largely to the {{wipe|United States}} EPA’s Acid Rain Program, the U.S. has had a 33% decrease in emissions between 1983 and 2002.  This improvement resulted in part from [[Physics:Flue-gas desulfurization|flue-gas desulfurization]], a technology that enables SO<sub>2</sub> to be chemically bound in power plants burning sulfur-containing [[Earth:Coal|coal]] or [[Chemistry:Petroleum|oil]]. In particular, [[Chemistry:Calcium oxide|calcium oxide (lime)]] reacts with sulfur dioxide to form [[Chemistry:Calcium sulfite|calcium sulfite]]:
:CaO + SO<sub>2</sub> →  CaSO<sub>3</sub>
Aerobic oxidation of the CaSO<sub>3</sub> gives CaSO<sub>4</sub>, [[Chemistry:Anhydrite|anhydrite]].  Most gypsum sold in Europe comes from flue-gas desulfurization.

Sulfur can be removed from coal during burning by using limestone as a bed material in [[Physics:Fluidized bed combustion|fluidized bed combustion]].<ref>{{cite book |last=Lindeburg |first=Michael R. |title=Mechanical Engineering Reference Manual for the PE Exam |location=Belmont, C.A. |publisher=Professional Publications, Inc |year=2006 |pages=27–3 |isbn=978-1-59126-049-3 }}</ref>

Sulfur can also be removed from fuels before burning, preventing formation of SO<sub>2</sub> when the fuel is burnt. The [[Chemistry:Claus process|Claus process]] is used in refineries to produce sulfur as a byproduct.  The [[Chemistry:Stretford process|Stretford process]] has also been used to remove sulfur from fuel. [[Chemistry:Redox|Redox]] processes using iron oxides can also be used, for example, Lo-Cat<ref>[https://web.archive.org/web/20100304170107/http://www.gtp-merichem.com/support/index.php FAQ’s About Sulfur Removal and Recovery using the LO-CAT® Hydrogen Sulfide Removal System]. gtp-merichem.com</ref> or Sulferox.<ref>[http://www.netl.doe.gov/technologies/coalpower/gasification/pubs/pdf/SFA%20Pacific_Process%20Screening%20Analysis_Dec%202002.pdf Process screening analysis of alternative gas treating and sulfur removal for gasification]. (December 2002) Report by SFA Pacific, Inc. prepared for U.S. Department of Energy (PDF) . Retrieved on 2011-10-31.</ref>
[[File:SulufrDioxide2017.png|thumb|Sulfur dioxide in the world on April 15th, 2017. Note that sulfur dioxide moves through the atmosphere with prevailing winds and thus local sulfur dioxide distributions vary day to day with weather patterns and seasonality.]]
Fuel additives such as [[Chemistry:Calcium|calcium]] additives and magnesium carboxylate may be used in marine engines to lower the emission of sulfur dioxide gases into the atmosphere.<ref>May, Walter R.  [http://www.fuelspec.com/library/Marine%20Emissions%20Abatement.pdf Marine Emissions Abatement] {{Webarchive|url=https://web.archive.org/web/20150402130001/http://www.fuelspec.com/library/Marine%20Emissions%20Abatement.pdf |date=2015-04-02 }}. SFA International, Inc., p. 6.</ref>

As of 2006, {{wipe|China}} was the world's largest sulfur dioxide polluter, with 2005 emissions estimated to be {{convert|25,490,000|ST|Mt|sigfig=3}}. This amount represents a 27% increase since 2000, and is roughly comparable with U.S. emissions in 1980.<ref>[http://www.physorg.com/news78159898.html China has its worst spell of acid rain], United Press International (2006-09-22).</ref>

==Safety==
[[File:20180519 USGS Leilani Estates Hawaii Volcanic EruptionDSC 0411 medium.jpg|thumb|US Geological Survey volunteer tests for sulfur dioxide after the 2018 lower Puna eruption]]

===Inhalation===
Inhaling sulfur dioxide is associated with increased respiratory symptoms and disease, difficulty in breathing, and premature death.<ref>[http://www.epa.gov/air/sulfurdioxide/ Sulfur Dioxide][broken link] U.S. Environmental Protection Agency</ref> In 2008, the [[Organization:American Conference of Governmental Industrial Hygienists|American Conference of Governmental Industrial Hygienists]] reduced the [[Chemistry:Short-term exposure limit|short-term exposure limit]] to 0.25 [[Parts per million|parts per million]] (ppm). The [[Organization:Occupational Safety and Health Administration|OSHA]] [[Earth:Permissible exposure limit|PEL]] is currently set at 5 ppm (13 &nbsp;mg/m<sup>3</sup>) time-weighted average. NIOSH has set the IDLH at 100 ppm.<ref name=NIOSH>{{cite web |url=https://www.cdc.gov/niosh/npg/npgd0575.html |title=NIOSH Pocket Guide to Chemical Hazards }}</ref>  In 2010, the EPA "revised the primary SO<sub>2</sub> NAAQS by establishing a new one-hour standard at a level of 75 parts per billion (ppb). EPA revoked the two existing primary standards because they would not provide additional public health protection given a one-hour standard at 75 ppb."<ref name="EPA" />

