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	{{chembox		{{chembox
			| Verifiedfields = changed
	| verifiedrevid = 413729438
			| Watchedfields = changed
	| ImageFile1 = Sodium metabisulfite.jpg
			| verifiedrevid = 448850808
	| ImageName1 = Sodium metabisulfite
			| Name =
	| ImageFile2 = Sodium-metabisulfite-2D.png
			| ImageFile =
	| ImageName2 = Structure of sodium metabisulfite
			| ImageFile1 = Sodium metabisulfite.jpg
	| OtherNames = Sodium pyrosulfite<br/>Sodium disulfite
			| ImageName1 = Sodium metabisulfite
	| Section1 = {{Chembox Identifiers
			| ImageFile2 = Sodium-metabisulfite-2D.png
	| CASNo = 7681-57-4
			| ImageName2 = Structure of sodium metabisulfite
	| CASNo_Ref = {{cascite|correct|CAS}}
			| OtherNames = {{Unbulleted list|Sodium pyrosulfite|Sodium disulfite}}
	| PubChem = 24346
	| EINECS = 231-673-0		| IUPACName =
			| SystematicName =
	| RTECS = UX8225000
			| Section1 = {{Chembox Identifiers
			| CASNo = 7681-57-4
			| CASNo_Ref = {{cascite|correct|CAS}}
			| UNII_Ref = {{fdacite|correct|FDA}}
			| UNII = 4VON5FNS3C
			| PubChem = 656671
			| ChEMBL_Ref = {{ebicite|changed|EBI}}
			| ChEMBL = 2016976
			| ChEBI_Ref = {{ebicite|changed|EBI}}
			| ChEBI = 114786
			| EINECS = 231-673-0
			| RTECS = UX8225000
			| SMILES = S(=O)S(=O)(=O)..
			| InChI = 1S/2Na.H2O5S2/c;;1-6(2)7(3,4)5/h;;(H,1,2)(H,3,4,5)/q2*+1;/p-2
	}}		}}
	| Section2 = {{Chembox Properties		| Section2 = {{Chembox Properties
	| Formula = Na<sub>2</sub>S<sub>2</sub>O<sub>5</sub>, Na-O-(S=O)-O-(S=O)-O-Na		| Formula = Na<sub>2</sub>S<sub>2</sub>O<sub>5</sub>
	| MolarMass = 190.107 g/mol		| MolarMass = 190.107{{nbsp}}g/mol
	| Appearance = white powder		| Appearance = White to yellow powder
	| Density = 1.48 g/cm<sup>3</sup>		| Odor = Faint ]
			| Density = 1.48{{nbsp}}g/cm<sup>3</sup>
	| Solubility = 54 g/100 ml
			| Solubility = {{ubl
	| MeltingPt = >170 °C (begins at 150 °C)
			| 45.1{{nbsp}}g/100{{thinsp}}mL (0{{nbsp}}°C)
	| BoilingPt =
			| 65.3{{nbsp}}g/100{{thinsp}}mL (20{{nbsp}}°C)
			| 81.7{{nbsp}}g{{thinsp}}100 mL (100{{nbsp}}°C)
			}}
			| SolubleOther = Very soluble in ] <br>Slightly soluble in ]
			| MeltingPtC = 170
			| MeltingPt_notes = decomposition begins at 150 °C
			| BoilingPt =
	}}		}}
			| Section3 =
	| Section7 = {{Chembox Hazards
			| Section4 =
	| ExternalMSDS =
			| Section5 =
	| EUIndex = 016-063-00-2
			| Section6 =
	| EUClass = Harmful ('''Xn''')<br/>Irritant ('''Xi''')
			| Section7 = {{Chembox Hazards
	| NFPA-H = 2
			| ExternalSDS =
	| NFPA-F = 0
	| NFPA-R = 0		| NFPA-H = 2
	| NFPA-O =		| NFPA-F = 0
			| NFPA-R = 1
	| RPhrases = {{R22}} {{R31}} {{R41}}
