Oxalate
Oxalate (IUPAC: ethanedioate), sometimes abbreviated as ox when a ligand, 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.
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” feeling 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.
| Common high-oxalate foods[3] | ||
|---|---|---|
| Food Item | Serving (oz.) |
Oxalate Content (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 |
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 metallic ions such as Ca2+ and Fe2+ to form crystals of the corresponding oxalates which are then excreted in urine. 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] Mg oxalate is 567 times more soluble than Ca oxalate, so that Ca oxalate crystals are more likely to precipitate out when Mg levels are low and Ca and oxalate levels are high. Mg oxalate is a million times more soluble than Hg oxalate. Oxalate solubility: Mg>>Ca>Cd>Zn>Mn, Ni, Fe, Cu>As, Sb, Pb> Hg. The highly insoluble Fe oxalate appears to play a major role in the nucleation and growth of the otherwise extremely soluble Na urate, to form the Na urate crystals in gout, which is how gout usually appears after 40 years of age, when ferritin levels (which reflect Fe reserves) exceed 100 ng/dl. Beer is rich in oxalate and Fe and ethanol increases Fe absorption and Mg elimination (both Mg urate and oxalate are very soluble and prevent the precipitation of Ca oxalate and Na urate crystals), so beer intake greatly increases the risk of a gout attack. Cadmium catalyzed the transformation of vitamin C into oxalic acid and high Cd levels are often high in smokers, people who eat produce tainted with Cd or who are exposed industrially to Cd.
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).[dubious – discuss] Powdered oxalate is used to eliminate the bee mite that can destroy complete hives. 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. 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.
See also
Raphides
Oxalate salts
- sodium oxalate - Na2C2O4
- calcium oxalate - CaC2O4, a major component of kidney stones
Oxalate complexes
- potassium ferrioxalate - K3[Fe(C2O4)3], an iron complex with oxalate ligands
Oxalate esters
- diphenyl oxalate - (C6H5)2C2O4
- dimethyl oxalate - (CH3)2C2O4
References
- ^ Wilhelm Riemenschneider, Minoru Tanifuji "Oxalic Acid" in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a18_247.
- ^ Streitweiser, Andrew Jr.; Heathcock, Clayton H.: Introduction to Organic Chemistry, Macmillan 1976, p 737
- ^ Resnick, Martin I. (1990). Urolithiasis, A Medical and Surgical Reference. W.B. Saunders Company. p. 158. ISBN 0721624391.
{{cite book}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Coe FL, Evan A, Worcester E. (2005). "Kidney stone disease". J Clin Invest. 115 (10): 2598–608. doi:10.1172/JCI26662. PMC 1236703. PMID 16200192.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ 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.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ 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.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ 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.
- ^ 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.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ 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.
{{cite journal}}: CS1 maint: multiple names: authors list (link)