Oxalate Minerals
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Oxalate (systematic IUPAC name: ethanedioate) is an anion with the
chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...
. This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of
salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
s, for example sodium oxalate (), and several
esters In chemistry, an ester is a chemical compound, compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds c ...
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
proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
s 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 In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
with this anion is sometimes called an acid oxalate, monobasic oxalate, or hydrogen oxalate. The equilibrium constant ( ''K''a) for loss of the first proton is ( p''K''a = 1.27). The loss of the second proton, which yields the oxalate ion, has an equilibrium constant of (p''K''a = 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 dihedrals approach 90° with approximate D2d symmetry. When chelated to cations, oxalate adopts the planar, D2h conformation. However, in the structure of caesium oxalate the O–C–C–O dihedral angle is 81(1)°. Therefore, is more closely approximated by a D2d 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 (which formally corresponds to the difference in energy between the planar and staggered forms) 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 Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
of saccharides. Several plant foods such as the root and/or leaves of
spinach Spinach (''Spinacia oleracea'') is a leafy green flowering plant native to Central Asia, Central and Western Asia. It is of the order Caryophyllales, family Amaranthaceae, subfamily Chenopodioideae. Its leaves are a common vegetable consumed eit ...
, 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. Other edible plants with significant concentrations of oxalate include, in decreasing order, star fruit ( carambola),
black pepper Black pepper (''Piper nigrum'') is a flowering vine in the family Piperaceae, cultivated for its fruit (the peppercorn), which is usually dried and used as a spice and seasoning. The fruit is a drupe (stonefruit) which is about in diameter ...
, parsley,
poppy seed Poppy seed is an oilseed obtained from the poppy plant (''Papaver somniferum''). The tiny, kidney-shaped seeds have been harvested from dried seed pods by various civilizations for thousands of years. It is still widely used in many countries, ...
,
amaranth ''Amaranthus'' is a cosmopolitan distribution, cosmopolitan group of more than 50 species which make up the genus of annual plant, annual or short-lived perennial plants collectively known as amaranths. Some names include "prostrate pigweed" an ...
, chard,
beet The beetroot (British English) or beet (North American English) is the taproot portion of a '' Beta vulgaris'' subsp. ''vulgaris'' plant in the Conditiva Group. The plant is a root vegetable also known as the table beet, garden beet, dinner ...
s, cocoa,
chocolate Chocolate is a food made from roasted and ground cocoa beans that can be a liquid, solid, or paste, either by itself or to flavoring, flavor other foods. Cocoa beans are the processed seeds of the cacao tree (''Theobroma cacao''); unprocesse ...
, most nuts, most
berries A berry is a small, pulpy, and often edible fruit. Typically, berries are juicy, rounded, brightly colored, sweet, sour or tart, and do not have a stone fruit, stone or pit (fruit), pit although many wikt:pip#Etymology 2, pips or seeds may be p ...
, fishtail palms, New Zealand spinach ('' Tetragonia tetragonioides''), and
bean A bean is the seed of some plants in the legume family (Fabaceae) used as a vegetable for human consumption or animal feed. The seeds are often preserved through drying (a ''pulse''), but fresh beans are also sold. Dried beans are traditi ...
s. Leaves of the tea plant ('' Camellia sinensis'') 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.


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 A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
of the
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
''
Aspergillus ' () is a genus consisting of several hundred mold species found in various climates worldwide. ''Aspergillus'' was first catalogued in 1729 by the Italian priest and biologist Pier Antonio Micheli. Viewing the fungi under a microscope, Miche ...
'' 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 Oxaliplatin, sold under the brand name Eloxatin among others, is a cancer medication (platinum-based antineoplastic class) used to treat colorectal cancer. It is given by intravenous, infusion into a vein. Common side effects include paresth ...
exhibits improved water solubility relative to older
platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...
-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,
black tea Black tea (also literally translated as red tea from various East Asian languages) is a type of tea that is more tea processing, oxidized than oolong, yellow tea, yellow, white tea, white, and green tea, green teas. Black tea is generally st ...
) may result in
kidney disease Kidney disease, or renal disease, technically referred to as nephropathy, is damage to or disease of a kidney. Nephritis is an Inflammation, inflammatory kidney disease and has several types according to the location of the inflammation. Infla ...
or even
death Death is the end of life; the irreversible cessation of all biological functions that sustain a living organism. Death eventually and inevitably occurs in all organisms. The remains of a former organism normally begin to decompose sh ...
due to oxalate poisoning. ''
The New England Journal of Medicine ''The New England Journal of Medicine'' (''NEJM'') is a weekly medical journal published by the Massachusetts Medical Society. Founded in 1812, the journal is among the most prestigious peer-reviewed medical journals. Its 2023 impact factor w ...
'' 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. '' Oxalobacter formigenes'' in the gut flora may help alleviate this.


Congenital

Primary hyperoxaluria Primary hyperoxaluria is a rare condition (autosomal recessive) resulting in increased excretion of oxalate (up to 600 mg a day from normal 50 mg a day), with oxalate stones being common. Signs and symptoms Primary hyperoxaluria is an a ...
is a rare, inherited condition, resulting in increased excretion of oxalate, with oxalate stones being common.


References


Further reading

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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 {{Oxalates Anticoagulants Carboxylate anions Oxocarbon anions