Oxalic acid is an

organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are re ...

with the systematic name ethanedioic acid and

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 ...

, also written as or or . It is the simplest

dicarboxylic acid In organic chemistry, a dicarboxylic acid is an organic compound containing two carboxyl groups (). The general molecular formula for dicarboxylic acids can be written as , where R can be aliphatic or aromatic.Boy Cornils, Peter Lappe "Dicarbox ...

. It is a white crystalline solid that forms a colorless solution in water. Its name is derived from early investigators who isolated oxalic acid from

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...

s of the genus ''

Oxalis ''Oxalis'' ( (British English) or (American 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 ...

'', commonly known as wood-sorrels. It occurs naturally in many foods. Excessive ingestion of oxalic acid or prolonged skin contact can be dangerous. Oxalic acid is a much stronger acid than

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

. It is a

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are common reducing agents include hydrogen, carbon ...

and its

conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid gives a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as it loses a hydrogen ion in the reve ...

hydrogen oxalate Hydrogenoxalate or hydrogen oxalate ''(IUPAC name: 2-Hydroxy-2-oxoacetate)'' 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 n ...

() and

oxalate 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 ...

() are

chelating agents Chelation () is a type of bonding of ions and their molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These l ...

for metal cations. It is used as a cleaning agent, especially for the removal of

rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH) ...

, because it forms a water-soluble ferric iron complex, the

ferrioxalate Ferrioxalate or trisoxalatoferrate(III) is a trivalent anion with formula . It is a transition metal complex consisting of an iron atom in the +3 oxidation state and three bidentate oxalate ions anions acting as ligands. The ferrioxalate anio ...

ion. Oxalic acid typically occurs as the

dihydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...

with the formula .

History

The preparation of salts of oxalic acid from plants had been known since at least 1745, when the Dutch botanist and physician

Herman Boerhaave Herman Boerhaave (, 31 December 1668 – 23 September 1738Underwood, E. Ashworth. "Boerhaave After Three Hundred Years." ''The British Medical Journal'' 4, no. 5634 (1968): 820–25. .) was a Dutch chemist, botanist, Christian humanist, and ph ...

isolated a salt from

wood sorrel ''Oxalis'' ( (British English) or (American 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 ...

, akin to

kraft process The kraft process (also known as kraft pulping or sulfate process) is a process for conversion of wood into wood pulp, which consists of almost pure cellulose fibres, the main component of paper. The kraft process involves treatment of wood chip ...

. By 1773, François Pierre Savary of Fribourg, Switzerland had isolated oxalic acid from its salt in sorrel. In 1776, Swedish chemists

Carl Wilhelm Scheele Carl Wilhelm Scheele (, ; 9 December 1742 – 21 May 1786) was a Swedish Pomerania, German-Swedish pharmaceutical chemist. Scheele discovered oxygen (although Joseph Priestley published his findings first), and identified the elements molybd ...

and

Torbern Olof Bergman Torbern Olof Bergman (''KVO'') (20 March 17358 July 1784) was a Swedish chemist and mineralogist noted for his 1775 ''Dissertation on Elective Attractions'', containing the largest chemical affinity tables ever published. Bergman was the first ...

produced oxalic acid by reacting sugar with concentrated

nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...

; Scheele called the acid that resulted ''socker-syra'' or ''såcker-syra'' (sugar acid). By 1784, Scheele had shown that "sugar acid" and oxalic acid from natural sources were identical. The modern name was introduced along with many other acid names by de Morveau,

Lavoisier Antoine-Laurent de Lavoisier ( ; ; 26 August 17438 May 1794),
CNRS (

Friedrich Wöhler Friedrich Wöhler Royal Society of London, FRS(For) HonFRSE (; 31 July 180023 September 1882) was a German chemist known for his work in both organic chemistry, organic and inorganic chemistry, being the first to isolate the chemical elements be ...

obtained oxalic acid by reacting

cyanogen Cyanogen is the chemical compound with the chemical formula, formula . Its structure is . The simplest stable carbon nitride, it is a Transparency and translucency, colorless and highly toxic gas with a pungency, pungent odor. The molecule is a ...

with ammonia in aqueous solution. This experiment may represent the first synthesis of a

natural product A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical s ...

Production

Industrial

Oxalic acid is mainly manufactured by the oxidation of

carbohydrate A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...

s or

glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

using

or air in the presence of

vanadium pentoxide Vanadium(V) oxide (''vanadia'') is the inorganic compound with the formula V2 O5. Commonly known as vanadium pentoxide, it is a dark yellow solid, although when freshly precipitated from aqueous solution, its colour is deep orange. Because of ...

