View source for Oxalic acid

{{Chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 476995784
| ImageFile = Oxalsäure3.svg
|  ImageSize = 140
|  ImageName = Structural formula of oxalic acid
| ImageFileL1 = Oxalic-acid-3D-balls.png
|  ImageSizeL1 =
| ImageFileR1 = Oxalic-acid-3D-vdW.png
|  ImageSizeR1 = 110
|  ImageNameR1 = Space-filling model of oxalic acid
| IUPACName = ethanedioic acid
|  OtherNames = oxalic acid
| Section1 = {{Chembox Identifiers
|  CASNo = 144-62-7
|   CASNo_Ref = {{cascite|correct|CAS}}
|  PubChem = 971
|   PubChem_Ref = {{Pubchemcite|correct|PubChem}}
|  ChemSpiderID = 946
|   ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
|  UNII = 9E7R5L6H31
|   UNII_Ref = {{fdacite|correct|FDA}}
|  EINECS = 205-634-3
|  UNNumber = 3261
|  DrugBank_Ref = {{drugbankcite|correct|drugbank}}
| DrugBank = DB03902
|  KEGG_Ref = {{keggcite|changed|kegg}}
| KEGG = C00209
|  MeSHName = Oxalic+acid
|  ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 16995
|  RTECS = RO2450000
|  ATCvet = yes
|  ATCCode_prefix = P53
|  ATCCode_suffix = AG03
|  SMILES = C(=O)(C(=O)O)O
|  ChEMBL_Ref = {{ebicite|correct|EBI}}
| ChEMBL = 146755
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/C2H2O4/c3-1(4)2(5)6/h(H,3,4)(H,5,6)
|  StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = MUBZPKHOEPUJKR-UHFFFAOYSA-N
|  Beilstein = 385686
|  Gmelin = 2208
|  3DMet = B00059}}
| Section2 = {{Chembox Properties
|  C = 2
|  H = 2
|  O = 4
|  MolarMass_notes =<br /> (anhydrous) <br /> 126.07 g mol<sup>−1</sup> (dihydrate)
|  ExactMass = 89.995308552 g mol<sup>−1</sup>
|  Appearance = White crystals
|  Density = 1.90 g cm<sup>−3</sup> (anhydrous) <br /> 1.653g cm<sup>−3</sup> (dihydrate)
|  MeltingPtC = 102
|  MeltingPtCH = 103
|  Melting_notes = {{convert|101.5|C|F}} dihydrate
|  Solubility = 14.3 g/100ml (25 °C)
|  SolubleOther = 23.7 g/100ml (15 °C) in [[ethanol]] <hr> 1.4 g/100ml (15 °C) in [[diethyl ether]]  <ref>{{cite web|last=Radiant Agro Chem|title=Oxalic Acid MSDS|url=http://www.racpl.com/oa_msds.htm}}</ref>
|  pKa = 1.25, 4.14<ref>Bjerrum, J., et al. ''Stability Constants'', Chemical Society, London, 1958.</ref>
|  }}
| Section7 = {{Chembox Hazards
|   MainHazards = Toxic
|   NFPA-H = 3
|   NFPA-F = 1
|   NFPA-R =
|   FlashPtC = 166
|   ExternalMSDS = [http://hazard.com/msds/mf/baker/baker/files/o6044.htm External MSDS]
  }}
| Section8 = {{Chembox Related
|   OtherCpds = [[oxalyl chloride]]<br />[[disodium oxalate]]<br />[[calcium oxalate]]<br />[[phenyl oxalate ester]]
  }}
}}

'''Oxalic acid''' is an [[organic compound]] with the [[chemical formula|formula]] H<sub>2</sub>C<sub>2</sub>O<sub>4</sub>. It is a colorless crystalline solid that dissolves in water to give colorless solutions.  It is classified as a [[dicarboxylic acid]]. In terms of acid strength, it is much stronger than [[acetic acid]]. Oxalic acid is a [[reducing agent]] and its [[conjugate base]], known as [[oxalate]] (C<sub>2</sub>O<sub>4</sub><sup>2&minus;</sup>), is a [[chelating agent]] for metal cations. Typically, oxalic acid occurs as the [[water of crystallization|dihydrate]] with the formula H<sub>2</sub>C<sub>2</sub>O<sub>4</sub>·2H<sub>2</sub>O.  Oral consumption of oxalic acid in excess or prolonged skin contact can be dangerous.