			| NFPA-S =
	| SPhrases = {{S2}} {{S26}} {{S39}} {{S46}}
			| GHSPictograms = {{GHS05}}{{GHS07}}
			| GHSSignalWord = Danger
			| HPhrases = {{H-phrases|302|318}}
			| PPhrases = {{P-phrases|264|270|280|301+312|305+351+338|310|330|501}}
			| PEL = None<ref name=PGCH>{{PGCH|0566}}</ref>
			| IDLH = N.D.<ref name=PGCH/>
			| REL = TWA 5{{nbsp}}mg/m<sup>3</sup><ref name=PGCH/>
	}}		}}
	| Section8 = {{Chembox Related		| Section8 = {{Chembox Related
	| OtherAnions = ]<br/>]		| OtherAnions = ]<br/>]
	| OtherCations = ]		| OtherCations = ]
	| OtherCpds = ]<br/>]<br/>]		| OtherCompounds = ]<br/>]<br/>]
	}}		}}
	}}		}}
	'''Sodium metabisulfite''' or '''sodium pyrosulfite''' (IUPAC spelling; Br. E. '''sodium metabisulphite''' or '''sodium pyrosulphite''') is an ] of chemical formula Na<sub>2</sub>S<sub>2</sub>O<sub>5</sub>. The substance is sometimes referred to as ''disodium'' (metabisulfite). It is used as a ], ] and ] agent.		'''Sodium metabisulfite''' or '''sodium pyrosulfite''' (IUPAC spelling; Br. E. '''sodium metabisulphite''' or '''sodium pyrosulphite''') is an ] of chemical formula Na<sub>2</sub>S<sub>2</sub>O<sub>5</sub>. The substance is sometimes referred to as disodium metabisulfite. It is used as a ], ], and ] agent.<ref>{{Ullmann|doi=10.1002/14356007.a25_477|isbn=3527306730|title=Sulfites, Thiosulfates, and Dithionitesl Chemistry|year=2000|last1=Barberá|first1=José Jiménez|last2=Metzger|first2=Adolf|last3=Wolf|first3=Manfred}}</ref> When dissolved in water it forms ].
			==Preparation==
			{{see also|Wellman–Lord process}}
			Sodium metabisulfite can be prepared by treating a solution of ] with ].<ref name="InorgChem">{{cite book
			| title = Inorganic Chemistry, 3rd Edition
			| chapter = Chapter 16: The group 16 elements
			| author1 = Catherine E. Housecroft
			| author2 = Alan G. Sharpe
			| publisher = Pearson
			| year = 2008
			| isbn = 978-0-13-175553-6
			| page = 520
			}}</ref> When conducted in warm water, Na<sub>2</sub>SO<sub>3</sub> initially precipitates as a yellow solid. With more SO<sub>2</sub>, the solid dissolves to give the disulfite, which crystallises upon cooling.<ref>{{cite book|doi=10.1002/9780470132333.ch49|isbn=9780470132333|chapter=Sulfites and Pyrosulfites of the Alkali Metals|year=1946|last1=Johnstone|first1=H. F.|title=Inorganic Syntheses|pages=162–167|volume=2}}</ref>
			:SO<sub>2</sub> + 2 NaOH → Na<sub>2</sub>SO<sub>3</sub> + H<sub>2</sub>O
			:SO<sub>2</sub> + Na<sub>2</sub>SO<sub>3</sub> → Na<sub>2</sub>S<sub>2</sub>O<sub>5</sub>
			which yields a residue of colourless solid Na<sub>2</sub>S<sub>2</sub>O<sub>5</sub>.
