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Oxalic acid
Systematic name	ethanedioic acid
Chemical formula	C2H2O4 (anhydrous) C2H2O4.2H2O (dihydrate)
SMILES	OC(=O)C(O)=O
Molecular mass	90.03 g/mol (anhydrous) 126.07 g/mol (dihydrate)
Appearance	white crystals
CAS number	[144-62-7] (anhydrous) [6153-56-6] (dihydrate)
Properties
Density	1.90 g/cm3 (anhydrous) 1.653 g/cm3 (dihydrate)
Solubility in water	9.5 g/100 mL (15 °C) 14.3 g /100 mL (25 °C?) 120 g/100 mL (100 °C)
Solubility in Ethanol	23.7 /100 mL (15 °C) (dihydrate)
Solubility in Diethyl ether	1.37 g/100 ml (15 °C) (dihydrate)
Sublimation point	157 °C
Decomposition	189.5 °C
pKa 25°C	1.23 (pK1) 4.19 (pK2)
Crystal Structure	rhombic (anhydrous) monoclinic (dihydrate)
Hazards
LD50	7.5 g/kg
NFPA 704	1 3 0
Flash point	166 °C
Related compounds
Related compounds	oxalyl chloride disodium oxalate calcium oxalate phenyl oxalate ester
Disclaimer and references

Oxalic acid is the chemical compound with the formula H₂C₂O₄. This dicarboxylic acid is better described with the formula HOOCCOOH. It is a relatively strong organic acid, being about 10,000 times stronger than acetic acid. The anion is also a reducing agent. The dianion of oxalic acid is known as oxalate.

Contents 1 Preparation 2 Chemical reactions 2.1 Affinity for metal ions 3 Occurrence in nature 4 Safety 4.1 Oxalic acid-containing foods 5 Uses 6 Tests for oxalic acid 7 External links 8 References

Oxalic acid can be conveniently prepared in the laboratory by oxidizing sucrose using nitric acid as the oxidizer and a small amount of vanadium pentoxide as a catalyst.^[1] On a large scale, sodium oxalate is manufactured by absorbing carbon monoxide under pressure in hot sodium hydroxide.^[2]

Oxalate is an excellent ligand for metal ions, where it usually binds as a bidentate ligand forming a 5-membered MO₂C₂ ring. An illustrative complex is [Fe(C₂O₄)₃]^3-. The affinity of divalent metal ions is sometimes reflected in their tendency to form insoluble precipitates. Thus, oxalic acid also combines with metals such as calcium, iron, sodium, magnesium, and potassium in the body to form crystals of the corresponding oxalates, which irritate the gut and kidneys. Because it binds vital nutrients such as calcium, long-term consumption of foods high in oxalic acid can lead to nutrient deficiencies. Healthy individuals can safely consume such foods in moderation, but those with kidney disorders, gout, rheumatoid arthritis, or certain forms of chronic vulvar pain (vulvodynia) are typically advised to avoid foods high in oxalic acid or oxalates. Conversely, calcium supplements taken along with foods high in oxalic acid can cause oxalic acid to precipitate in the gut and drastically reduce the levels of oxalate absorbed by the body (by 97% in some cases.)^[3][4] The calcium oxalate precipitate (better known as kidney stones) obstruct the kidney tubules.

Oxalic acid and oxalates are abundantly present in many plants, most notably fat hen (lamb's quarters) and sorrel. The root and/or leaves of rhubarb and buckwheat are listed being high in oxalic acid.^{[citation needed]}

Foods that are edible, but that still contain significant concentrations of oxalic acid include - in decreasing order - star fruit (carambola), black pepper, parsley, poppy seed, rhubarb stalks, amaranth, spinach, chard, beets, cocoa, chocolate, most nuts, most berries, and beans. The gritty “mouth feel” one experiences when drinking milk with a rhubarb dessert is caused by precipitation of calcium oxalate. Thus even dilute amounts of oxalic acid can readily "crack" the casein found in various dairy products.

