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. On a large scale, sodium oxalate is manufactured by absorbing carbon monoxide under pressure in hot sodium hydroxide.
Typically oxalic acid is obtained as the dihydrate. This solid can be dehydrated with heat or by azeotropic distillation. 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.
Oxalic acid is a relatively strong weak acid with pKa1=1.27 and pKa2=4.28. Oxalic acid exhibits many of the reactions characteristic of other carboxylic acids. It forms esters such as dimethyloxalate (m.p. 52.5–53.5 °C). It forms an acid chloride called oxalyl chloride.
Oxalate, the conjugate base of oxalic acid, is an excellent ligand for metal ions. It usually binds as a bidentate ligand forming a 5-membered MO2C2 ring. An illustrative complex is potassium ferrioxalate, K3[Fe(C2O4)3]. The drug Oxaliplatin exhibits improved water solubility relative to older platinum-based drugs, avoiding the dose-limiting side-effect of nephrotoxicity.
Occurrence in nature
Oxalic acid and oxalates are abundantly present in many plants, most notably fat hen (lamb’s quarters), sorrel, and Oxalis species. The root and/or leaves of rhubarb and buckwheat are listed being high in oxalic acid.
Other edible plants that contain significant concentrations of oxalic acid include—in decreasing order—star fruit (carambola), black pepper, parsley, poppy seed, 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) 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.
Chemical structure of Oxalic acid.
The affinity of divalent metal ions is sometimes reflected in their tendency to form insoluble precipitates. Thus in the body, oxalic acid also combines with metals ions such as Ca2+, Fe2+, and Mg2+ to deposit 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 be problematic. 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 calcium oxalate to precipitate out in the gut and drastically reduce the levels of oxalate absorbed by the body (by 97% in some cases.) The calcium oxalate precipitate (better known as kidney stones) obstruct the kidney tubules.
Oxalic acid can also be produced by the metabolism of ethylene glycol (“antifreeze”), glyoxylic acid or ascorbic acid (vitamin C). Under certain conditions of concentration and pH, oxalic acid can precipitate in the kidneys as calcium oxalate crystals, forming an estimated 80% of kidney stones.
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.)
Methods to reduce the oxalate content in food are of current interest.
- 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 and for the restoration of antiques.
- As a recommended surface pretreatment for stainless steels (surface etch) before application of solid metal or polymer self-lubricating coatings.
- For polishing stones and marble.
- Used in the acid treatment for destroying warts.
Tests for oxalic acid
Titration with potassium permanganate can reveal the presence of oxalic acid. Ascorbate interferes with this test which is based on reducing power. For this reason, a second test for strong reductants using, for example, iodine can be done.
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