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Oxalic acid

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Title: Oxalic acid  
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Subject: Titration curve, Wöhler synthesis, Carl Wilhelm Scheele, Ethylene glycol poisoning, Diphenyl oxalate
Collection: Dicarboxylic Acids, Household Chemicals, Oxalates
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Oxalic acid

Oxalic acid
Structural formula of oxalic acid
Skeletal formula of oxalic acid
Space-filling model of oxalic acid
Oxalic acid dihydrate
IUPAC name
ethanedioic acid
Other names
oxalic acid
ATCvet code QP53
ChemSpider  Y
DrugBank  Y
EC number 205-634-3
Jmol-3D images Image
RTECS number RO2450000
UN number 3261
Molar mass 90.03 g·mol−1
126.07 g mol−1 (dihydrate)
Appearance White crystals
Odor odorless
Density 1.90 g cm−3 (anhydrous)
1.653 g cm−3 (dihydrate)
Melting point 102 to 103 °C (216 to 217 °F; 375 to 376 K)
101.5 °C (214.7 °F; 374.7 K) dihydrate
143 g/L (25 °C)
Solubility 237 g/L (15 °C) in ethanol
14 g/L (15 °C) in diethyl ether [1]
Vapor pressure <0.001 mmHg (20 °C)[2]
Acidity (pKa) 1.25, 4.14[3]
Main hazards Toxic T
Safety data sheet External MSDS
NFPA 704
Flash point 166 °C (331 °F; 439 K)
Lethal dose or concentration (LD, LC):
1000 mg/kg (dog, oral)
1400 mg/kg (rat)
7500 mg/kg (rat, oral)[4]
US health exposure limits (NIOSH):
PEL (Permissible)
TWA 1 mg/m3[2]
REL (Recommended)
TWA 1 mg/m3 ST 2 mg/m3[2]
500 mg/m3[2]
Related compounds
Related compounds
oxalyl chloride
disodium oxalate
calcium oxalate
phenyl oxalate ester
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
 N  (: Y/N?)

Oxalic acid is an formula H2C2O4 (also written as HOOCCOOH). It is a colorless crystalline solid that forms a colorless solution in water. 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 [5] 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. Excessive ingestion of oxalic acid or prolonged skin contact can be dangerous.


  • Preparation 1
    • Laboratory methods 1.1
  • Structure 2
  • Reactions 3
  • Occurrence 4
    • Biosynthesis 4.1
    • Occurrence in foods and plants 4.2
    • Other 4.3
  • Biochemistry 5
  • Applications 6
    • Cleaning 6.1
    • Extractive metallurgy 6.2
    • Niche uses 6.3
  • Content in food items 7
  • Toxicity and safety 8
  • References 9
  • External links 10


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.[6] 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 tonnes are produced annually.[7]

Historically oxalic acid was obtained exclusively by using caustics, such as sodium or potassium hydroxide, on sawdust.[8]

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.[9]

The hydrated solid can be dehydrated with heat or by azeotropic distillation.[10]

Developed in the Netherlands, an electrocatalysis by a copper complex helps reduce carbon dioxide to oxalic acid;[11] this conversion uses carbon dioxide as a feedstock to generate oxalic acid.

Of historical interest, Wöhler prepared oxalic acid by hydrolysis of cyanogen in 1824. This experiment may represent the first synthesis of a natural product.[7]


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.[12] Because the anhydrous material is both acidic and hydrophilic (water seeking), it is used in esterifications.


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

C2O4H2 → C2O4H + H+; pKa = 1.27
C2O4H → C2O42− + H+; pKa = 4.27

Oxalic acid undergoes many of the reactions characteristic of other carboxylic acids. It forms esters such as dimethyl oxalate (m.p. 52.5 to 53.5 °C (126.5 to 128.3 °F)).[13] 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.[14]



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:[15]

[O2CC(O)CH2CO2]2− + H2O → C2O42− + CH3CO2

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 stones. Early investigators isolated oxalic acid from wood-sorrel (Oxalis). Members of the spinach family and the brassicas (cabbage, broccoli, brussels sprouts) are high in oxalates, as are sorrel and umbellifers like parsley.[16] 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.[7]

Plants of the Fenestraria genus produce optical fibers made from crystalline oxalic acid to transmit light to subterranean photosynthetic sites.


Oxidized bitumen or bitumen exposed to gamma rays also contains oxalic acid among its degradation products. Oxalic acid may increase the leaching of radionuclides conditioned in bitumen for radioactive waste disposal.[17]


The conjugate base of oxalic acid (oxalate) is a competitive inhibitor of the lactate dehydrogenase (LDH) enzyme.[18] LDH catalyses the conversion of pyruvate to lactic acid (end product of the fermentation (anaerobic) process) oxidising the coenzyme NADH to NAD+ and H+ concurrently. Restoring NAD+ levels is 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,[19] thus is an interesting potential course of cancer treatment.


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.[7]


Oxalic acid's main applications include cleaning or bleaching, especially for the removal of rust (iron complexing agent). 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, ferrioxalate ion.

Extractive metallurgy

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

Niche uses

Honeybee coated with oxalic acid crystals

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.[20]

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.[21][22]

Oxalic acid is used as a bleach for wood, removing black stains caused by water penetration.

