Oxalic acid

Oxalic acid
Oxalic acid
CAS number 144-62-7 YesY
PubChem 971
ChemSpider 946 YesY
UNII 9E7R5L6H31 YesY
EC number 205-634-3
UN number 3261
DrugBank DB03902
KEGG C00209 N
MeSH Oxalic+acid
ChEBI CHEBI:16995 YesY
RTECS number RO2450000
ATCvet code QP53AG03
Beilstein Reference 385686
Gmelin Reference 2208
3DMet B00059
Jmol-3D images Image 1
Molecular formula C2H2O4
Molar mass 90.03 g mol−1
Exact mass 89.995308552 g mol-1
Appearance White crystals
Density 1.90 g cm-3
Melting point

189-191 °C, 462-464 K, 372-376 °F

Solubility in water 90 g dm-3 (at 20 °C)
Acidity (pKa) 1.25, 4.14[1]
MSDS External MSDS
NFPA 704
NFPA 704.svg
Flash point 166 °C
Related compounds
Related compounds oxalyl chloride
disodium oxalate
calcium oxalate
phenyl oxalate ester
 N (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Oxalic acid is an organic compound with the formula H2C2O4. This colourless solid is a dicarboxylic acid. In terms of acid strength, it is about 3,000 times stronger than acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate (C2O42−), is a chelating agent for metal cations. Typically oxalic acid occurs as the dihydrate with the formula H2C2O4·2H2O.



Oxalic acid is mainly manufactured by the oxidation of carbohydrates 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.[2] A newer method entails oxidative carbonylation of alcohols to give the diesters of oxalic acid:

4 ROH + 4 CO + O2 → 2 (CO2R)2 + 2 H2O

These diesters are subsequently hydrolyzed to oxalic acid. Approximately 120,000 metric tons are produced annually.[3]

Laboratory methods

Although it can be readily purchased, oxalic acid can be prepared in the laboratory by oxidizing sucrose using nitric acid in the presence of a small amount of vanadium pentoxide as a catalyst.[4]

The hydrated solid can be dehydrated with heat or by azeotropic distillation.[5]

Of historical interest, Wöhler prepared oxalic acid by hydrolysis of cyanogen in 1824. This experiment may represent the first synthesis of a natural product.[3]


Anhydrous oxalic acid exists as two polymorphs; 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.[6] Because the anhydrous material is both acidic and hygroscopic (water seeking), it is used in esterifications.


Oxalic acid is a relatively strong acid, despite being a carboxylic acid:

C2O4H2 → C2O4H + H+; pKa = 1.38
C2O4H → C2O42− + H+; pKa = 4.28

Oxalic acid undergoes many of the reactions characteristic of other carboxylic acids. It forms esters such as dimethyl oxalate (m.p. 52.5–53.5 °C).[7] 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.[8]

Occurrence in nature

Oxalic acid and oxalates are present in many plants, including black tea, and occur naturally in animals. Calcium oxalate is the most common component of kidney stones. Early investigators isolated oxalic acid from wood-sorrel (Oxalis). Its presence makes it dangerous to eat unripe carambola or monstera fruits. Members of the spinach family are high in oxalates, as is sorrel, and a, "steady diet of raw leaves," is not recommended.[9] 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 carbohydrates.[3]

Oxidized bitumen or bitumen exposed to gamma-rays also contains oxalic acid amongst its degradation products. Oxalic acid may increase the leaching of radionuclides conditioned in bitumen for radioactive waste disposal.


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. About 25% of produced oxalic acid is used as a mordant in dyeing processes. It is used in bleaches, especially for pulpwood. It is also used in baking powder.[3]

Extractive metallurgy

Oxalic acid is also 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 non-lanthanide elements. Lanthanide oxalates figure importantly in commercial processing of lanthanides, and are used to recover lanthanides from solution after separation. After thermal decomposition and oxalate combustion, lanthanide oxalates convert to the oxides, which are the most common form in which the lanthanides are marketed.

Miscellaneous uses

Oxalic acid is used in the restoration of old wood. Its reducing properties are used in platinotype, the early photographic platinum/palladium printing process. Oxalic acid is also used for cleaning 'grubbyness' from dirty leather to get back to the flesh of the leather, before reintroducing preservatives.

Vaporized oxalic acid, or a 3.2% solution of oxalic acid in sugar syrup, is used by some beekeepers as a miticide against the parasitic varroa mite.

Oxalic acid is rubbed onto completed marble sculptures to seal the surface and introduce a shine.

Toxicity and safety

In humans, oxalic acid has an oral LDLo (lowest published lethal dose) of 600 mg/kg (human).[10]

The main toxicity of oxalic acid is due to the precipitation of calcium oxalate in the kidneys when urine becomes supersaturated with respect to this salt. Oxalic acid is also a metabolism product of the degradation of ethylene glycol if accidentally ingested and, as such, directly represents a danger for the kidneys in case of glycol poisoning.


