Oxalate

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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. 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.

Contents
[hide]
• 1 Relationship to oxalic acid
• 2 Occurrence in nature
○ 2.1 Physiological effects
• 3 As a ligand
• 4 Safety
 • 5 See also
○ 5.1 Raphides
○ 5.2 Oxalate salts
○ 5.3 Oxalate complexes
○ 5.4 Oxalate esters
• 6 References

[edit] Relationship to oxalic acid

The dissociation 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 is -log(Ka) = 1.27. The second
ionization, which yields the oxalate anion, has -log(Ka) = 4.28. These values imply that, in
solutions with neutral pH, there is no oxalic acid, and only trace amounts of hydrogen oxalate.[1]
The literature is often unclear on the distinction between H2C2O4, HC2O4-, and C2O42-, and the
collection of species is referred to oxalic acid.
[edit] Occurrence in nature
Oxalate occurs in many plants, where it is synthesized via the incomplete oxidation of
carbohydrates.
Oxalate-rich plants include fat hen ("lamb's quarters"), sorrel, and several Oxalis species. The
root and/or leaves of rhubarb and buckwheat are high in oxalic acid.[2] 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
(Tetragoniatetragonioides) and beans.[citation needed] 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.
[show]Common high-oxalate foods[3]
Oxalate
Serving
Food Item Content
(oz.)
(mg)
Beet greens, cooked 1/2 cup 916
Purslane, leaves, cooked 1/2 cup 910
Rhubarb, stewed, no sugar 1/2 cup 860
Spinach, cooked 1/2 cup 750
Beets, cooked 1/2 cup 675
Chard, Swiss, leaves cooked 1/2 cup 660
Rhubarb, canned 1/2 cup 600
Spinach, frozen 1/2 cup 600
Beets, pickled 1/2 cup 500
Poke Greens, cooked 1/2 cup 476
Endive, raw 20 long leaves 273
Cocoa, dry 1/3 cup 254
Dandelion greens, cooked 1/2 cup 246
Okra, cooked 8 - 9 pods 146
Potatoes, sweet, cooked 1/2 cup 141
Kale, cooked 1/2 cup 125
Peanuts, raw 1/3 cup (1-3/4 oz) 113
Turnip greens, cooked 1/2 cup 110
Chocolate, unsweetened 1 oz. 91
Parsnips, diced, cooked 1/2 cup 81
Collard greens, cooked 1/2 cup 74
Pecans, halves, raw 1/3 cup (1-1/4 oz) 74
Tea, leaves (4 min. infusion) 1 level tsp in 7 oz water 72
Cereal germ, toasted 1/4 cup 67
Gooseberries 1/2 cup 66
Potato, Idaho white, baked 1 medium 64
Carrots, cooked 1/2 cup 45
Apple, raw with skin 1 medium 41
Brussel sprouts, cooked 6 - 8 medium 37
Strawberries, raw 1/2 cup 35
Celery, raw 2 stalks 34
Milk chocolate bar 1 bar (1.02 oz) 34
Raspberries, black, raw 1/2 cup 33
Orange, edible portion 1 medium 24
Green beans, cooked 1/2 cup 23
Chives, raw, chopped 1 tablespoon 19
Leeks, raw 1/2 medium 15
Blackberries, raw 1/2 cup 13
Concord grapes 1/2 cup 13
Blueberries, raw 1/2 cup 11
Currants, red 1/2 cup 11
Apricots, raw 2 medium 10
Raspberries, red, raw 1/2 cup 10
Broccoli, cooked 1 large stalk 6
Cranberry juice 1/2 cup (4 oz) 6
The "gritty mouth" feeling one experiences when drinking milk with a rhubarb dessert is caused
by precipitation of calcium, abstracted from the casein in dairy products, as calcium oxalate.[citation
needed]

[edit] Physiological effects

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 than can obstruct the kidney
tubules. An estimated 80% of kidney stones are formed from calcium oxalate.[4] 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.[5]
Magnesium (Mg2+) oxalate is 567 times more soluble than calcium oxalate, so the latter is more
likely to precipitate out when magnesium levels are low and calcium and oxalate levels are high.
Magnesium oxalate is a million times more soluble than mercury oxalate. Oxalate solubility for
other metals decreases in the order Ca> Cd > Zn > {Mn,Ni,Fe,Cu} > {As,Sb,Pb} > Hg. The
highly insoluble iron(II) oxalate appears to play a major role in gout, in the nucleation and
growth of the otherwise extremely soluble sodium urate. This explains why gout usually appears
after age 40, when ferritin levels in blood exceed 100 ng/dl. Beer is rich in oxalate and iron, and
ethanol increases iron absorption and magnesium elimination, so beer intake greatly increases
the risk of a gout attack.
Cadmiumcatalyzes the transformation of vitamin C into oxalic acid. This can be a problem for
people exposed to high levels of cadmium in the diet, in the workplace, or through smoking.
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.)[6][7]
Oxalic acid can also be produced by the metabolism of ethylene glycol ("antifreeze"), glyoxylic
acid, or ascorbic acid (vitamin C).[8][dubious – discuss]
Powdered oxalate is used as a pesticide in beekeping to combat the bee mite.
Some fungi of the genusAspergillus produce oxalic acid, which reacts with calcium from the
blood or tissue to precipitate calcium oxalate.[9]
There is some preliminary evidence that the administration of probiotics can affect oxalic acid
excretion rates[10] (and presumably oxalic acid levels as well.)
[edit] 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.
[edit] 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 Oxalobacterformigenes in the
gut flora can prevent this. Cadmium catalyzes the transformation of vitamin C into oxalic acid
and can result from smoking heavily, ingesting produce tainted with Cd or from industrial
exposure to Cd.
[edit] See also
[edit] Raphides
 • raphides
[edit] Oxalate salts
• sodium oxalate - Na2C2O4
• calcium oxalate - CaC2O4, a major component of kidney stones
[edit] Oxalate complexes
• potassium ferrioxalate - K3[Fe(C2O4)3], an iron complex with oxalate ligands
[edit] Oxalate esters
• diphenyl oxalate - (C6H5)2C2O4
• dimethyl oxalate - (CH3)2C2O4
[edit] 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. ^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.
6. ^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 Res34: 168. doi:10.1007/s00240-006-0035-7. PMID 16444511.
7. ^Hossain RZ, Ogawa Y, Morozumi M, Hokama S, Sugaya K (2003). "Milk and calcium prevent
gastrointestinal absorption and urinary excretion of oxalate in rats". Front Biosci.8: a117–25.
doi:10.2741/1083. PMID 12700095.
8. ^Mandl J, Szarka A, Bánhegyi G (2009). "Vitamin C: update on physiology and pharmacology".
British Journal of Pharmacology157: 1097-1110. PMID 19508394.
9. ^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.
10.^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.
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Retrieved from "http://en.wikipedia.org/wiki/Oxalate"
Categories: Oxalates | Carboxylate anions
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