It is a white solid used in removal of certain kinds of stains, in removing calcium ions from solutions, and in tanning leather. It occurs naturally and is toxic. The potassium and calcium salts of oxalic acid are found naturally in cabbage, spinach, and rhubarb leaves, and are also found in the bark of some species of eucalyptus trees. The metabolism of sugar by many species of mold results in the production of oxalic acid. Ingestion of large amounts can cause kidney damage, convulsions, and death.
The most common uses of oxalic acid are in tanning leather and removing rust and ink stains. In stain removal, it acts as a reducing agent (a substance that donates electrons to other substances) and is relied on by most dry cleaners for this purpose. Iron rust stains contain iron in its oxidized form (Fe III); the oxalic acid reduces it to its colorless reduced form (Fe II). Oxalic acid is also used to clean metals in many industries and is also used in the purification of glycerol (glycerin).
Few people ingest toxic amounts of oxalic acid directly. However, if a child or pet swallows antifreeze (which typically contains ethylene glycol and has a sweet taste), enzymes in the body will metabolize the ethylene glycol to oxalic acid, which is the reason antifreeze is toxic.
In many industrial processes oxalic acid is used to remove calcium ions from solutions. The reaction of calcium ions with oxalic acid produces an insoluble solid, calcium oxalate.