A 2011 [[Systematic review|systematic review]] concluded that exposure to sulfur dioxide is associated with [[Medicine:Preterm birth|preterm birth]].<ref name="Shah-2011">{{Cite journal | title = Air pollution and birth outcomes: a systematic review | journal = Environ Int | volume = 37 | pages = 498–516 | year = 2011 | doi = 10.1016/j.envint.2010.10.009 | pmid = 21112090 | author = Shah PS, Balkhair T, Knowledge Synthesis Group on Determinants of Preterm/LBW Births | issue = 2 }}</ref>

===Ingestion===
In the United States, the [[Organization:Center for Science in the Public Interest|Center for Science in the Public Interest]] lists the two food preservatives, sulfur dioxide and [[Chemistry:Sodium bisulfite|sodium bisulfite]], as being safe for human consumption except for certain [[Medicine:Asthma|asthma]]tic individuals who may be sensitive to them, especially in large amounts.<ref>{{cite web | title = Center for Science in the Public Interest – Chemical Cuisine | url = http://www.cspinet.org/reports/chemcuisine.htm | accessdate = March 17, 2010}}</ref>  Symptoms of sensitivity to [[Chemistry:Sulfite#Health risks|sulfiting]] agents, including sulfur dioxide, manifest as potentially life-threatening trouble breathing within minutes of ingestion.<ref>{{cite web | title = California Department of Public Health: Food and Drug Branch: Sulfites | url = http://www.cdph.ca.gov/pubsforms/Guidelines/Documents/fdb%20Sulfites.pdf | accessdate = September 27, 2013 | deadurl = yes | archiveurl = https://web.archive.org/web/20120723065412/http://www.cdph.ca.gov/pubsforms/Guidelines/Documents/fdb%20Sulfites.pdf | archivedate = July 23, 2012 | df =  }}</ref>

==See also==
* [[Chemistry:Sulfur trioxide|Sulfur trioxide]]
* [[Chemistry:Sulfur–iodine cycle|Sulfur–iodine cycle]]
* [[Earth:National Ambient Air Quality Standards|National Ambient Air Quality Standards]]
* Bunker fuel

==References==
{{Reflist|30em}}

==External links==
*[https://earth.nullschool.net/#current/chem/surface/level/overlay=so2smass/winkel3 Global map of sulfur dioxide distribution]
*[http://www.epa.gov/air/airtrends/sulfur.html United States Environmental Protection Agency Sulfur Dioxide page]
*[http://www.inchem.org/documents/icsc/icsc/eics0074.htm International Chemical Safety Card 0074]
*[https://web.archive.org/web/20090325155828/http://monographs.iarc.fr/ENG/Monographs/vol54/volume54.pdf IARC Monographs. "Sulfur Dioxide and some Sulfites, Bisulfites and Metabisulfites" v54. 1992. p131.]
*[https://www.cdc.gov/niosh/npg/npgd0575.html NIOSH Pocket Guide to Chemical Hazards]
*[https://www.cdc.gov/niosh/topics/SulfurDioxide/ CDC – Sulfure Dioxide – NIOSH Workplace Safety and Health Topic]
*[http://www.chm.bris.ac.uk/motm/so2/so2h.htm Sulfur Dioxide, Molecule of the Month]
{{Oxides}}
{{Molecules detected in outer space}}
{{sulfur compounds}}

{{DEFAULTSORT:Sulfur Dioxide}}
[[Category:Acidic oxides]]
[[Category:Industrial gases]]
[[Category:Interchalcogens]]
[[Category:Preservatives]]
[[Category:Refrigerants]]
[[Category:Smog]]
[[Category:Sulfur oxides]]
[[Category:Gaseous signaling molecules]]
[[Category:Triatomic molecules]]
[[Category:Reducing agents]]
[[Category:Inorganic solvents]]
[[Category:Hypervalent molecules]]

{{Sourceattribution|Sulfur dioxide}}