Oxalate (IUPAC: ethanedioate) is an
anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
with the formula C
2O
42−. This
dianion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
is colorless. It occurs naturally, including in some foods. It forms a variety of
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
s, for example
sodium oxalate
Sodium oxalate, or disodium oxalate, is the sodium salt of oxalic acid with the formula Na2C2O4. It is a white, crystalline, odorless solid, that decomposes above 290 °C.
Disodium oxalate can act as a reducing agent, and it may be used as a pr ...
(Na
2C
2O
4), and several esters such as
dimethyl oxalate
Dimethyl oxalate is the organic compound with the formula (CO2CH3)2. It is the dimethyl ester of oxalic acid. Dimethyl oxalate is a colorless or white solid that is soluble in water.
Production
Dimethyl oxalate can be obtained by esterificatio ...
(C
2O
4(CH
3)
2). It is a
conjugate base
A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...
of
oxalic acid
Oxalic acid is an organic acid with the systematic name ethanedioic acid and formula . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name comes from the fact that early inve ...
. 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 , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s from oxalic acid proceeds in a stepwise manner; as for other
polyprotic acids, loss of a single proton results in the monovalent
hydrogenoxalate
Hydrogenoxalate or hydrogen oxalate is an anion with chemical formula or , derived from oxalic acid by the loss of a single proton; or, alternatively, from the oxalate anion by addition of a proton. The name is also used for any salt containing ...
anion . A
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
with this
anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
is sometimes called an acid oxalate, monobasic oxalate, or hydrogen oxalate. The
equilibrium constant
The equilibrium constant of a chemical reaction is the value of its reaction quotient at chemical equilibrium, a state approached by a dynamic chemical system after sufficient time has elapsed at which its composition has no measurable tendency ...
(
''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 H
2C
2O
4, , 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 Cs
2C
2O
4 the O–C–C–O dihedral angle is 81(1)°.
Therefore, Cs
2C
2O
4 is more closely approximated by a D
2d symmetry structure because the two CO
2 planes are staggered. Two structural forms of Rb
2C
2O
4 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
carbon–carbon bond
A carbon–carbon bond is a covalent bond between two carbon atoms. The most common form is the single bond: a bond composed of two electrons, one from each of the two atoms. The carbon–carbon single bond is a sigma bond and is formed b ...
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, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
of
saccharide
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...
s.
Several plant foods such as the root and/or leaves of
spinach
Spinach (''Spinacia oleracea'') is a leafy green flowering plant native to central and western Asia. It is of the order Caryophyllales, family Amaranthaceae, subfamily Chenopodioideae. Its leaves are a common edible vegetable consumed either f ...
,
rhubarb, and
buckwheat
Buckwheat (''Fagopyrum esculentum''), or common buckwheat, is a flowering plant in the knotweed family Polygonaceae cultivated for its grain-like seeds and as a cover crop. The name "buckwheat" is used for several other species, such as '' Fago ...
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
Sorrel (''Rumex acetosa''), also called common sorrel or garden sorrel, is a perennial herbaceous plant in the family Polygonaceae. Other names for sorrel include spinach dock and narrow-leaved dock ('dock' being a common name for the genus '' ...
, and several ''
Oxalis
''Oxalis'' ( (American English) or (British English)) is a large genus of flowering plants in the wood-sorrel family Oxalidaceae, comprising over 550 species. The genus occurs throughout most of the world, except for the Polar region, polar are ...
'' 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 (
carambola
Carambola, also known as star fruit, is the fruit of '' Averrhoa carambola'', a species of tree native to tropical Southeast Asia. The mildly poisonous fruit is commonly consumed in parts of Brazil, Southeast Asia, South Asia, the South Pacif ...
),
black pepper
Black pepper (''Piper nigrum'') is a flowering vine in the family Piperaceae, cultivated for its fruit, known as a peppercorn, which is usually dried and used as a spice and seasoning. The fruit is a drupe (stonefruit) which is about in diame ...
