Oxalic acid, also known as ethanedioic acid or clover acid, is the simplest form of dicarboxylic acid. With the formula C2H2O4, it is a crystalline solid that is colorless and odorless. The physical state of oxalic acid is solid.
What is oxalic acid?
As a reducing agent, oxalic acid is determined by titration with oxidizing agents such as potassium permanganate. During this process, carbon dioxide is formed as the oxidation product. The salts of oxalic acid are known by the systematic name ethandioates, oxalates. Oxalic acid was discovered as early as 1769, when Johann Christian Wiegleb discovered this acid in wood sorrel as a potassium salt. Derived from wood sorrel, the historical name clover acid therefore remained in linguistic usage. In 1776, Carl Wilhelm Scheele and Torbern Olof Bergman succeeded in producing oxalic acid by oxidizing sugar together with nitric acid. However, this process is not a synthesis, but merely the degradation of a natural substance. The artificial production of oxalic acid succeeded for the first time in 1824. Friedrich Wöhler produced oxalic acid artificially from inorganic base materials by a synthesis through saponification of dicyan.
Function, action and tasks
Oxalic acid is an ideal agent for removing rust stains and is also used as a bleaching agent. Beekeepers use oxalic acid, especially in winter, to control the varroa mite. The 3.5 percent sugar solution, which is very watery, is sprayed or drizzled on bees. However, beekeepers who treat varroasis with oxalic acid face dangers from direct contact with the acid. Oxalic acid is also used in the laboratory as a primary titrant for manganometry, since the dihydrate of oxalic acid can be used for this purpose. Furthermore, oxalic acid is suitable as a uranium titer substance for the accurate determination of the content of alkaline measuring solutions such as sodium hydroxide solution. Due to the formation of a sparingly soluble calcium salt, oxalic acid is useful for the gravimetric determination of a calcium ion as calcium oxalate. Furthermore, oxalic acid, which is extracted from the wood sorrel of the Fichtelgebirge, is used for the effective bleaching of quartz, i.e. rock crystal, the occurrence of which is proven in particular under the town of Weißenstadt. Oxalic acid helps polish marble to a shine and is used in the woodworking industry as a mild bleaching agent. It removes reactively formed stains as a reaction of tannic acids or by contact with wood containing tannic acid by iron tools. Pharmaceutical chemistry benefits from the action of oxalic acid through its property as a salt former. By means of protonation of basic amines, so-called oxalates are formed. Oxalic acid is also found in minute quantities in black tea, peppermint tea, and in the roots and barks of many plants as insoluble calcium oxalates. In small quantities, oxalic acid is not harmful to health, but in higher concentrations it is very harmful. Calcium oxalates are often formed in nature by the death of plant cells. Kidney stones also consist of this calcium oxalate as well as uric acid, but the formation of kidney stones is prevented by eating fruits that contain citric acid.
Formation, occurrence, properties and optimal values
Oxalic acid as well as potassium salt are found in larger amounts in knotweed plants such as rhubarb, which can contain about 180 to 765 milligrams per 100 grams of fresh weight and stems. In sorrel, most of the oxalic acid is found in the leaves. Starfruit contains between 40 to 1000 milligrams per 100 grams of fresh weight, similar to wood sorrel. Swiss chard contains between 110 to 940 milligrams per 100 grams of fresh weight. Spinach contains 120 to 1330 milligrams, whereas parsley has only up to 185 milligrams on 100 grams fresh weight. Oxalic acid and its potassium salt can also be found in cocoa at 338 to 480 milligrams per 100 grams, and in chocolate at 80 to 200 milligrams. 100 grams of fresh beets contain between 17 to 329 milligrams. Many fungi excrete oxalic acid, which is promoted by the production of alkaline reaction of the nutrient solution. Oxalic acid is obtained by rapidly heating sodium formate to 360 degrees Celsius. Approximately 140,000 tons of oxalic acid along with esters are produced annually worldwide. The strong acid decomposes at temperatures above 150 degrees Celsius to form carbon dioxide, carbon monoxide and water.
Diseases and disorders
Oxalic acid impairs the absorption of iron in the intestine, so one should reduce consumption of foods containing oxalic acid. There may be a reduction in calcium in the affected tissues, which in the worst case could damage the heart. In general, even minor poisoning due to clogged renal tubules leads to kidney damage. Regular direct contact with oxalic acid can lead to health hazards, such as for beekeepers who use oxalic acid to control varroasis. Although oxalic acid has a bad reputation in connection with food, it is assumed that the acid has positive properties in the defense against diseases such as aflatoxins and Aspergillus. It has probably been proven that too high a level of oxalate has a negative effect on the human organism, as it reduces substances important for the body such as magnesium, zinc, potassium and calcium. To a small extent, there should be no harmful effects from eating oxalic acid in foods, as various types of vegetables such as chard, rhubarb or sorrel are healthy. In moderation, foods high in oxalates and oxalic acid do indeed have healthy effects, such as spinach, or even moderate consumption of tea does have its positive influences.