==Preparation==
Oxalic acid is mainly manufactured by the oxidation of [[carbohydrate]]s or [[glucose]] using [[nitric acid]] or air in the presence of [[vanadium pentoxide]]. A variety of precursors can be used including [[glycolic acid]] and [[ethylene glycol]].<ref>Yonemitsu Eiichi, Isshiki Tomiya, Suzuki Tsuyoshi, Yashima Yukio, Process for the production of oxalic acid, {{cite patent|US|3678107}}</ref>  A newer method entails oxidative [[carbonylation]] of [[alcohol]]s to give the diesters of oxalic acid:

:4 ROH  +  4 CO  +  O<sub>2</sub>   →   2 (CO<sub>2</sub>R)<sub>2</sub>  +  2 H<sub>2</sub>O

These diesters are subsequently hydrolyzed to oxalic acid.  Approximately 120,000 [[tonne]]s are produced annually.<ref name=Ullmann>Wilhelm Riemenschneider, Minoru Tanifuji "Oxalic acid" in ''Ullmann's Encyclopedia of Industrial Chemistry'', 2002, Wiley-VCH, Weinheim. {{DOI| 10.1002/14356007.a18_247}}.</ref>

Historically oxalic acid was obtained exclusively by using caustics, such as sodium or [[potassium hydroxide]], on sawdust.<ref>{{cite book|last=Von Wagner|first=Rudolf|title=Manual of chemical technology|year=1897|publisher=D. Appleton & Co.|location=New York|page=499|url=http://babel.hathitrust.org/cgi/pt?id=uc2.ark:/13960/t3tt4gz1p;view=1up;seq=527}}</ref>

===Laboratory methods===
Although it can be readily purchased, oxalic acid can be prepared in the laboratory by [[oxidizer|oxidizing]] [[sucrose]] using [[nitric acid]] in the presence of a small amount of [[vanadium pentoxide]] as a [[catalyst]].<ref name="cohen">''Practical Organic Chemistry'' by Julius B. Cohen, 1930 ed. preparation #42</ref>

The hydrated solid can be dehydrated with heat or by [[azeotropic distillation]].<ref>{{OrgSynth | author = Clarke H. T.;. Davis, A. W. | title = Oxalic acid (anhydrous) | collvol = 1 | pages = 421 | year = 1941 | prep = CV1P0421}}</ref>

Of historical interest, [[Friedrich Wöhler|Wöhler]] prepared oxalic acid by [[hydrolysis]] of [[cyanogen]] in 1824.  This experiment may represent the first synthesis of a [[natural product]].<ref name=Ullmann/>

==Structure==
Anhydrous oxalic acid exists as two [[Polymorphism (materials science)|polymorph]]s; in one the [[hydrogen-bonding]] results in a chain-like structure whereas the hydrogen bonding pattern in the other form defines a sheet-like structure.<ref>Wells, A.F. (1984) ''Structural Inorganic Chemistry'', Oxford: Clarendon Press. ISBN 0-19-855370-6.</ref>  Because the anhydrous material is both acidic and [[hydrophilic]] (water seeking), it is used in [[esterification]]s.

==Reactions==
Oxalic acid is a relatively strong acid, despite being a [[carboxylic acid]]:
:C<sub>2</sub>O<sub>4</sub>H<sub>2</sub>   →   C<sub>2</sub>O<sub>4</sub>H<sup>&minus;</sup>  +  H<sup>+</sup>;   pK<sub>a</sub> = 1.27
:C<sub>2</sub>O<sub>4</sub>H<sup>&minus;</sup>   →   C<sub>2</sub>O<sub>4</sub><sup>2&minus;</sup>  +  H<sup>+</sup>;   pK<sub>a</sub> = 4.27

Oxalic acid undergoes many of the reactions characteristic of other carboxylic acids.  It forms esters such as [[dimethyl oxalate]] ([[melting point|m.p.]] 52.5–53.5 °C).<ref>{{OrgSynth | author = Bowden, E. | title = Methyl oxalate | collvol = 2 | pages = 414 | year = 1943 | prep = CV2P0414}}</ref>  It forms an acid chloride called [[oxalyl chloride]].

Oxalate, the conjugate base of oxalic acid, is an excellent [[ligand]] for metal ions, e.g. the drug [[oxaliplatin]].