Content in food items

This table was originally published in Agriculture Handbook No. 8-11, Vegetables and Vegetable Products, 1984.[23]

Vegetable Oxalic acid (g/100 g)
Amaranth 1.09
Asparagus 0.13
Beans, snap 0.36
Beet leaves 0.61
Broccoli 0.19
Brussels sprouts 0.36
Cabbage 0.10
Carrot 0.50
Cassava 1.26
Cauliflower 0.15
Celery 0.19
Chicory 0.2
Chives 1.48
Collards 0.45
Coriander 0.01
Corn, sweet 0.01
Cucumber 0.02
Eggplant 0.19
Endive 0.11
Garlic 0.36
Kale 0.02
Lettuce 0.33
Okra 0.05
Onion 0.05
Parsley 1.70
Parsnip 0.04
Pea 0.05
Bell pepper 0.04
Potato 0.05
Purslane 1.31
Radish 0.48
Rutabaga 0.03
Spinach 0.97
Squash 0.02
Sweet potato 0.24
Tomato 0.05
Turnip 0.21
Turnip greens 0.05
Watercress 0.31

This table references other source material

Vegetable Oxalic acid (g/100 g)
Rhubarb leaves[24] 0.52

Toxicity and safety

Oxalic acid in concentrated form can have harmful effects through contact and if ingested; manufacturers provide details in Material Safety Data Sheets (MSDS). It is not identified as mutagenic or carcinogenic; there is a possible risk of congenital malformation in the fetus; may be harmful if inhaled, and is extremely destructive to tissue of mucous membranes 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.[25]

In humans, ingested oxalic acid has an oral LDLo (lowest published lethal dose) of 600 mg/kg.[26] It has been reported that the lethal oral dose is 15 to 30 grams.[27]

The toxicity of oxalic acid is due to kidney failure caused by precipitation of solid calcium oxalate,[28] 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.


  1. ^ Radiant Agro Chem. "Oxalic Acid MSDS". 
  2. ^ a b c d "NIOSH Pocket Guide to Chemical Hazards #0474".  
  3. ^ Bjerrum, J., et al. (1958) Stability Constants, Chemical Society, London.
  4. ^ "Oxalic acid". Immediately Dangerous to Life and Health.  
  5. ^ Ullmann's Encyclopedia of Industrial Chemistry. Wiley. 2005. pp. 17624/28029.  
  6. ^ Eiichi, Yonemitsu; Tomiya, Isshiki; Tsuyoshi, Suzuki and Yukio, Yashima "Process for the production of oxalic acid", U.S. Patent 3,678,107, priority date March 15, 1969
  7. ^ 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.
  8. ^ Von Wagner, Rudolf (1897). Manual of chemical technology. New York: D. Appleton & Co. p. 499. 
  9. ^ Practical Organic Chemistry by Julius B. Cohen, 1930 ed. preparation #42
  10. ^ Clarke H. T.;. Davis, A. W. (1941). "Oxalic acid (anhydrous)".  
  11. ^ Bouwman, Elisabeth; Angamuthu, Raja; Byers, Philip; Lutz, Martin; Spek, Anthony L. (July 15, 2010). "Electrocatalytic CO2 Conversion to Oxalate by a Copper Complex". Science 327 (5393): 313–315.  
  12. ^ Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.
  13. ^ Bowden, E. (1943). "Methyl oxalate".  
  14. ^ 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.  
  15. ^ Dutton, M. V.; Evans, C. S. (1996). "Oxalate production by fungi: Its role in pathogenicity and ecology in the soil environment". Canadian Journal of Microbiology 42 (9): 881–895.  .
  16. ^ Rombauer, Rombauer Becker, and Becker (1931/1997). Joy of Cooking, p.415. ISBN 0-684-81870-1.
  17. ^ EPJ Web of Conferences
  18. ^ Novoa, William; Alfred Winer; Andrew Glaid; George Schwert (1958). "Lactic Dehydrogenase V. inhibition by Oxamate and Oxalate". Journal of Biological Chemistry 234 (5): 1143–8.  
  19. ^ Le, Anne; Charles Cooper; Arvin Gouw; Ramani Dinavahi; Anirban Maitra; Lorraine Deck; Robert Royer; David Vander Jagt; Gregg Semenza; Chi Dang (14 December 2009). "Inhibition of lactate dehydrogenase A induces oxidative stress and inhibits tumor progression". Proceedings of the National Academy of Sciences 107: 2037–2042.  
  20. ^ Exploring New Methods for Varroa Mite Control, Yu-Lun Lisa Fu
  21. ^ Jackson, Faith. "Quartz Crystal Cleaning".
  22. ^ "Rock Currier – Cleaning Quartz".
  23. ^ "Nutrient Data : Oxalic Acid Content of Selected Vegetables".
  24. ^ Pucher, GW; Wakeman, AJ; Vickery, HB (1938). ). III. The behavior of the organic acids during culture of excised leaves"Rheum hybridium"The organic acids of rhubarb (. Journal of Biological Chemistry 126 (1): 43. 
  25. ^ Oxalic acid dihydrate. MSDS.
  26. ^ "Oxalic Acid Material Safety Data Sheet" (PDF). Radiant Indus Chem. Retrieved 2014-05-20. 
  27. ^ "CDC – Immediately Dangerous to Life or Health Concentrations (IDLH): Oxalic acid – NIOSH Publications and Products".
  28. ^ EMEA Committee for veterinary medicinal products, oxalic acid summary report, December 2003

External links

  • Oxalic acid MS Spectrum
  • International Chemical Safety Card 0529
  • NIOSH Guide to Chemical Hazards (CDC)
  • "Oxalic acid". 
  • Table: Oxalic acid content of selected vegetables (USDA)
  • Alternative link: Table: Oxalic Acid Content of Selected Vegetables (USDA)
  • About rhubarb poisoning (The Rhubarb Compendium)
  • Oxalosis & Hyperoxaluria Foundation (OHF) The Oxalate Content of Food 2008 (PDF)
  • Oxalosis & Hyperoxaluria Foundation (OHF) Diet Information
  • Calculator: Water and solute activities in aqueous oxalic acid
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