,
parsley
Parsley, or garden parsley (''Petroselinum crispum'') is a species of flowering plant in the family Apiaceae that is native to the central and eastern Mediterranean region (Sardinia, Lebanon, Israel, Cyprus, Turkey, southern Italy, Greece, Por ...
,
poppy seed
Poppy seed is an oilseed obtained from the opium poppy (''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,
chard
Chard or Swiss chard (; ''Beta vulgaris'' subsp. ''vulgaris'', Cicla Group and Flavescens Group) is a green leafy vegetable. In the cultivars of the Flavescens Group, the leaf stalks are large and often prepared separately from the leaf blade; ...
,
beet
The beetroot is the taproot portion of a beet plant, usually known in North America as beets while the vegetable is referred to as beetroot in British English, and also known as the table beet, garden beet, red beet, dinner beet or golden beet ...
s,
cocoa
Cocoa may refer to:
Chocolate
* Chocolate
* ''Theobroma cacao'', the cocoa tree
* Cocoa bean, seed of ''Theobroma cacao''
* Chocolate liquor, or cocoa liquor, pure, liquid chocolate extracted from the cocoa bean, including both cocoa butter and ...
,
chocolate
Chocolate is a food made from roasted and ground cacao seed kernels that is available as a liquid, solid, or paste, either on its own or as a flavoring agent in other foods. Cacao has been consumed in some form since at least the Olmec civ ...
, 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 or pit, although many pips or seeds may be present. Common examples are strawberries, raspb ...
,
fishtail palms, New Zealand spinach (''
Tetragonia tetragonioides
''Tetragonia tetragonioides'', commonly called New Zealand spinach, Warrigal greens and other local names, is a flowering plant in the fig-marigold family (Aizoaceae). It is often cultivated as a leafy vegetable.
It is a widespread species, nat ...
''), and
bean
A bean is the seed of several plants in the family Fabaceae, which are used as vegetables for human or animal food. They can be cooked in many different ways, including boiling, frying, and baking, and are used in many traditional dishes th ...
s.
Leaves of the
tea
Tea is an aromatic beverage prepared by pouring hot or boiling water over cured or fresh leaves of ''Camellia sinensis'', an evergreen shrub native to East Asia which probably originated in the borderlands of southwestern China and north ...
plant (''
Camellia sinensis'') contain among the greatest measured concentrations of oxalic acid relative to other plants. However, the beverage derived by
infusion
Infusion is the process of extracting chemical compounds or flavors from plant material in a solvent such as water, oil or alcohol, by allowing the material to remain suspended in the solvent over time (a process often called steeping). An inf ...
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
![Surface of a kidney stone](https://upload.wikimedia.org/wikipedia/commons/c/cb/Surface_of_a_kidney_stone.jpg)
Excess consumption has been linked to gout and kidney stones. Many metal ions form insoluble precipitates with oxalate, a prominent example being
calcium oxalate
Calcium oxalate (in archaic terminology, oxalate of lime) is a calcium salt of oxalic acid with the chemical formula . It forms hydrates , where ''n'' varies from 1 to 3. Anhydrous and all hydrated forms are colorless or white. The monohydrate ...
, the primary constituent of the most common kind of
kidney stone
Kidney stone disease, also known as nephrolithiasis or urolithiasis, is a crystallopathy where a solid piece of material (kidney stone) develops in the urinary tract. Kidney stones typically form in the kidney and leave the body in the urine s ...
s.
The highly insoluble
iron(II) oxalate
Ferrous oxalate, or iron(II) oxalate, is an inorganic compound with the formula FeC2O4 where is typically 2. These are orange compounds, poorly soluble in water.
Structure
The dihydrate FeC2O4 is a coordination polymer, consisting of chains of ...
appears to play a major role in
gout
Gout ( ) is a form of inflammatory arthritis characterized by recurrent attacks of a red, tender, hot and swollen joint, caused by deposition of monosodium urate monohydrate crystals. Pain typically comes on rapidly, reaching maximal intensit ...