Oxalic acid and oxalates can be oxidized by [[permanganate]] in an [[autocatalytic]] reaction.<ref>{{cite journal | title = Revising the mechanism of the permanganate/oxalate reaction | author = Kovacs K.A., Grof P., Burai L., Riedel M. | journal = J. Phys. Chem. A | doi = 10.1021/jp047061u | year = 2004 | volume = 108 | pages = 11026–11031 | issue = 50}}</ref>

==Occurrence==

===Biosynthesis===
At least two pathways exist for the enzyme-mediated formation of oxalate.  In one pathway, oxaloacetate, a component of the [[Krebs citric acid cycle]], is hydrolyzed to oxalate and acetic acid by the enzyme [[oxaloacetase]]:<ref>Martin V. Dutton, Christine S. Evans "Oxalate production by fungi: its role in pathogenicity and ecology in the soil environment" Canadian Journal of Microbiology, 1996, vol. 42, p. 881-895.  {{DOI|10.1139/m96-114}}.</ref>
:[O<sub>2</sub>CC(O)CH<sub>2</sub>CO<sub>2</sub>]<sup>2−</sup>   +  H<sub>2</sub>O   →   C<sub>2</sub>O<sub>4</sub><sup>2−</sup>  +  CH<sub>3</sub>CO<sub>2</sub><sup>−</sup>
It also arises from the dehydrogenation of [[glycolic acid]], which is produced by the metabolism of [[ethylene glycol]].

===Occurrence in foods and plants===
[[Calcium oxalate]] is the most common component of [[kidney stone]]s.  Early investigators isolated oxalic acid from [[Oxalis|wood-sorrel]] (''Oxalis'').  Members of the [[spinach]] family are high in oxalates, as is [[sorrel]].<ref>Rombauer, Rombauer Becker, and Becker (1931/1997). ''Joy of Cooking'', p.415. ISBN 0-684-81870-1.</ref> [[Rhubarb]] leaves contain about 0.5% oxalic acid and jack-in-the-pulpit (''[[Arisaema triphyllum]]'') contains [[calcium oxalate]] crystals. Bacteria produce oxalates from oxidation of [[carbohydrate]]s.<ref name=Ullmann/>

===Other===
Oxidized [[bitumen]] or bitumen exposed to [[gamma rays]] also contains oxalic acid among its degradation products. Oxalic acid may increase the leaching of [[radionuclide]]s conditioned in bitumen for [[radioactive waste disposal]].{{citation needed|date=September 2012}}

==Biochemistry==
The [[conjugate base]] of oxalic acid ([[oxalate]]) is a [[competitive inhibitor]] of the [[lactate dehydrogenase]](LDH) enzyme.<ref>{{cite journal|last=Novoa|first=William|coauthors=Alfred Winer, Andrew Glaid, George Schwert|title=Lactic Dehydrogenase V. inhibiton by Oxamate and Oxalate|journal=Journal of Biological Chemistry|year=1958}}</ref> LDH catalyses the conversion of [[pyruvate]] to [[lactic acid]] (End product of the Fermentation (Anaerobic) Process) oxidising the coenzyme [[NADH]] to [[NAD+]] and [[Hydron (chemistry)|H+]] concurrently. Restoring NAD+ levels are essential to the continuation of anaerobic energy metabolism through [[glycolysis]]. As cancer cells preferentially use anaerobic metabolism (see [[Warburg effect]]) inhibition of LDH has been shown to inhibit tumor formation and growth,<ref>{{cite journal|last=Le|first=Anne|coauthors=Charles Cooper, Arvin Gouw, Ramani Dinavahi, Anirban Maitra, Lorraine Deck, Robert Royer, David Vander Jagt, Gregg Semenza, Chi Dang|title=Inhibition of lactate dehydrogenase A induces oxidative stress and inhibits tumor progression|journal=Proceedings of the National Academy of Sciences|date=14 December 2009|url=http://www.pnas.org/content/107/5/2037.abstract}}</ref> thus is an interesting potential course of cancer treatment.

==Applications==
About 25% of produced oxalic acid is used as a [[mordant]] in dyeing processes.  It is used in [[Bleach (chemical)|bleach]]es, especially for [[pulpwood]]. It is also used in baking powder.<ref name=Ullmann/>

===Cleaning===
Oxalic acid's main applications include cleaning or bleaching, especially for the removal of rust (iron complexing agent), e.g. [[Bar Keepers Friend]] is an example of a household cleaner containing oxalic acid. Its utility in rust removal agents is due to its forming a stable, water soluble salt with ferric iron, [[Sodium ferrioxalate|ferrioxalate]] ion.

