Oxalic acid is an organic compound with the formula C2H2O4. It is a white crystalline solid that forms a colorless solution in water. Its condensed formula is HOOCCOOH, reflecting its classification as the simplest dicarboxylic acid.
|Preferred IUPAC name
|Systematic IUPAC name
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||90.034 g·mol−1 |
126.065 g·mol−1 (dihydrate)
|Density||1.90 g·cm−3 (anhydrous, at 17 °C)|
1.653 g·cm−3 (dihydrate)
|Melting point|| 189 to 191 °C (372 to 376 °F; 462 to 464 K) |
101.5 °C (214.7 °F; 374.6 K) dihydrate
|90-100 g/L (20 °C)|
|Solubility||237 g/L (15 °C) in ethanol |
14 g/L (15 °C) in diethyl ether 
|Vapor pressure||<0.001 mmHg (20 °C)|
|Acidity (pKa)||1.25, 4.14|
|Safety data sheet||External MSDS|
|NFPA 704 (fire diamond)|
|Flash point||166 °C (331 °F; 439 K)|
|Lethal dose or concentration (LD, LC):|
LDLo (lowest published)
|1000 mg/kg (dog, oral)|
1400 mg/kg (rat)
7500 mg/kg (rat, oral)
|NIOSH (US health exposure limits):|
|TWA 1 mg/m3|
|TWA 1 mg/m3 ST 2 mg/m3|
IDLH (Immediate danger)
phenyl oxalate ester
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Its acid strength is much greater than that of acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate (C
4), is a chelating agent for metal cations. Typically, oxalic acid occurs as the dihydrate with the formula C2H2O4·2H2O.
It occurs naturally in many foods, but excessive ingestion of oxalic acid or prolonged skin contact can be dangerous.
The preparation of salts of oxalic acid from plants had been known, at the latest, since 1745, when the Dutch botanist and physician Herman Boerhaave isolated a salt from sorrel. By 1773, François Pierre Savary of Fribourg, Switzerland had isolated oxalic acid from its salt in sorrel.
In 1776, Swedish chemists Carl Wilhelm Scheele and Torbern Olof Bergman produced oxalic acid by reacting sugar with concentrated nitric acid; Scheele called the acid that resulted socker-syra or såcker-syra (sugar acid). By 1784, Scheele had shown that "sugar acid" and oxalic acid from natural sources were identical.
In 1824, the German chemist Friedrich Wöhler obtained oxalic acid by reacting cyanogen with ammonia in aqueous solution. This experiment may represent the first synthesis of a natural product.
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. 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
Historically oxalic acid was obtained exclusively by using caustics, such as sodium or potassium hydroxide, on sawdust. Pyrolysis of sodium formate (ultimately prepared from carbon monoxide), leads to the formation of sodium oxalate, easily converted to oxalic acid.
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. 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− ⇌ C
4 + 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)). It forms an acid chloride called oxalyl chloride.
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:
- [O2CC(O)CH2CO2]2− + H2O → C
4 + CH
2 + H+
Occurrence in foods and plantsEdit
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. Rhubarb leaves contain about 0.5% oxalic acid, and jack-in-the-pulpit (Arisaema triphyllum) contains calcium oxalate crystals. Similarly, the Virginia creeper, a common decorative vine, produces oxalic acid in its berries as well as oxalate crystals in the sap, in the form of raphides. Bacteria produce oxalates from oxidation of carbohydrates.
Carambola, also known as starfruit, also contains oxalic acid along with caramboxin. Citrus juice contains small amounts of oxalic acid. Citrus fruits produced in organic agriculture contain less oxalic acid than those produced in conventional agriculture.
The formation of naturally occurring calcium oxalate patinas on certain limestone and marble statues and monuments has been proposed to be caused by the chemical reaction of the carbonate stone with oxalic acid secreted by lichen or other microorganisms.
Production by fungiEdit
Many soil fungus species secrete oxalic acid, resulting in greater solubility of metal cations, increased availability of certain soil nutrients, and can lead to the formation of calcium oxalate crystals.
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.
The conjugate base of oxalic acid is the hydrogenoxalate anion, and its conjugate base (oxalate) is a competitive inhibitor of the lactate dehydrogenase (LDH) enzyme. 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, thus is an interesting potential course of cancer treatment.
About 25% of produced oxalic acid will be used as a mordant in dyeing processes. It is used in bleaches, especially for pulpwood. It is also used in baking powder and as a third reagent in silica analysis instruments.
Oxalic acid's main applications include cleaning or bleaching, especially for the removal of rust (iron complexing agent). Its utility in rust removal agents is due to its forming a stable, water-soluble salt with ferric iron, ferrioxalate ion.
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.
Oxalic acid is sometimes used in the aluminum anodizing process, with or without sulfuric acid. Compared to sulfuric acid anodizing, the coatings obtained are thinner and exhibit lower surface roughness.
Content in food itemsEdit
|Spinach||0.97 (ranges from .65 to 1.3 grams per 100 grams on fresh weight basis)|
|Swiss Chard, green||0.96 |
Oxalic acid in concentrated form can have harmful effects through contact and if ingested. It is not identified as mutagenic or carcinogenic, although there is a study suggesting it might cause breast cancer; 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.
Oxalate may enter cells where it is known to cause mitochondrial dysfunction.
The toxicity of oxalic acid is due to kidney failure caused by precipitation of solid calcium oxalate, the main component of calcium 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.
^a Unless otherwise cited, all measurements are based on raw vegetable weights with original moisture content.
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- See also Wikipedia's articles "Oxalis acetosella" and "Potassium hydrogen oxalate".
- François Pierre Savary, Dissertatio Inauguralis De Sale Essentiali Acetosellæ [Inaugural dissertation on the essential salt of wood sorrel] (Jean François Le Roux, 1773). (in Latin) Savary noticed that when he distilled sorrel salt (potassium hydrogen oxalate), crystals would sublimate onto the receiver. From p. 17: "Unum adhuc circa liquorem acidum, quem sal acetosellae tam sincerissimum a nobis paratum quam venale destillatione fundit phoenomenon erit notandum, nimirum quod aliquid ejus sub forma sicca crystallina lateribus excipuli accrescat, ..." (One more [thing] will be noted regarding the acid liquid, which furnished for us sorrel salt as pure as commercial distillations, [it] produces a phenomenon, that evidently something in dry, crystalline form grows on the sides of the receiver, ...) These were crystals of oxalic acid.
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- Torbern Bergman, Opuscula Physica et Chemica, (Leipzig (Lipsia), (Germany): I.G. Müller, 1776), volume 1, "VIII. De acido Sacchari," pp. 238-263.
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The scientists analyzed oxalate concentrations in 310 spinach varieties—300 USDA germplasm accessions and 10 commercial cultivars. “These spinach varieties and cultivars displayed oxalate concentrations from 647.2 to 1286.9 mg/100 g on a fresh weight basis,” says Mou.
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- EMEA Committee for veterinary medicinal products, oxalic acid summary report, December 2003
|Wikimedia Commons has media related to Oxalic acid.|
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- Alternative link: Table: Oxalic Acid Content of Selected Vegetables (USDA)
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- Oxalosis & Hyperoxaluria Foundation (OHF) The Oxalate Content of Food 2008 (PDF)
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