Oxalate

Oxalate 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 protons 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 oxalate, monobasic oxalate, or hydrogen oxalate. The equilibrium constant for loss of the first proton is . The loss of the second proton, which yields the oxalate ion, has an equilibrium constant of . 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 dihedrals 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°. 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,. Such results are consistent with the interpretation that the central C−C bond is regarded as a single bond with minimal π interactions between the two units. This barrier to rotation about the C−C bond may be attributed to electrostatic interactions as unfavorable O−O repulsion is maximized in the planar form.

Occurrence in nature

Oxalate occurs in many plants, where it is synthesized by the incomplete oxidation of saccharides. 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, sorrel, and several Oxalis species. The root and/or leaves of rhubarb and buckwheat are high in oxalic acid. Other edible plants with significant concentrations of oxalate include, in decreasing order, star fruit, black pepper, parsley, poppy seed, amaranth, chard, beets, cocoa, chocolate, most nuts, most berries, fishtail palms, New Zealand spinach, and beans.
Leaves of the tea plant contain among the greatest measured concentrations of oxalic acid relative to other plants. However, the drink derived by infusion in hot water typically contains only low to moderate amounts of oxalic acid due to the small mass of leaves used for brewing.

Food item	Serving	Oxalate content
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
Poke greens, cooked	cup	476
Almond, raw	1 oz	296
Endive, raw	20 long leaves	273
Cocoa, dry	cup	254
Dandelion greens, cooked	cup	246
Okra, cooked	8–9 pods	146
Sweet potato, cooked	cup	141
Kale, cooked	cup	125
Peanuts, raw	cup	113
Turnip greens, cooked	cup	110
Chocolate, unsweetened	1 oz	91
Parsnips, diced, cooked	cup	81
Collard greens, cooked	cup	74
Pecans, halves, raw	cup	74
Tea, leaves	1 level tsp in 7 fl oz water	72
Cereal germ, toasted	cup	67
Gooseberries	cup	66
Potato, Idaho white, baked	1 medium	64
Carrots, cooked	cup	45
Apple, raw with skin	1 medium	41
Brussels sprouts, cooked	6–8 medium	37
Strawberries, raw	cup	35
Celery, raw	2 stalks	34
Milk chocolate bar	1 bar	34
Raspberries, black, raw	cup	33
Orange, edible portion	1 medium	24
Green beans, cooked	cup	23
Chives, raw, chopped	1 tablespoon	19
Leeks, raw	medium	15
Blackberries, raw	cup	13
Concord grapes	cup	13
Blueberries, raw	cup	11
Redcurrants	cup	11
Apricots, raw	2 medium	10
Raspberries, red, raw	cup	10
Broccoli, cooked	1 large stalk	6
Cranberry juice	cup	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 fungi 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 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". 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 and with large amounts of calcium oxalate in urine sediment. Oxalobacter formigenes in the gut flora may help alleviate this.

Congenital

is a rare, inherited condition, resulting in increased excretion of oxalate, with oxalate stones being common.