===Extractive metallurgy===
Oxalic acid is an important reagent in [[lanthanide]] chemistry. Hydrated lanthanide oxalates form readily in strongly acidic solutions in a densely [[crystalline]], easily filtered form, largely free of contamination by nonlanthanide elements. Thermal decomposition of these oxalate gives the [[oxide]]s, which is the most commonly marketed form of these elements.

===Niche uses===
Vaporized oxalic acid, or a 3.2% solution of oxalic acid in sugar syrup, is used by some [[beekeeper]]s as a [[miticide]] against the parasitic [[Varroa destructor|varroa mite]].

Oxalic acid is rubbed onto completed marble sculptures to seal the surface and introduce a shine. Oxalic acid is also used to clean iron and manganese deposits from [[quartz]] crystals.<ref>[http://bluemooncrystals.com/Crystal_Cleaning.html]</ref><ref>[http://www.mindat.org/article.php/403/Cleaning+Quartz]</ref>

==Content in food items==
This table was originally published in ''Agriculture Handbook No. 8-11, Vegetables and Vegetable Products'', 1984.<ref>http://www.ars.usda.gov/Services/docs.htm?docid=9444</ref>

{| class="wikitable sortable"
|-
! Vegetable !! Oxalic acid (g/100 g)
|-
| [[Amaranth]]               || {{ntsh|1.09}} 1.09
|-
| [[Asparagus]]              || {{ntsh|.13}}   0.13
|-
| [[Snap beans|Beans, snap]] || {{ntsh|.36}}   0.36
|-
| [[Beet]] leaves            || {{ntsh|.61}}   0.61
|-
| [[Broccoli]]               || {{ntsh|.19}}   0.19
|-
| [[Brussels sprouts]]       || {{ntsh|.36}}   0.36
|-
| [[Cabbage]] || {{ntsh|.10}} 0.10
|-
| [[Carrot]] || {{ntsh|.50}} 0.50
|-
| [[Cassava]] || {{ntsh|1.26}} 1.26
|-
| [[Cauliflower]] || {{ntsh|.15}} 0.15
|-
| [[Celery]] || {{ntsh|.19}} 0.19
|-
| [[Chicory]] || {{ntsh|.21}} 0.21
|-
| [[Chives]] || {{ntsh|1.48}} 1.48
|-
| [[Collards]] || {{ntsh|.45}} 0.45
|-
| [[Coriander]] || {{ntsh|.01}} 0.01
|-
| [[Sweet corn|Corn, sweet]] || {{ntsh|.01}} 0.01
|-
| [[Cucumbers]] || {{ntsh|.02}} 0.02
|-
| [[Eggplant]] || {{ntsh|.19}} 0.19
|-
| [[Endive]] || {{ntsh|.11}} 0.11
|-
| [[Garlic]] || {{ntsh|.36}} 0.36
|-
| [[Kale]] || {{ntsh|.02}} 0.02
|-
| [[Lettuce]] || {{ntsh|.33}} 0.33
|-
| [[Okra]] || {{ntsh|.05}} 0.05
|-
| [[Onion]] || {{ntsh|.05}} 0.05
|-
| [[Parsley]] || {{ntsh|1.70}} 1.70
|-
| [[Parsnip]] || {{ntsh|.04}} 0.04
|-
| [[Pea]] || {{ntsh|.05}} 0.05
|-
| [[Bell pepper]] || {{ntsh|.04}} 0.04
|-
| [[Potato]] || {{ntsh|.05}} 0.05
|-
| [[Portulaca oleracea|Purslane]] || {{ntsh|1.31}} 1.31
|-
| [[Radish]] || {{ntsh|.48}} 0.48
|-
| [[Rutabaga]] || {{ntsh|.03}} 0.03
|-
| [[Spinach]] || {{ntsh|.97}} 0.97
|-
| [[Squash (plant)|Squash]] || {{ntsh|.02}} 0.02
|-
| [[Sweet potato]] || {{ntsh|.24}} 0.24
|-
| [[Tomato]] || {{ntsh|.05}} 0.05
|-
| [[Turnip]] || {{ntsh|.21}} 0.21
|-
| [[Turnip greens]] || {{ntsh|.05}} 0.05
|-
| [[Watercress]]    || {{ntsh|.31}} 0.31
|}

