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Oxalates - Oxalic acid

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Oxalate (see first figure) with the formula C2O42−, also written (COO)22−. Either name is often used for derivatives, such as salts of oxalic acid (right mist figure), for example sodium oxalate Na2C2O4, or dimethyl oxalate ((CH3)2C2O4). Oxalate also forms coordination compounds where it is sometimes abbreviated as ox.  Many metal ions form insoluble precipitates with oxalate, a prominent example being calcium oxalate, the primary constituent of the most common kind of kidney stones.  In the body, oxalic acid combines with divalent metallic cations such as calcium (Ca2+) and iron(II) (Fe2+) to form crystals of the corresponding oxalates which are then excreted in urine as minute crystals. These oxalates can form larger kidney stones that can obstruct the kidney tubules. An estimated 80% of kidney stones are formed from calcium oxalate. Those with kidney disorders, gout, rheumatoid arthritis, or certain forms of chronic vulvar pain (vulvodynia) are typically advised to avoid foods high in oxalic acid. Methods to reduce the oxalate content in food are of current interest.

Oxalic acid is an organic compound with the formula H2C2O4. It is a colorless crystalline solid that forms a colorless solution in water. Its condensed formula is HOOCCOOH, reflecting its classification as the simplest dicarboxylic acid. Its acid strength is much greater than that of acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate (C2O42−), is a chelating agent for metal cations. 

The preparation of salts of oxalic acid from plants had been known at least since 1745, when the Dutch botanist and physician Herman Boerhaave isolated a salt from sorrel.[6] By 1773, François Pierre Savary of Fribourg, Switzerland had isolated oxalic acid from its salt in sorrel.

In 1776, Swedish chemists Carl Wilhelm Scheele and Torbern Olof Bergman[8] produced oxalic acid by reacting sugar with concentrated nitric acid; Scheele called the acid that resulted socker-syra or såcker-syra (sugar acid). By 1784, Scheele had shown that "sugar acid" and oxalic acid from natural sources were identical.

In 1824, the German chemist Friedrich Wöhler obtained oxalic acid by reacting cyanogen with ammonia in aqueous solution. This experiment may represent the first synthesis of a natural product.


  • When Healthy Isn’t: The Risks of High-Oxalate Foods by Sally K. Norton, MPH, (download as pdf from here).  This article was originally printed in the July/August 2015 issue of Well Being Journal and is reprinted here with permission of the publisher.  An overview article on Oxalates in food.




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