Oxalic acid

Oxalic acid
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	1.77 g/cm3
Solubility in water	14.3 g / 100 ml
Sublimation point	157 °C
Decomposition	189.5 °C
pKa	1.27 4.27
Hazards
LD50	7.5 g/kg
NFPA 704	NFPA 704 safety square 3 1
Flash point	166 °C
Related compounds
Related compounds	oxalyl chloride disodium oxalate calcium oxalate phenyl oxalate ester
Disclaimer and references

Oxalic acid (IUPAC name: ethanedioic acid, formula H₂C₂O₄) is a dicarboxylic acid with structure (HOOC)-(COOH). Because of the joining of two carboxyl groups, this is one of the strongest organic acids. It is also a reducing agent. The anions of oxalic acid as well as its salts and esters are known as oxalates.

Biological hazards

Main article: oxalate poisoning

Oxalic acid and oxalates are mild nephrotoxic acids that are abundantly present in many plants, most notably fat hen (lamb's quarters), rhubarb and sorrel. 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. Oxalic acid can also be present in the body due to the consumption of another toxin, ethylene glycol (generally known as automobile antifreeze), because over time, the body metabolizes ethylene glycol into oxalic acid. Estimated fatal dose is 5 to 15 grams.

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. Some Aspergillus species produce oxalic acid, which reacts with blood or tissue calcium to precipitate calcium oxalate. There is some preliminary evidence that the administration of probiotics can affect oxalic acid excretion rates (and presumably oxalic acid levels as well.)

Action of poisoning

Oxalic acid also combines with metals such as calcium, iron, sodium, magnesium, and potassium in the body to form oxalate crystals which precipitate and irritate the gut and kidneys. The calcium oxalate preciptate (better known as kidney stones) obstruct the kidney tubules. 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.)

Oxalic acid containing foods

The root and leaves of rhubarb contain dangerously high concentrations of oxalic acid.

Foods that are edible, but still contain significant concentrations of oxalic acid include - in decreasing order - buckwheat, star fruit (carambola), black pepper, parsley, poppy seed, rhubarb stalks, amaranth, spinach, chard, beets, cocoa, chocolate, most nuts, most berries, and beans. The gritty feel one gets in the mouth when drinking milk with rhubarb desserts 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.

Research is being done on methods to safely reduce oxalate in food.

Uses

• 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 a mordant in dyeing processes.
• Vaporized oxalic acid is used by some beekeepers as an insecticide against the parasitic Varroa mite.

Tests for oxalic acid

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 test based on reducing power: the solution is to run a second test for strong reductants using, for example, iodine.

External links

• 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)
• 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)
• Medical Terminology Dictionary
• Low-Oxalate Diet (PDF)