  1. ^ Bjerrum, J., et al. Stability Constants, Chemical Society, London, 1958.
  2. ^ http://www.freepatentsonline.com/3678107.html Process for the production of oxalic acid
  3. ^ a b c d Wilhelm Riemenschneider, Minoru Tanifuji "Oxalic acid" in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. doi: 10.1002/14356007.a18_247.
  4. ^ Practical Organic Chemistry by Julius B. Cohen, 1930 ed. preparation #42
  5. ^ Clarke H. T.;. Davis, A. W. (1941), "Oxalic acid (anhydrous)", Org. Synth.: 421, http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV1P0421 ; Coll. Vol. 1 
  6. ^ Wells, A.F. (1984) Structural inorganic chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.
  7. ^ Bowden, E. (1943), "Methyl oxalate", Org. Synth.: 414, http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV2P0414 ; Coll. Vol. 2 
  8. ^ Kovacs K.A., Grof P., Burai L., Riedel M. (2004). "Revising the mechanism of the permanganate/oxalate reaction". J. Phys. Chem. A 108 (50): 11026–11031. doi:10.1021/jp047061u. 
  9. ^ Rombauer, Rombauer Becker, and Becker (1931/1997). Joy of Cooking, p.415. ISBN 0-684-81870-1.
  10. ^ Safety Officer in Physical Chemistry (August 13, 2005). "Safety (MSDS) data for oxalic acid dihydrate". Oxford University. http://msds.chem.ox.ac.uk/OX/oxalic_acid_dihydrate.html. Retrieved December 30, 2009. 

External links

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Look at other dictionaries:

  • Oxalic acid — Oxalic Ox*al ic, a. [From {Oxalis}: cf. F. oxalique.] (Chem.) Pertaining to, derived from, or contained in, sorrel, or oxalis; specifically, designating an acid found in, and characteristic of, oxalis, and also certain plant of the Buckwheat… …   The Collaborative International Dictionary of English

  • oxalic acid — [äk sal′ik] n. [Fr oxalique < L oxalis: see OXALIS] a colorless, poisonous, crystalline acid, (COOH) 2, found in oxalis and other plants or prepared synthetically and used in dyeing, bleaching, etc …   English World dictionary

  • oxalic acid — Chem. a white, crystalline, water soluble, poisonous acid, H2C2O4·2H2O, first discovered in the juice of the wood sorrel species of oxalis and obtained by reacting carbon monoxide with sodium hydroxide or certain carbohydrates with acids or… …   Universalium

  • oxalic acid — An acid, HOOC–COOH, found in many plants and vegetables, particularly in buckwheat (family Polygoniaceae) and Oxalis (family Oxalidaceae); used as a hemostatic in veterinary medicine, but toxic in elevated levels when ingested by humans; also… …   Medical dictionary

  • oxalic acid — an extremely poisonous acid, C2H2O4. It is a component of some bleaching powders and is found in many plants, including sorrel and the leaves of rhubarb. Oxalic acid is a powerful local irritant; when swallowed it produces burning sensations in… …   The new mediacal dictionary

  • oxalic acid — oksalo rūgštis statusas T sritis chemija formulė (HOOC)₂ atitikmenys: angl. oxalic acid rus. щавелевая кислота ryšiai: sinonimas – etano dirūgštis sinonimas – rūgštynių rūgštis …   Chemijos terminų aiškinamasis žodynas

  • oxalic acid — /ɒkˌsælɪk ˈæsəd/ (say ok.salik asuhd) noun a white, crystalline, dibasic acid, (COOH)2.2H2O, first discovered in the juice of a species of oxalis (wood sorrel), used in textile and dye manufacturing, in bleaching, etc. {oxalic, from French… …   Australian-English dictionary

  • oxalic acid — noun A colourless, crystalline dicarboxylic acid, (COOH), found in rhubarb, spinach and other plants, often as crystals of the calcium salt. Syn: acid of sugar, ethandioic acid …   Wiktionary

  • oxalic acid — noun Etymology: French (acide) oxalique, from Latin oxalis Date: 1790 a poisonous strong acid (COOH)2 or H2C2O4 that occurs in various plants (as spinach) as oxalates and is used especially as a bleaching or cleaning agent and as a chemical… …   New Collegiate Dictionary

  • oxalic acid — [ɒk salɪk] noun Chemistry a poisonous crystalline organic acid, present in rhubarb leaves, wood sorrel, and other plants. Origin C18: from Fr. oxalique, via L. from Gk oxalis wood sorrel …   English new terms dictionary

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