|Molar mass||90.03 g·mol−1
126.07 g mol−1 (dihydrate)
|Density||1.90 g cm−3 (anhydrous)
1.653 g cm−3 (dihydrate)
|Melting point||102 °C (216 °F; 375 K) 101.5 °C (214.7 °F) dihydrate|
|143 g/L (25 °C)|
|Solubility||237 g/L (15 °C) in ethanol
14 g/L (15 °C) in diethyl ether 
|Acidity (pKa)||1.25, 4.14|
|Flash point||166 °C (331 °F; 439 K)|
phenyl oxalate ester
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
Oxalic acid is an organic compound with the 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  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.
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
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− → 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)). 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 → C2O42− + CH3CO2−
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 umbellifers like parsley, and sorrel. 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.
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.
The conjugate base of oxalic acid (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.
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.
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 oxides, which is the most commonly marketed form of these elements.
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.
|Vegetable||Oxalic acid (g/100 g)|
This table references other source material
|Vegetable||Oxalic acid (g/100 g)|
Toxicity and safety
Oxalic acid has toxic 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.
The toxicity of oxalic acid is due to kidney failure caused by precipitation of solid calcium oxalate, 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.
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|Wikimedia Commons has media related to Oxalic acid.|
- Oxalic acid MS Spectrum
- International Chemical Safety Card 0529
- NIOSH Guide to Chemical Hazards (CDC)
- "Oxalic acid". ChemicalLand21.com.[dead link]
- 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