Oxalate
Overview
Oxalate is the dianion with formula C2O42− also written (COO)22−. Either name is often used for derivatives, such as disodium oxalate
Disodium oxalate
Disodium oxalate, often called simply sodium oxalate, is a sodium salt of oxalic acid with the molecular formula Na2C2O4. It is usually a white, crystalline, odorless powder, that decomposes at 250–270 °C....

, (Na+)2C2O42−, or an ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

 of oxalic acid (such as dimethyl oxalate, (CH3)2C2O4. Oxalate also forms coordination compounds where it is sometimes abbreviated as ox.

Many metal ions form insoluble precipitates with oxalate, a prominent example being calcium oxalate
Calcium oxalate
Calcium oxalate is a chemical compound that forms needle-shaped crystals, known in plants as raphides. A major constituent of human kidney stones, the chemical is also found in beerstone, a scale that forms on containers used in breweries...

, the primary constituent of the most common kind of kidney stone
Kidney stone
A kidney stone, also known as a renal calculus is a solid concretion or crystal aggregation formed in the kidneys from dietary minerals in the urine...

s.
The dissociation of proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s from oxalic acid
Oxalic acid
Oxalic acid is an organic compound with the formula H2C2O4. This colourless solid is a dicarboxylic acid. In terms of acid strength, it is about 3,000 times stronger than acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate , is a chelating agent for metal cations...

 proceeds in a stepwise manner as for other polyprotic acids.
Encyclopedia
Oxalate is the dianion with formula C2O42− also written (COO)22−. Either name is often used for derivatives, such as disodium oxalate
Disodium oxalate
Disodium oxalate, often called simply sodium oxalate, is a sodium salt of oxalic acid with the molecular formula Na2C2O4. It is usually a white, crystalline, odorless powder, that decomposes at 250–270 °C....

, (Na+)2C2O42−, or an ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

 of oxalic acid (such as dimethyl oxalate, (CH3)2C2O4. Oxalate also forms coordination compounds where it is sometimes abbreviated as ox.

Many metal ions form insoluble precipitates with oxalate, a prominent example being calcium oxalate
Calcium oxalate
Calcium oxalate is a chemical compound that forms needle-shaped crystals, known in plants as raphides. A major constituent of human kidney stones, the chemical is also found in beerstone, a scale that forms on containers used in breweries...

, the primary constituent of the most common kind of kidney stone
Kidney stone
A kidney stone, also known as a renal calculus is a solid concretion or crystal aggregation formed in the kidneys from dietary minerals in the urine...

s.

Relationship to oxalic acid

The dissociation of proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s from oxalic acid
Oxalic acid
Oxalic acid is an organic compound with the formula H2C2O4. This colourless solid is a dicarboxylic acid. In terms of acid strength, it is about 3,000 times stronger than acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate , is a chelating agent for metal cations...

 proceeds in a stepwise manner as for other polyprotic acids. Loss of a single proton results in the monovalent hydrogenoxalate
Hydrogenoxalate
Hydrogenoxalate or hydrogen oxalate is an anion with chemical formula HC2O4− or HO2C–CO2−, derived from oxalic acid by the loss of a single proton; or, alternatively, from the oxalate anion C2O42− by addition of a proton....

 anion HC2O4. A salt with this anion is sometimes called an acid oxalate, monobasic oxalate, or hydrogen oxalate. The equilibrium constant (Ka
Acid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...

) for loss of the first proton is 5.37×10-2 (pKa = 1.27). The loss of the second proton, which yields the oxolate ion has a equilibrium constant of 5.25×10-5 (pKa = 4.28). These values imply that, in solutions with neutral pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

, there is no oxalic acid, and only trace amounts of hydrogen oxalate. 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 in many plants, where it is synthesized via the incomplete oxidation of carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

s.

Oxalate-rich plants include fat hen
Chenopodium album
Chenopodium album is a fast-growing weedy annual plant in the genus Chenopodium.Though cultivated in some regions, the plant is elsewhere considered a weed...

 ("lamb's quarters"), sorrel
Sorrel
Common sorrel or garden sorrel , often simply called sorrel, is a perennial herb that is cultivated as a garden herb or leaf vegetable...

