Oxalates: Wikis

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The structure of the oxalate anion
A ball-and stick model of oxalate

Oxalate (IUPAC: ethanedioate) is the dianion with formula C2O42− also written (COO)22−. Either name is often used for derivatives, such as disodium oxalate, (Na+)2C2O42−, or an ester of oxalic acid (such as dimethyl oxalate, (CH3)2C2O4). 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.

Contents

Relationship to oxalic acid

The ionization of protons from oxalic acid proceeds in a stepwise manner as for other polyprotic acids. Loss of a single proton results in the monovalent hydrogenoxalate anion HC2O4. A salt with this anion is sometimes called an acid oxalate, monobasic oxalate, or hydrogen oxalate. The equilibrium constant for this ionization {log(Ka) = 1.27) indicates that oxalic acid does not exist near neutral pH. The second ionization occurs more reluctantly (-log(Ka) = 4.28) also indicates that only trace amounts of HC2O4- exist in neutral solutions.[1] The literature is often unclear on the distinction between H2C2O4-, HC2O4-, and C2O42-, and the collection of species is referred to oxalic acid.

Occurrence in nature

Oxalate occurs widely in the plant kingdom, e.g. fat hen (lamb's quarters), sorrel, and Oxalis species. The root and/or leaves of rhubarb and buckwheat are listed as being high in oxalic acid.[2] It arises biosynthetically via the incomplete oxidation of carbohydrates.

Other edible plants that contain significant concentrations of oxalate include—in decreasing order—star fruit (carambola), black pepper, parsley, poppy seed, amaranth, spinach, chard, beets, cocoa, chocolate, most nuts, most berries, fishtail palms, New Zealand spinach (Tetragonia tetragonioides) and beans.[citation needed] The gritty “mouth feel” one experiences when drinking milk with a rhubarb dessert is caused by precipitation of calcium oxalate.[citation needed] The calcium is abstracted from the casein in dairy products.

Leaves of the tea plant (Camellia sinensis) contain among the greatest measured concentrations of oxalic acid relative to other plants. However the infusion beverage typically contains only low to moderate amounts of oxalic acid per serving, due to the small mass of leaves used for brewing.

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Physiological effects

The affinity of divalent metal ions is sometimes reflected in their tendency to form insoluble precipitates. Thus in the body, oxalic acid also combines with metals ions such as Ca2+, Fe2+, and Mg2+ to form crystals of the corresponding oxalates which are then excreted in Urea . 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. The calcium oxalate crystals or precipitate (better known as kidney stones) can obstruct the kidney tubules. An estimated 80% of kidney stones are formed from calcium oxalate.[4]

In studies with rats, calcium supplements given along with foods high in oxalic acid can cause calcium oxalate to precipitate out in the gut and reduce the levels of oxalate absorbed by the body (by 97% in some cases.)[5][6]

Oxalic acid can also be produced by the metabolism of ethylene glycol ("antifreeze"), glyoxylic acid or ascorbic acid (vitamin C).

Some Aspergillus species produce oxalic acid, which reacts with blood or tissue calcium to precipitate calcium oxalate.[7] There is some preliminary evidence that the administration of probiotics can affect oxalic acid excretion rates[8] (and presumably oxalic acid levels as well.)

Methods to reduce the oxalate content in food are of current interest.[9]

As a ligand

Oxalate, the conjugate base of oxalic acid, is an excellent ligand for metal ions. It usually binds as a bidentate ligand forming a 5-membered MO2C2 ring. An illustrative complex is potassium ferrioxalate, K3[Fe(C2O4)3]. The drug Oxaliplatin exhibits improved water solubility relative to older platinum-based drugs, avoiding the dose-limiting side-effect of nephrotoxicity. Oxalic acid and oxalates can be oxidized by permanganate in an autocatalytic reaction. One of the main applications of oxalic acid is a rust-removal, which arises because oxalate forms water soluble derivatives with the ferric ion.

Safety

Although unusual, consumption of oxalates (for example, the grazing of animals on oxalate-containing plants such as greasewood or human consumption of Sorrel) may result in kidney disease or even death due to oxalate poisoning. The presence of Oxalobacter formigenes in the gut flora can prevent this.

See also

Oxalate salts

Oxalate complexes

Oxalate esters

References

  1. ^ Wilhelm Riemenschneider, Minoru Tanifuji "Oxalic Acid" in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. doi: 10.1002/14356007.a18_247.
  2. ^ Streitweiser, Andrew Jr.; Heathcock, Clayton H.: Introduction to Organic Chemistry, Macmillan 1976, p 737
  3. ^ Resnick, Martin I.; Pak, Charles Y. C. (1990). Urolithiasis, A Medical and Surgical Reference. W.B. Saunders Company. pp. 158. ISBN 0721624391. 
  4. ^ Coe FL, Evan A, Worcester E. (2005). "Kidney stone disease". J Clin Invest. 115 (10): 2598–608. doi:10.1172/JCI26662. PMID 16200192. 
  5. ^ Morozumi M, Hossain RZ, Yamakawa KI, Hokama S, Nishijima S, Oshiro Y, Uchida A, Sugaya K, Ogawa Y (2006). "Gastrointestinal oxalic acid absorption in calcium-treated rats". Urol Res 34: 168. doi:10.1007/s00240-006-0035-7. PMID 16444511. 
  6. ^ Hossain RZ, Ogawa Y, Morozumi M, Hokama S, Sugaya K. "Milk and calcium prevent gastrointestinal absorption and urinary excretion of oxalate in rats". Front Biosci.. PMID 12700095. 
  7. ^ Pabuccuoglu U. (2005). "Aspects of oxalosis associated with aspergillosis in pathology specimens". Pathol Res Pract. 201 (5): 363–8. doi:10.1016/j.prp.2005.03.005. PMID 16047945. 
  8. ^ Lieske JC, Goldfarb DS, De Simone C, Regnier C. (2005). "Use of a probiotic to decrease enteric hyperoxaluria". Kidney Int. 68 (3): 1244–9. doi:10.1111/j.1523-1755.2005.00520.x. PMID 16105057. 
  9. ^ Betsche, T.; Fretzdorff, B. (2005). "Biodegradation of oxalic acid from spinach using cereal radicles". J Agric Food Chem. 53 (25): 9751–8. doi:10.1021/jf051091s. PMID 16332126. 

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