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

Oxalic acid and oxalates are mild toxins found in many plants. Oxalic acid is a strong acid that irritates the lining of the gut when consumed, and can prove fatal in large doses. 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. Oxalic acid also combines with metals such as calcium in the body to form oxalates which further irritate the gut and kidneys. The most common kind of kidney stone is made of calcium oxalate.

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, or rheumatoid arthritis are typically advised to avoid foods high in oxalic acid or oxalates.

Foods that contain significant quantities of oxalic acid include cocoa, chocolate, most nuts, most berries, rhubarb, beans, and beets, among many others.

In addition to its natural occurrence in plants, oxalic acid may also be found in household chemical products such as Bar Keeper's Friend, some bleaches, and rustproofing treatments. It is also used in wood restorers where the acid dissolves away a layer of dry surface wood to expose fresh material underneath.

Other uses

Vaporized oxalic acid is used by some beekeepers as an insecticide against the parasitic Varroa mite.