As oxalates systematically Ethandioate, the salts and esters of oxalic acid, are referred to.

  • 3.1 Occurrence in Plants
  • 3.2 Occurrence in minerals


Many salt-like oxalates are sparingly soluble in water. The best in water are soluble ammonium oxalate ( (NH4 ) 2 ( COO ) 2) and Alkalioxalate as sodium oxalate. The precipitation of the sparingly soluble calcium serves as a common detection of calcium ions. As oxalate is also the anion of oxalic acid is called.

Since oxalic acid, as well as their salts, calcium (needed for bone ) forms a sparingly soluble salt, it can be excreted only slowly. Therefore, oxalic acid and its salts should be consumed only in small doses. Oxalic acid is also present in chocolate, nuts, cocoa powder, spinach leaves and rhubarb.


Detection methods for oxalates

The oxalate solution is buffered with acetic acid / acetate buffer, the pH range is between 4 and 6, then added calcium chloride solution, as a result, a white precipitate of calcium oxalate ( in rhombic crystal form ). As a white precipitate is formed by other ions, the precipitate is filtered off and dissolve it in dilute sulfuric acid, dripping a drop of potassium permanganate solution and heated to the test. By heating the colored by potassium permanganate solution must discolor.

Quantitative determination

In aqueous solution, the concentration of oxalate ions can be determined by titration with KMnO4 solution, but you have the oxalate solution previously heated to 70 ° C. In the titration reaction is as follows:


The esters of oxalic acid, oxalic acid esters are also known, has the general formula R 1 O -CO -CO- OR 2, wherein R are alkyl or aryl radicals. Acid ester with an organic group are not only stable but can exist as stable salts. The neutral reaction, double esters are used as solvents. Important esters are dimethyl oxalate and diethyl oxalate.


Occurrence in plants

As a product of the incomplete carbohydrate breakdown oxalates are found in almost all plants.

Known plants with a very high content of oxalates are lambsquarters and Sorrel. The roots and leaves of rhubarb and buckwheat contain very high concentrations of oxalates.

Other edible plants with significant amounts of oxalate are the star fruit, black pepper, parsley, poppy seed, amaranth, spinach, chard, beets, blueberries, and most nuts. Cocoa also contains considerable amounts of oxalates. The leaves of the tea bush (Camellia sinensis) take the oxalate even a top position, although it must be borne in mind that a tea made from these leaves has ultimately only comparatively low oxalate concentrations, partly because of the small amount of tea leaves for the preparation are needed, on the other hand, because many oxalates are only moderately soluble in water.

Occurrence in minerals

As the salts of an organic acid oxalates come in only a few, rare minerals. The classification of minerals according to Strunz gives an overview. Even if it is, for the occurring as natural minerals oxalates (eg Whewellite ) salts of an organic acid is concerned, shall, at their formation not necessarily biological processes have been involved - the possibility of formation of organic substances ( to amino acids) to purely abiotic way has now been confirmed by numerous experiments.

Physiological Effects

In the body of higher organisms, the oxalate anions form with divalent metal ions such as calcium (Ca2 ) and divalent iron ( Fe2 ) in the renal excretion of small crystals. By further aggregation can form larger kidney stones logically. About 80 % of all kidney stones are composed of calcium oxalate. In addition to other kidney diseases oxalates also for gout, rheumatoid arthritis and vulvodynia may be the cause.

Cadmium catalyzes the conversion of Vitamin C in the oxalic acid. This may in humans that are exposed to high loads of cadmium, for example, Smokers who cause problems.