	==Chemical structure==		==Chemical structure==
	The anion is a hybride of dithionite (S<sub>2</sub>O<sub>4</sub><sup>2-</sup>) and dithionate (S<sub>2</sub>O<sub>6</sub><sup>2-</sup>). The anion consists of an SO<sub>2</sub> group linked to an SO<sub>3</sub> group, with the negative charge more localized on the SO<sub>3</sub> end. The S-S bond length is 2.22 Å and the "thionate" and thionite" S-O distances are 1.46 and 1.50 Å, respectively.<ref>K. L. Carter, T. A. Siddiquee, K. L. Murphy, D. W. Bennett "The surprisingly elusive crystal structure of sodium metabisulfite" Acta Cryst. (2004). B60, 155-162. {{doi|10.1107/S0108768104003325}}</ref>		The anion ] consists of an SO<sub>2</sub> group linked to an SO<sub>3</sub> group, with the negative charge more localised on the SO<sub>3</sub> end. The S–S bond length is 2.22 Å, and the "thionate" and "thionite" S–O distances are 1.46 and 1.50 Å, respectively.<ref>K. L. Carter, T. A. Siddiquee, K. L. Murphy, D. W. Bennett "The surprisingly elusive crystal structure of sodium metabisulfite" Acta Crystallogr. (2004). B60, 155–162. {{doi|10.1107/S0108768104003325}}</ref>
	==Uses==		==Reactivity==
			{{main|Bisulfite#Reactions}}
	===Food additive===
			Upon dissolution in water, bisulfite is generated:
	It is used as a preservative and antioxidant in food and is also known as ].<ref>http://www.eufic.org/upl/1/en/doc/EUFIC%20e-number%20mini%20guide%202.pdf</ref>
			:Na<sub>2</sub>S<sub>2</sub>O<sub>5</sub> + H<sub>2</sub>O → 2 Na<sup>+</sup> + 2 HSO<sub>3</sub><sup>−</sup>
			== Uses ==
	It may cause allergic reactions in those who are sensitive to ], including ] reactions in ], ] and other allergic reactions in sensitive individuals.<ref>Dean D. Metcalfe, Ronald A. Simon, ''Food allergy: adverse reactions to food and food additives'', Wiley-Blackwell 2003, pp 324-339</ref><ref>http://www.ific.org/publications/brochures/asthmabroch.cfm</ref>
			Sodium and ] have many major and niche uses. It is widely used for ].
			*Sodium metabisulfite is added as an ] to medications which contain adrenaline (]), in order to prevent the oxidation of adrenaline.<ref name="McGee" /> For example, it is added to ] formulations which contain a ] and adrenaline,<ref name="McGee">{{cite book |last1=McGee |first1=Douglas L. |editor1-last=Roberts |editor1-first=James R. |editor2-last=Jerris |editor2-first=R. Hedges |title=Roberts and Hedges' Clinical Procedures in Emergency Medicine |date=2013 |publisher=Elsevier/Saunders |isbn=9781455748594 |page=519 |edition=6th |chapter-url=https://books.google.com/books?id=slyLreFkHuIC&pg=PA519 |chapter=Local and topical anesthesia}}</ref> and to the formulation in ]s, such as the EpiPen.<ref>{{cite book |last1=Niazi |first1=Safaraz K. |title=Handbook of Pharmaceutical Manufacturing Formulations. Volume 6, Sterile Products |date=2009 |publisher=Informa Healthcare |location=New York |isbn=9781420081312 |page=410 |edition=2nd |url=https://books.google.com/books?id=HQbLBQAAQBAJ&pg=PA410}}</ref> This lengthens the shelf life of the formulation,<ref name="McGee" /> although the sodium metabisulfite reacts with adrenaline, causing it to degrade and form epinephrine sulfonate.<ref>{{cite book |last=Barnes |first=Andrew R. |chapter=Chapter 48. Chemical stability in dosage forms|editor1-last=Aulton |editor1-first=Michael E. |editor2-last=Taylor |editor2-first=Kevin M.G. |title=Aulton's pharmaceutics : the design and manufacture of medicines |date=2013 |publisher=Churchill Livingstone/Elsevier |isbn=9780702053931 |page=833 |edition=4th |chapter-url=https://books.google.com/books?id=SZ43AAAAQBAJ&pg=PA833}}</ref>
			*In combination with ] it is used as a rust-stain remover<ref>{{cite web |title=Ingredients |url=https://summitbrands.com/ingredients/ |website=Summit Brands |date=31 October 2019 |access-date=25 April 2021}}</ref>
			*It is used in photography.<ref>{{cite book|last1=Anchell|first1=Steve|title=The darkroom cookbook|url=https://archive.org/details/darkroomcookbook00anch|url-access=limited|date=2008|publisher=Focal Press|location=Amsterdam|isbn=978-0240810553|pages=|edition=3rd}}</ref>
			*Concentrated sodium metabisulfite can be used to remove ]s. Some brands contain 98% sodium metabisulfite, and cause degradation of ] in the stumps, facilitating removal.<ref> {{dead link|date=January 2022}}</ref>
			*It is also used as an ] in some ], such as ].
			*A very important health related aspect of this substance is that it can be added to a blood smear in a test for ] (and other similar forms of ] ]). The substance causes defunct cells to sickle (through a complex ]) hence confirming disease.
