Oxalic acid
Oxalic acid is the chemical compound with the formula H2C2O4. This dicarboxylic acid is better described with the formula HO2CCO2H.  It is a relatively strong organic acid, being about 10,000 times stronger than acetic acid. The di-anion, known as oxalate, is also a reducing agent as well as a ligand in coordination chemistry.  Many metal ions form insoluble precipitates with oxalate, a prominent example being calcium oxalate, which is the primary constituent of the most common kind of kidney stone.

# Preparation
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.

# Reactions
Oxalic acid exhibits many of the reactions characteristic of other carboxylic acids.  It forms esters such as dimethyloxalate (m.p. 52.553.5 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. 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), sour grass, and sorrel (including oxalis).  The root and/or leaves of rhubarb and buckwheat are listed being high in oxalic acid.
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) 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.

## Physiological effects
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 is also biosynthesized via the metabolism of ethylene glycol ("antifreeze"), glyoxylic acid, and ascorbic acid (vitamin C).  The latter pathway presents a potential health risk for long term "megadosers" of vitamin C supplements.  An estimated 80% of kidney stones are 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.)
Methods to reduce the oxalate content in food are of current interest.

# Other 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 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.

# 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.