==Toxicity and safety==
Oxalic acid has toxic effects through contact and if [[Ingestion|ingested]]; manufacturers provide details in [[Material Safety Data Sheet]]s (MSDS). It is not identified as [[mutagenic]] or [[carcinogenic]]; there is a possible risk of congenital malformation in the fetus; may be harmful if [[Inhalation|inhaled]], and is extremely destructive to  [[Biological tissue|tissue]] of [[mucous membrane]]s and upper [[respiratory tract]]; harmful if swallowed; harmful to and destructive of tissue and causes burns if absorbed through the skin or is in contact with the eyes. Symptoms and effects include a burning sensation, cough, wheezing, [[laryngitis]], shortness of breath, [[spasm]], inflammation and [[edema]] of the [[larynx]], inflammation and edema of the [[bronchi]], [[pneumonitis]], [[pulmonary edema]].<ref>[http://www.sigmaaldrich.com/MSDS/MSDS/DisplayMSDSPage.do?country=GB&language=en&productNumber=247537&brand=SIAL&PageToGoToURL=http%3A%2F%2Fwww.sigmaaldrich.com%2Fcatalog%2Fsearch%3Finterface%3DAll%26term%3Doxalic%2Bacid%26lang%3Den%26region%3DGB%26focus%3Dproduct%26N%3D0%2B220003048%2B219853121%2B219853286%26mode%3Dmatch%2520partialmax Sigma-Aldrich: Oxalic acid dihydrate Material Safety Data Sheet (MSDS)]</ref>

In humans, ingested oxalic acid has an oral [[Lowest published lethal dose|LD<sub>Lo</sub>]] (lowest published lethal dose) of 600&nbsp;mg/kg.<ref>{{cite web
  | author = Safety Officer in Physical Chemistry
  | title = Safety (MSDS) data for oxalic acid dihydrate
  | work = 
  | publisher = Oxford University
  | date = August 13, 2005
  | url = http://msds.chem.ox.ac.uk/OX/oxalic_acid_dihydrate.html
  | accessdate = December 30, 2009
}}</ref>{{Dead link|date=May 2013}}  It has been reported that the lethal oral dose is 15 to 30 grams.<ref>[http://www.cdc.gov/niosh/idlh/144627.html CDC: Documentation for Immediately Dangerous To Life or Health Concentrations (IDLHs), Oxalic acid, May 1994]</ref>

The toxicity of oxalic acid is due to kidney failure caused by precipitation of solid [[calcium oxalate]],<ref>[http://www.ema.europa.eu/ema/pages/includes/document/open_document.jsp?webContentId=WC500015217 EMEA Committee for veterinary medicinal products, oxalic acid summary report, December 2003]</ref> the main component of kidney stones.  Oxalic acid can also cause [[joint pain]] due to the formation of similar precipitates in the joints.  Ingestion of [[ethylene glycol]] results in oxalic acid as a metabolite which can also cause acute kidney failure.

==References==
{{Reflist|2}}

== External links ==
{{commons|Oxalic acid|Oxalic acid}}
* [http://gmd.mpimp-golm.mpg.de/Spectrums/ad304d2f-132a-4bba-8e72-d931e77e1c6e.aspx Oxalic acid MS Spectrum]
*{{ICSC|0529|05}}
* [http://www.cdc.gov/niosh/npg/npgd0474.html NIOSH Guide to Chemical Hazards (CDC)]
* {{cite web | url = http://www.chemicalland21.com/arokorhi/industrialchem/organic/OXALIC%20ACID.htm | title = Oxalic acid | publisher = ChemicalLand21.com}}
*[http://www.nal.usda.gov/fnic/foodcomp/Data/Other/oxalic.html Table: Oxalic acid content of selected vegetables (USDA)]
*[http://www.ars.usda.gov/Services/docs.htm?docid=9444 Alternative link: Table: Oxalic Acid Content of Selected Vegetables (USDA)]
*[http://www.rhubarbinfo.com/rhubarb-poison.html About rhubarb poisoning (The Rhubarb Compendium)]
* [http://www.ohf.org/docs/Oxalate2008.pdf Oxalosis & Hyperoxaluria Foundation (OHF) The Oxalate Content of Food 2008 (PDF)]
* [http://www.ohf.org/diet.html Oxalosis & Hyperoxaluria Foundation (OHF) Diet Information]
*[http://www.aim.env.uea.ac.uk/aim/accent2/inputpage.php Calculator: Water and solute activities in aqueous oxalic acid]

[[Category:Oxalates]]
[[Category:Household chemicals]]
[[Category:Dicarboxylic acids]]