, and several Oxalis
Oxalis
Oxalis is by far the largest genus in the wood-sorrel family Oxalidaceae: of the approximately 900 known species in the Oxalidaceae, 800 belong here...

species. The root and/or leaves of rhubarb
Rhubarb
Rhubarb is a group of plants that belong to the genus Rheum in the family Polygonaceae. They are herbaceous perennial plants growing from short, thick rhizomes. They have large leaves that are somewhat triangular-shaped with long fleshy petioles...

 and buckwheat
Buckwheat
Buckwheat refers to a variety of plants in the dicot family Polygonaceae: the Eurasian genus Fagopyrum, the North American genus Eriogonum, and the Northern Hemisphere genus Fallopia. Either of the latter two may be referred to as "wild buckwheat"...

 are high in oxalic acid. Other edible plants that contain significant concentrations of oxalate include—in decreasing order—star fruit (carambola
Carambola
Carambola, also known as starfruit, is the fruit of Averrhoa carambola, a species of tree native to the Philippines, Indonesia, Malaysia, India, Bangladesh and Sri Lanka. The fruit is a popular food throughout Southeast Asia, the South Pacific and parts of East Asia...

), black pepper
Black pepper
Black pepper is a flowering vine in the family Piperaceae, cultivated for its fruit, which is usually dried and used as a spice and seasoning. The fruit, known as a peppercorn when dried, is approximately in diameter, dark red when fully mature, and, like all drupes, contains a single seed...

, parsley
Parsley
Parsley is a species of Petroselinum in the family Apiaceae, native to the central Mediterranean region , naturalized elsewhere in Europe, and widely cultivated as an herb, a spice and a vegetable.- Description :Garden parsley is a bright green hairless biennial herbaceous plant in temperate...

, poppy
Poppy
A poppy is one of a group of a flowering plants in the poppy family, many of which are grown in gardens for their colorful flowers. Poppies are sometimes used for symbolic reasons, such as in remembrance of soldiers who have died during wartime....

 seed, amaranth
Amaranth
Amaranthus, collectively known as amaranth, is a cosmopolitan genus of herbs. Approximately 60 species are recognized, with inflorescences and foliage ranging from purple and red to gold...

, spinach
Spinach
Spinach is an edible flowering plant in the family of Amaranthaceae. It is native to central and southwestern Asia. It is an annual plant , which grows to a height of up to 30 cm. Spinach may survive over winter in temperate regions...

, chard
Chard
Chard , is a leafy green vegetable often used in Mediterranean cooking. While the leaves are always green, chard stalks vary in color. Chard has been bred to have highly nutrious leaves at the expense of the root...

, beet
Beet
The beet is a plant in the Chenopodiaceae family which is now included in Amaranthaceae family. It is best known in its numerous cultivated varieties, the most well known of which is the purple root vegetable known as the beetroot or garden beet...

s, cocoa
Cocoa solids
Cocoa solids are the low-fat component of chocolate. When sold as an end product, it may also be called cocoa powder, cocoa, and cacao....

, chocolate
Chocolate
Chocolate is a raw or processed food produced from the seed of the tropical Theobroma cacao tree. Cacao has been cultivated for at least three millennia in Mexico, Central and South America. Its earliest documented use is around 1100 BC...

, most nut
Nut (fruit)
A nut is a hard-shelled fruit of some plants having an indehiscent seed. While a wide variety of dried seeds and fruits are called nuts in English, only a certain number of them are considered by biologists to be true nuts...

s, most berries
Berry
The botanical definition of a berry is a fleshy fruit produced from a single ovary. Grapes are an example. The berry is the most common type of fleshy fruit in which the entire ovary wall ripens into an edible pericarp. They may have one or more carpels with a thin covering and fleshy interiors....

, fishtail palms
Caryota
Caryota is a genus of palm trees. They are often known as fishtail palms because of the shape of their leaves. There are about 13 species native to Asia and the South Pacific. One of the more widely known species is Caryota urens, which yields sap used to make an unrefined sugar called jaggery, and...

, New Zealand spinach (Tetragonia tetragonioides) and bean
Bean
Bean is a common name for large plant seeds of several genera of the family Fabaceae used for human food or animal feed....

s.
Leaves of the tea
Tea
Tea is an aromatic beverage prepared by adding cured leaves of the Camellia sinensis plant to hot water. The term also refers to the plant itself. After water, tea is the most widely consumed beverage in the world...

 plant (Camellia sinensis
Camellia sinensis
Camellia sinensis is the species of plant whose leaves and leaf buds are used to produce Chinese tea. It is of the genus Camellia , a genus of flowering plants in the family Theaceae. White tea, green tea, oolong, pu-erh tea and black tea are all harvested from this species, but are processed...