			*It is used as a bleaching agent in the production of coconut cream
			*It (or liquid SO<sub>2</sub>) is commonly used as an antimicrobial and antioxidant in winemaking; bottled wine indicates its use with the label "Contains Sulfites" in the US.
			*It is used as a reducing agent to break sulfide bonds in shrunken items of clothing made of natural fibres, thus allowing the garment to go back to its original shape after washing
			*It is used as an SO<sub>2</sub> source (mixed with air or oxygen) for the destruction of cyanide in commercial ] processes.
			*It is used as an SO<sub>2</sub> source (mixed with air or oxygen) for the precipitation of elemental gold in ] (aqua regia) solutions.
			*It is used in the water treatment industry to quench residual chlorine.
			*It is used in tint etching iron-based metal samples for microstructural analysis.<ref>{{cite web | url=https://vacaero.com/information-resources/metallography-with-george-vander-voort/991-color-metallography.html | title=Color Metallography| date=2011-05-04}}</ref><ref>{{cite web|url=https://www.asminternational.org/documents/10192/1874035/htp00102p025.pdf/ace8f01d-bf9a-4048-b948-a3aeb2d8a536|format=PDF|title=ETCHING ISOTHERMALLY TREATED STEELS|author=George F. Vander Voort|website=Asminternational.org|access-date=24 January 2022|archive-date=16 October 2022|archive-url=https://web.archive.org/web/20221016054106/https://www.asminternational.org/documents/10192/1874035/htp00102p025.pdf/ace8f01d-bf9a-4048-b948-a3aeb2d8a536|url-status=dead}}</ref>
			*It is used as a fungicide for anti-microbe and mould prevention during shipping of consumer goods such as shoes and clothing. Plastic stickers and packaging (such as Micro-Pak''']''') containing the anhydrous, sodium metabisulfite solid active ingredient are added prior to shipping. The devices absorb moisture from the atmosphere during shipping and release low levels of sulfur dioxide.<ref>{{cite web | url=http://hawaii.gov/hdoa/labels/8979.1.pdf | title=Micro-Pak Enhanced Packaging Stickers| date=2020-05-05}}</ref>
			*It is used for preserving fruit during shipping.<ref>{{cite book | chapter-url=https://www.sciencedirect.com/science/article/pii/B9781845697358500152 | title=Postharvest Biology and Technology of Tropical and Subtropical Fruits| chapter=15 - Litchi (Litchi chinensis Sonn.)| series=Woodhead Publishing Series in Food Science, Technology and Nutrition| doi=10.1533/9780857092885.361 |date=2020-05-05| pages=361–409e| publisher=Woodhead| isbn=9781845697358| last1=Sivakumar| first1=D.| last2=Korsten| first2=L.}}</ref>
			*It is used as a solvent in the extraction of starch from tubers,<ref>{{cite journal|last1=Manek|first1=Rahul V.|last2=Builders|first2=Philip F.|last3=Kolling|first3=William M.|last4=Emeje|first4=Martins|last5=Kunle|first5=Olobayo O.|date=June 2012|title=Physicochemical and binder properties of starch obtained from Cyperus esculentus|journal=AAPS PharmSciTech|volume=13|issue=2|pages=379–388|doi=10.1208/s12249-012-9761-z|issn=1530-9932|pmc=3364391|pmid=22350737}}</ref> fruit,<ref>{{cite journal|last1=Nawab|first1=Anjum|last2=Alam|first2=Feroz|last3=Haq|first3=Muhammad Abdul|last4=Hasnain|first4=Abid|date=2016|title=Biodegradable film from mango kernel starch: Effect of plasticizers on physical, barrier, and mechanical properties|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/star.201500349|journal=Starch - Stärke|language=en|volume=68|issue=9–10|pages=919–928|doi=10.1002/star.201500349|issn=1521-379X}}</ref> and cereal crops.<ref>{{cite journal|last1=Nyakabau|first1=Tatenda|last2=Wokadala|first2=Obiro Cuthbert|last3=Emmambux|first3=Mohammad Naushad|date=2013|title=Effect of steeping additives on tef starch extraction and its quality|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/star.201200241|journal=Starch - Stärke|language=en|volume=65|issue=9–10|pages=738–746|doi=10.1002/star.201200241|issn=1521-379X|hdl=2263/41924|hdl-access=free}}</ref><ref>{{cite journal|last1=Park|first1=S. H.|last2=Bean|first2=S. R.|last3=Wilson|first3=J. D.|last4=Schober|first4=T. J.|date=2006|title=Rapid Isolation of Sorghum and Other Cereal Starches Using Sonication|url=https://onlinelibrary.wiley.com/doi/abs/10.1094/CC-83-0611|journal=Cereal Chemistry|language=en|volume=83|issue=6|pages=611–616|doi=10.1094/CC-83-0611|issn=1943-3638}}</ref>
			*It is used as a pickling agent to treat high pressure reverse osmosis and nanofiltration water desalination membranes for extended storage periods between uses.