, in the nucleation and growth of the otherwise extremely soluble
sodium urate. This explains why gout usually appears after age 40, when
ferritin
Ferritin is a universal intracellular protein that stores iron and releases it in a controlled fashion. The protein is produced by almost all living organisms, including archaea, bacteria, algae, higher plants, and animals. It is the primary ' ...
levels in blood exceed 1 μg/L . Foods high in oxalate are often avoided by people at risk of gout.
In studies with rats, calcium supplements given along with foods high in oxalic acid can cause calcium oxalate to precipitate in the gut and reduce the levels of oxalate absorbed by the body (by 97% in some cases).
Some
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
of the
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
''
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 compound
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
s where it is sometimes abbreviated as ox. It is commonly encountered as a
bidentate ligand
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
, such as in
potassium ferrioxalate
Potassium ferrioxalate, also called potassium trisoxalatoferrate or potassium tris(oxalato)ferrate(III) is a chemical compound with the formula []. It often occurs as the trihydrate . Both are crystalline compounds, lime green in colour.A. Sarith ...
. When the oxalate chelates to a single metal center, it always adopts the planar conformation. As a bidentate ligand, it forms a 5-membered MC
2O
2 ring. An illustrative complex is
potassium ferrioxalate
Potassium ferrioxalate, also called potassium trisoxalatoferrate or potassium tris(oxalato)ferrate(III) is a chemical compound with the formula []. It often occurs as the trihydrate . Both are crystalline compounds, lime green in colour.A. Sarith ...
, K
3[Fe(C
2O
4)
3]. The drug oxaliplatin exhibits improved water solubility relative to older
platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Platinu ...
-based drugs, avoiding the dose-limiting side-effect of
nephrotoxicity Nephrotoxicity is toxicity in the kidneys. It is a poisonous effect of some substances, both toxic chemicals and medications, on kidney function. There are various forms, and some drugs may affect kidney function in more than one way. Nephrotoxin ...
. 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
Hyperoxaluria is an excessive urinary excretion of oxalate. Individuals with hyperoxaluria often have calcium oxalate kidney stones. It is sometimes called Bird's disease, after Golding Bird, who first described the condition.
Causes
Hyperoxalur ...
.
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
''Oxalis'' ( (American English) or (British English)) is a large genus of flowering plants in the wood-sorrel family Oxalidaceae, comprising over 550 species. The genus occurs throughout most of the world, except for the polar areas; species d ...
or, specifically in excessive quantities,
black tea
Black tea, also translated to red tea in various East Asian languages, is a type of tea that is more oxidized than oolong, yellow, white and green teas. Black tea is generally stronger in flavour than other teas. All five types are made from ...
) 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 inflammatory kidney disease and has several types according to the location of the inflammation. Inflammation can ...
or even
death
Death is the irreversible cessation of all biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whole brain, including brainstem, and brain ...
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. It is among the most prestigious peer-reviewed medical journals as well as the oldest continuously published one.
His ...
'' 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 ). 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
Proteinuria is the presence of excess proteins in the urine. In healthy persons, urine contains very little protein; an excess is suggestive of illness. Excess protein in the urine often causes the urine to become foamy (although this symptom ma ...
(an excess of protein in the urine) and with large amounts of calcium oxalate in urine sediment. ''
Oxalobacter formigenes'' in the
gut flora
Gut microbiota, gut microbiome, or gut flora, are the microorganisms, including bacteria, archaea, fungi, and viruses that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the gut mi ...
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 ...
is a rare, inherited condition, resulting in increased excretion of oxalate, with oxalate stones being common.
References
Further reading
*
*
*
External links
Oxalate.org- Oxalate content of 750+ foods from university and government sources
{{Oxalates
Anticoagulants
Carboxylate anions