) 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.


The "gritty mouth" feeling one experiences when drinking milk with a rhubarb dessert is caused by precipitation of calcium, abstracted from the casein
Casein
Casein is the name for a family of related phosphoprotein proteins . These proteins are commonly found in mammalian milk, making up 80% of the proteins in cow milk and between 60% and 65% of the proteins in human milk....

 in dairy products, as calcium oxalate
Calcium oxalate
Calcium oxalate is a chemical compound that forms needle-shaped crystals, known in plants as raphides. A major constituent of human kidney stones, the chemical is also found in beerstone, a scale that forms on containers used in breweries...

.

Physiological effects

In the body, oxalic acid combines with divalent metallic cations such as calcium (Ca2+) and iron(II)
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 (Fe2+) to form crystals of the corresponding oxalates which are then excreted in urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

 as minute crystals. These oxalates can form larger kidney stone
Kidney stone
A kidney stone, also known as a renal calculus is a solid concretion or crystal aggregation formed in the kidneys from dietary minerals in the urine...

s that can obstruct the kidney tubules. An estimated 80% of kidney stones are formed from calcium oxalate. Those with kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

 disorders, gout
Gout
Gout is a medical condition usually characterized by recurrent attacks of acute inflammatory arthritis—a red, tender, hot, swollen joint. The metatarsal-phalangeal joint at the base of the big toe is the most commonly affected . However, it may also present as tophi, kidney stones, or urate...

, rheumatoid arthritis
Rheumatoid arthritis
Rheumatoid arthritis is a chronic, systemic inflammatory disorder that may affect many tissues and organs, but principally attacks synovial joints. The process produces an inflammatory response of the synovium secondary to hyperplasia of synovial cells, excess synovial fluid, and the development...

, or certain forms of chronic vulvar pain (vulvodynia
Vulvodynia
Vulvodynia is a chronic pain syndrome that affects the vulvar area and occurs without an identifiable cause or visible pathology categorized in the ICD-9 group 625—specifically ICD-9 625.7, which is for pain and other disorders of the female genital organs...

) are typically advised to avoid foods high in oxalic acid. Methods to reduce the oxalate content in food are of current interest.

Magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 (Mg2+) oxalate is 567 times more soluble than calcium oxalate, so the latter is more likely to precipitate out when magnesium levels are low and calcium and oxalate levels are high.

Magnesium oxalate is a million times more soluble than mercury oxalate. Oxalate solubility for other metals decreases in the order Ca > Cd > Zn > {Mn,Ni,Fe,Cu} > {As,Sb,Pb} > Hg. The highly insoluble iron(II) oxalate
Iron(II) oxalate
Ferrous oxalate, or iron oxalate, is a chemical compound consisting of one iron ion and one oxalate ion . It has the chemical formula Fe[C2O4]....

 appears to play a major role in gout, in the nucleation and growth of the otherwise extremely soluble sodium urate. This explains why gout usually appears after age 40, when ferritin
Ferritin
Ferritin is a ubiquitous intracellular protein that stores iron and releases it in a controlled fashion. The amount of ferritin stored reflects the amount of iron stored. The protein is produced by almost all living organisms, including bacteria, algae and higher plants, and animals...

 levels in blood exceed 100 ng/dl. Beer is rich in oxalate and iron, and ethanol increases iron absorption and magnesium elimination, so beer intake greatly increases the risk of a gout attack.

Cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...

 catalyzes the transformation of vitamin C
Vitamin C
Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...

 into oxalic acid. This can be a problem for people exposed to high levels of cadmium in the diet, in the workplace, or through smoking
Smoking
Smoking is a practice in which a substance, most commonly tobacco or cannabis, is burned and the smoke is tasted or inhaled. This is primarily practised as a route of administration for recreational drug use, as combustion releases the active substances in drugs such as nicotine and makes them...

.

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.)

Oxalic acid can also be produced by the metabolism of ethylene glycol
Ethylene glycol
Ethylene glycol is an organic compound widely used as an automotive antifreeze and a precursor to polymers. In its pure form, it is an odorless, colorless, syrupy, sweet-tasting liquid...

 ("antifreeze"), glyoxylic acid
Glyoxylic acid
Glyoxylic acid or oxoacetic acid is an organic compound. Together with acetic acid, glycolic acid, and oxalic acid, glyoxylic acid is one of the C2 carboxylic acids. It is a colourless solid that occurs naturally and is useful industrially....