			*It is used to create a bisulfite adduct from ketones to aid in separation of the ketone product. The usage of metabisulfite versus the sulfite is also more entropically favourable.
			== Safety ==
	Sodium metabisulfite and ] are the primary ingredients in ], used for ] and ] making.<ref name="isbn0-471-73518-3">{{cite book |author=Milne, George W. A. |title=Gardner's commercially important chemicals: synonyms, trade names, and properties |publisher=Wiley-Interscience |location=New York |year=2005 |pages=568 |isbn=0-471-73518-3 |oclc= |doi= |accessdate=}}</ref>
			Sodium metabisulfite, despite not being flammable, decomposes in 150 °C of heat releasing ] when decomposed. It is corrosive when dissolved in water. Some people who are sulfite sensitive may experience an allergic reaction to sodium meta bisulfite, sometimes severe, resulting in labeling requirements for food safety.<ref>{{cite web | url=https://www.webmd.com/allergies/sulfite-sensitivity | title=What is Sulfite Sensitivity? }}</ref> In 2024, it was named ‘allergen of the year 2024’ by the American Contact Dermatitis Society. <ref>{{cite web | url=https://www.liebertpub.com/doi/10.1089/derm.2023.0154 | title= Sulfites: Allergen of the Year 2024 }}</ref>
			==References==
	The ] is up to 0.7&nbsp;mg per kg of body weight.<ref>http://www.food-info.net/uk/e/e223.htm</ref> Sodium metabisulfite has no side effects; it is oxidised in the liver to harmless sulfate and excreted in urine.<ref></ref> {{Clarify|date=March 2010}}
			{{reflist}}
	===Sanitization and cleaning agent===
	It is commonly used in ] and ] to ] equipment. It is used as a cleaning agent for potable water ] membranes in ] systems. It is also used to remove ] from drinking water after treatment.
	===Other uses===
	*It is used in photography.<ref>http://silvergrain.com/labs/Metabisulfite</ref>
	*Concentrated sodium metabisulfite can be used to remove ]s. Some brands contain 98% sodium metabisulfite, and cause degradation of ] in the stumps, facilitating removal.<ref>http://www.bonideproducts.com/lbonide/msds/msds271.pdf</ref>
	*It is also used as an ] in some ], such as ]. Approximately 0.5&nbsp;mg is used in ]s such as the EpiPen.
	*A very important health related aspect of this substance is that it can be added to a blood smear in a test for ] (and other similar forms of ] ]). The substances causes defunct cells to sickle (through a complex ]) hence confirming disease.
	==Chemical properties==
	When mixed with water, sodium metabisulfite releases ] (SO<sub>2</sub>), a pungent, unpleasant smelling gas that can also cause breathing difficulties in some people. For this reason, sodium metabisulfite has fallen from common use in recent times, with agents such as ] becoming more popular for effective and odorless sterilization of equipment. Released sulfur dioxide however makes the water a strong reducing agent.
	Sodium metabisulfite releases sulfur dioxide in contact with ]:
	:Na<sub>2</sub>S<sub>2</sub>O<sub>5</sub> + 2 HCl → 2 NaCl + H<sub>2</sub>O + 2 SO<sub>2</sub>
	on heating it releases sulfur dioxide, leaving ] behind:
	:Na<sub>2</sub>S<sub>2</sub>O<sub>5</sub> → Na<sub>2</sub>O + 2 SO<sub>2</sub>
	== See also ==
	* ]
	==External links==		==External links==
	*		*
	*		*
			*
	==References==
	{{reflist}}
	{{Sodium compounds}}		{{Sodium compounds}}
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