Leaves of the tea plant (Camellia sinensis) are known to contain among the greatest measured concentrations of oxalic acid relative to other plants. However the infusion beverage typically contains only low to moderate amounts of oxalic acid per serving, due to the small mass of leaves used for brewing.

Bodily oxalic acid may also be synthesized via the metabolism of either glyoxylic acid or unused ascorbic acid (vitamin C), which is a serious health consideration for long term "megadosers" of vitamin C supplements. 80% of kidney stones are formed from calcium oxalate.^[5]

Some Aspergillus species produce oxalic acid, which reacts with blood or tissue calcium to precipitate calcium oxalate.^[6] There is some preliminary evidence that the administration of probiotics can affect oxalic acid excretion rates^[7] (and presumably oxalic acid levels as well.) Oxalic acid can also be present in the body due to the consumption of ethylene glycol ("antifreeze"), which is metabolized into oxalic acid. They are also used in anti-cancer drugs in oxaliplatin, where the leaving group attached to the platinum is an oxalic acid.

Oxalic acid irritates the lining of the gut when consumed, and can prove fatal in large doses. The LD50 for pure oxalic acid is predicted to be about 378 mg/kg body weight, or about 22 g for a 60 kg human.

Research is being done on methods to safely reduce oxalate in food.^[8]

• In household chemical products such as Bar Keeper's Friend, some bleaches, and rustproofing treatments.
• In wood restorers where the acid dissolves away a layer of dry surface wood to expose fresh material underneath.
• As an additive to automotive wheel cleaners.
• As a mordant in dyeing processes.
• Vaporized oxalic acid, or a 6% solution of oxalic acid in sugar syrup, is used by some beekeepers as an insecticide against the parasitic Varroa mite.
• As a rust remover in such applications as automotive shops, antiques restoration, beer can collecting.

Titration with potassium permanganate can reveal the presence of oxalic acid (as the acid is only a weak reductant, and needs an oxidant as strong as permanganate in order to react). However, this test will confuse ascorbate and oxalic acid, as will most tests based on reducing power: the solution is to run a second test for strong reductants using, for example, iodine.

• International Chemical Safety Card 0529
• Oxalic acid description, properties, manufacture, and applications (ChemicalLand21.com)
• Oxalic acid MSDS (Mallinckrodt Baker, Inc.)
• Table: Oxalic acid content of selected vegetables (USDA)
• Alternative link: Table: Oxalic Acid Content of Selected Vegetables (USDA)
• Query engine: Dr. Duke's Phytochemical and Ethnobotanical Databases (USDA ARS)
• About rhubarb poisoning (The Rhubarb Compendium)
• Article: 'Go Light on Oxalic Acid' (Dr. Mary Schrick's June 2004 newsletter)
• Oxalic acid and kidney disease
• Medical Terminology Dictionary
• Low-Oxalate Diet (PDF)
• Links to external chemical sources

1. ^ Practical Organic Chemistry by Julius B. Cohen, 1930 ed. preparation #42
2. ^ U.S. Patent 1602802 
3. ^ Gastrointestinal oxalic acid absorption in calcium-treated rats by Morozumi M, Hossain RZ, Yamakawa KI, Hokama S, Nishijima S, Oshiro Y, Uchida A, Sugaya K, Ogawa Y. on NIH website
4. ^ Milk and calcium prevent gastrointestinal absorption and urinary excretion of oxalate in rats by Hossain RZ, Ogawa Y, Morozumi M, Hokama S, Sugaya K. on NIH website
5. ^ Kidney stone disease by Coe FL, Evan A, Worcester E, on NIH website
6. ^ Aspects of oxalosis associated with aspergillosis in pathology specimens by Pabuccuoglu U. on NIH website
7. ^ Use of a probiotic to decrease enteric hyperoxaluria by Lieske JC, Goldfarb DS, De Simone C, Regnier C. on NIH website
8. ^ Biodegradation of oxalic acid from spinach using cereal radicles by Betsche T, Fretzdorff B. on NIH website