Oxalate (systematic IUPAC name: ethanedioate) is an anion with the chemical formula . This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (), and several esters such as dimethyl oxalate (). It is a conjugate base of oxalic acid. At neutral pH in aqueous solution, oxalic acid converts completely to oxalate.
Relationship to oxalic acid
The dissociation of
from oxalic acid proceeds in a determined order; as for other polyprotic acids, loss of a single proton results in the monovalent
hydrogenoxalate anion . A salt with this
anion is sometimes called an
Acid salt, monobasic oxalate, or
hydrogen oxalate. The equilibrium constant (
Ka) for loss of the first proton is (
pKa = 1.27). The loss of the second proton, which yields the oxalate ion, has an equilibrium constant of (p
Ka = 4.28). These values imply, in solutions with neutral pH, no oxalic acid and only trace amounts of hydrogen oxalate exist.
The literature is often unclear on the distinction between , , and , and the collection of species is referred to as oxalic acid.
Structure
The oxalate anion exists in a nonplanar conformation where the O–C–C–O
Dihedral angle approach 90° with approximate D
2d symmetry.
When chelated to cations, oxalate adopts the planar, D
2h conformation.
However, in the structure of
caesium oxalate the O–C–C–O dihedral angle is 81(1)°.
[In the figure 81(1)°, the (1) indicates that 1° is the standard uncertainty of the measured angle of 81°] Therefore, is more closely approximated by a D
2d symmetry structure because the two planes are staggered. Two structural forms of
rubidium oxalate have been identified by single-crystal X-ray diffraction: one contains a planar and the other a staggered oxalate.
The barrier to rotation about this bond is calculated to be roughly 2–6 kcal/mol for the free dianion, .
Occurrence in nature
Oxalate occurs in many plants, where it is synthesized by the incomplete
oxidation of
. Calcium oxalate crystals known as
raphides are also used by plants as a defense mechanism.
Several plant foods such as the root and/or leaves of spinach, rhubarb, and buckwheat are high in oxalic acid and can contribute to the formation of kidney stones in some individuals. Other oxalate-rich plants include fat hen ("lamb's quarters"), sorrel, and several Oxalis species (also sometimes called sorrels). The root and/or leaves of rhubarb and buckwheat are high in oxalic acid.
+Common high-oxalate foods |
|
| Beetroot greens, cooked | cup | 916 |
| Purslane, leaves, cooked | cup | 910 |
| Rhubarb, stewed, no sugar | cup | 860 |
| Spinach, cooked | cup | 750 |
| Beet, cooked | cup | 675 |
| Chard, Swiss, leaves cooked | cup | 660 |
| Rhubarb, canned | cup | 600 |
| Spinach, frozen | cup | 600 |
| Beet, pickled | cup | 500 |
| Pokeweed greens, cooked | cup | 476 |
| Almond, raw | 1 oz | 296 |
| Endive, raw | 20 long leaves | 273 |
| Cocoa solids, dry | cup | 254 |
| Dandelion greens, cooked | cup | 246 |
| Okra, cooked | 8–9 pods | 146 |
| Sweet potato, cooked | cup | 141 |
| Kale, cooked | cup | 125 |
| , raw | cup ( Ounce) | 113 |
| Turnip greens, cooked | cup | 110 |
| Chocolate, unsweetened | 1 oz | 91 |
| Parsnips, diced, cooked | cup | 81 |
| Collard greens, cooked | cup | 74 |
| , halves, raw | cup ( oz) | 74 |
| Tea, leaves (4-minute infusion) | 1 level teaspoon in 7 fluid ounce water | 72 |
| Cereal germ, toasted | cup | 67 |
| Gooseberry | cup | 66 |
| Potato, Idaho white, baked | 1 medium | 64 |
| , cooked | cup | 45 |
| Apple, raw with skin | 1 medium | 41 |
| , cooked | 6–8 medium | 37 |
| Strawberry, raw | cup | 35 |
| Celery, raw | 2 stalks | 34 |
| Milk chocolate bar | 1 bar (1.02 oz)
| 34 |
| Raspberry, black, raw | cup | 33 |
| Orange, edible portion | 1 medium | 24 |
| , cooked | cup | 23 |
| , raw, chopped | 1 tablespoon | 19 |
| , raw | medium | 15 |
| Blackberry, raw | cup | 13 |
| cup | 13 |
| Blueberry, raw | cup | 11 |
| cup | 11 |
| , raw | 2 medium | 10 |
| Raspberry, red, raw | cup | 10 |
| Broccoli, cooked | 1 large stalk | 6 |
| Cranberry juice | cup (4 oz) | 6 |
Physiological effects
Excess consumption of oxalate-rich foods has been linked to
kidney stone formation of metal ions, such as
calcium oxalate, a
risk factor for kidney stones.
Some fungus of the genus Aspergillus produce oxalic acid.
As a ligand for metal ions
Oxalate also forms coordination compounds where it is sometimes abbreviated as
ox. It is commonly encountered as a
bidentate ligand. When the oxalate chelates to a single metal center, it always adopts the planar conformation. As a bidentate ligand, it forms a 5-membered ring. An illustrative complex is potassium ferrioxalate, . 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 rust-removal, which arises because oxalate forms water-soluble derivatives with the ferric ion.
Excess
An excess oxalate level in the blood is termed
hyperoxalemia, and high levels of oxalate in the urine is termed
hyperoxaluria.
Acquired
Although unusual, consumption of oxalates (for example, the grazing of animals on oxalate-containing plants such as
Bassia hyssopifolia, or human consumption of
wood sorrel or, specifically in excessive quantities,
tea) may result in
kidney disease or even
death due to oxalate poisoning.
The New England Journal of Medicine reported acute oxalate nephropathy "almost certainly due to excessive consumption of iced tea" in a 56-year-old man, who drank "sixteen 8-ounce glasses of iced tea daily" (roughly one gallon or ). The authors of the paper hypothesized that acute oxalate nephropathy is an underdiagnosed cause of kidney failure and suggested thorough examination of patient dietary history in cases of unexplained kidney failure without
proteinuria (an excess of protein in the urine) and with large amounts of calcium oxalate in urine sediment.
in the
gut flora may help alleviate this.
Congenital
Primary hyperoxaluria is a rare, inherited condition, resulting in increased excretion of oxalate, with oxalate stones being common.
Further reading
External links
-
Oxalate.org - Oxalate content of 750+ foods from university and government sources
-
Oxalatecontent.com - Oxalate content database based on the latest trustworthy studies