, or ascorbic acid
Ascorbic acid
Ascorbic acid is a naturally occurring organic compound with antioxidant properties. It is a white solid, but impure samples can appear yellowish. It dissolves well in water to give mildly acidic solutions. Ascorbic acid is one form of vitamin C. The name is derived from a- and scorbutus , the...

 (vitamin C
Vitamin C
Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...

).

Powdered oxalate is used as a pesticide
Pesticide
Pesticides are substances or mixture of substances intended for preventing, destroying, repelling or mitigating any pest.A pesticide may be a chemical unicycle, biological agent , antimicrobial, disinfectant or device used against any pest...

 in beekeeping
Beekeeping
Beekeeping is the maintenance of honey bee colonies, commonly in hives, by humans. A beekeeper keeps bees in order to collect honey and other products of the hive , to pollinate crops, or to produce bees for sale to other beekeepers...

 to combat the bee mite.

Some fungi
Fungus
A fungus is a member of a large 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, Fungi, which is separate from plants, animals, and bacteria...

 of the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Aspergillus
Aspergillus
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...

produce oxalic acid, which reacts with calcium from the blood or tissue to precipitate calcium oxalate.

There is some preliminary evidence that the administration of probiotics can affect oxalic acid excretion rates (and presumably oxalic acid levels as well.)

As a ligand

Oxalate, the conjugate base of oxalic acid, is an excellent ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

 for metal ions. It usually binds as a bidentate ligand forming a 5-membered MO2C2 ring. An illustrative complex is potassium ferrioxalate
Potassium ferrioxalate
Potassium ferrioxalate, also known as potassium oxalatoferrate, is a chemical compound with the formula K3[Fe3], where iron is in the +3 oxidation state. It is an octahedral transition metal complex in which three bidentate oxalate ions are bound to an iron center. Potassium acts as a counterion,...

, K3[Fe(C2O4)3]. The drug Oxaliplatin
Oxaliplatin
Oxaliplatin is a coordination complex that is used in cancer chemotherapy. These platinum-based drugs are usually classified as alkylating agents, although they are not actually alkylating groups ....

 exhibits improved water solubility relative to older platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

-based drugs, avoiding the dose-limiting side-effect of nephrotoxicity
Nephrotoxicity
Nephrotoxicity is a poisonous effect of some substances, both toxic chemicals and medication, on the kidneys. There are various forms of toxicity. Nephrotoxicity should not be confused with the fact that some medications have a predominantly renal excretion and need their dose adjusted for the...

. 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
Greasewood
Greasewood, Sarcobatus, is a genus of one or two species of flowering plants. Traditionally it has been treated in the family Chenopodiaceae, but the APG II system, of 2003, places it in the family Sarcobataceae....

 or human consumption of Sorrel) may result in kidney disease or even death
Death
Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

 due to oxalate poisoning. The presence of Oxalobacter formigenes
Oxalobacter formigenes
Oxalobacter formigenes is an oxalate-degrading anaerobic bacterium that colonizes the large intestine of numerous vertebrates, including humans. O. formigenes and humans share a beneficial symbiosis....

in the gut flora
Gut flora
Gut flora consists of microorganisms that live in the digestive tracts of animals and is the largest reservoir of human flora. In this context, gut is synonymous with intestinal, and flora with microbiota and microflora....

 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.

Oxalate salts

  • sodium oxalate - Na2C2O4
  • calcium oxalate
    Calcium oxalate
    Calcium oxalate is a chemical compound that forms needle-shaped crystals, known in plants as raphides. A major constituent of human kidney stones, the chemical is also found in beerstone, a scale that forms on containers used in breweries...

     - CaC2O4, a major component of kidney stones

Oxalate complexes

  • potassium ferrioxalate
    Potassium ferrioxalate
    Potassium ferrioxalate, also known as potassium oxalatoferrate, is a chemical compound with the formula K3[Fe3], where iron is in the +3 oxidation state. It is an octahedral transition metal complex in which three bidentate oxalate ions are bound to an iron center. Potassium acts as a counterion,...

     - K3[Fe(C2O4)3], an iron complex with oxalate ligands

Oxalate esters

  • diphenyl oxalate - (C6H5)2C2O4
  • dimethyl oxalate - (CH3)2C2O4
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