oxalic acid - Preparation, Occurrence in nature, Safety, Tests for oxalic acid

IUPAC ethanedioic acid, HOOC–COOH, commonly occurs as the colourless crystalline dihydrate, melting point 101°C. It occurs in many plants, especially rhubarb, and is poisonous. It is a moderately strong acid, partially neutralized solutions having a pH of about 2·5. Its salts, oxalates, form chelates with transition metals, and are thus useful in removing rust and blood stains from clothing.

It is a relatively strong organic acid, being about 10,000 times stronger than acetic acid.

Preparation

Oxalic acid can be conveniently prepared in the laboratory by oxidizing sucrose using nitric acid as the oxidizer and a small amount of vanadium pentoxide as a catalyst.

Occurrence in nature

Oxalic acid and oxalates are abundantly present in many plants, most notably fat hen (lamb's quarters) and sorrel.

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.

Leaves of the tea plant (Camellia sinensis) are known to contain among the greatest measured concentrations of oxalic acid relative to other plants.

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.

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.) Oxalic acid can also be present in the body due to the consumption of ethylene glycol ("antifreeze"), which is metabolized into oxalic acid.

Safety

Oxalic acid irritates the lining of the gut when consumed, and can prove fatal in large doses.

Oxalic acid containing foods

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

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