. Another process uses oxygen to regenerate the nitric acid, using a variety of precursors including

glycolic acid Glycolic acid (or hydroxyacetic acid; chemical formula ) is a colorless, odorless and hygroscopic crystal, crystalline solid, highly solubility, soluble in water. It is used in various skin care, skin-care products. Glycolic acid is widespread in ...

and

ethylene glycol Ethylene glycol ( IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes: as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...

. As of 2011, this process was only used by

Mitsubishi The is a group of autonomous Japanese multinational companies in a variety of industries. Founded by Yatarō Iwasaki in 1870, the Mitsubishi Group traces its origins to the Mitsubishi zaibatsu, a unified company that existed from 1870 to 194 ...

in Japan. A newer method entails oxidative

carbonylation In chemistry, carbonylation refers to reactions that introduce carbon monoxide (CO) into organic and inorganic substrates. Carbon monoxide is abundantly available and conveniently reactive, so it is widely used as a reactant in industrial chemis ...

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

s to give the

diesters In chemistry, an ester is a 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 contain a distincti ...

of oxalic acid: : These diesters are subsequently hydrolyzed to oxalic acid. Approximately 120,000

tonne The tonne ( or ; symbol: t) is a unit of mass equal to 1,000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton in the United States to distinguish it from the non-metric units of the s ...

s are produced annually. Historically oxalic acid was obtained exclusively by using caustics, such as

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which utili ...

, on

sawdust Sawdust (or wood dust) is a by-product or waste product of woodworking operations such as sawing, sanding, milling and routing. It is composed of very small chips of wood. These operations can be performed by woodworking machinery, portable p ...

, followed by acidification of the oxalate by mineral acids, such as

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

. Oxalic acid can also be formed by the heating of

sodium formate Sodium formate, HCOONa, is the sodium salt of formic acid, HCOOH. It usually appears as a white deliquescent powder. Preparation For commercial use, sodium formate is produced by absorbing carbon monoxide under pressure in solid sodium hydroxide ...

in the presence of an

alkali In chemistry, an alkali (; from the Arabic word , ) is a basic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The a ...

ne catalyst.

Laboratory

Although it can be readily purchased, oxalic acid can be prepared in the laboratory by

oxidizing 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 ...

sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...

using

in the presence of a small amount of

as a

catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

.''Practical Organic Chemistry'' by Julius B. Cohen, 1930 ed. preparation #42 The dihydrate can be converted to the anhydrous form by heating or

azeotropic distillation In chemistry, azeotropic distillation is any of a range of techniques used to break an azeotrope in distillation. In chemical engineering, ''azeotropic distillation'' usually refers to the specific technique of adding another component to genera ...

Structure

Anhydrous

Anhydrous oxalic acid exists as two polymorphs; in one the

hydrogen-bonding In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently ...

results in a chain-like structure, whereas the hydrogen bonding pattern in the other form defines a sheet-like structure. Because the anhydrous material is both acidic and

hydrophilic A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are n ...

(water seeking), it is used in

esterification In chemistry, an ester is a 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 contain a distin ...

Dihydrate

The dihydrate has space group ''C''⁵_2''h''–''P''2₁/''n'', with

lattice parameter A lattice constant or lattice parameter is one of the physical dimensions and angles that determine the geometry of the unit cells in a crystal lattice, and is proportional to the distance between atoms in the crystal. A simple cubic crystal has ...

s , , , , . The main inter-atomic distances are: C−C 153 pm, C−O₁ 129 pm, C−O₂ 119 pm.

Reactions

Acid–base properties

Oxalic acid's p''K''_a values vary in the literature from 1.25 to 1.46 and from 3.81 to 4.40. The 100th ed of the CRC, released in 2019, has values of 1.25 and 3.81. Oxalic acid is relatively strong compared to other

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...

s: Oxalic acid undergoes many of the reactions characteristic for other carboxylic acids. It forms esters such as

dimethyl oxalate Dimethyl oxalate is an organic compound with the formula or . 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 esterification of oxa ...

( m.p. ). It forms an acid chloride called

oxalyl chloride Oxalyl chloride is an organic chemical compound with the formula . This colorless, sharp-smelling liquid, the diacyl chloride of oxalic acid, is a useful reagent in organic synthesis. Preparation Oxalyl chloride was first prepared in 1892 by the ...

Metal-binding properties

Transition metal oxalate complex Transition metal oxalate complexes are coordination complexes with oxalate (C2O42−) ligands. Some are useful commercially, but the topic has attracted regular scholarly scrutiny. Oxalate (C2O42-) is a kind of dicarboxylate ligand. As a small, sym ...

es are numerous, e.g. 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 ...

. Oxalic acid has been shown to reduce

manganese dioxide Manganese dioxide is the inorganic compound with the formula . This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese and a component of manganese nodules. The principal use for is for dry-cel ...

() in manganese ores to allow the leaching of the metal by

. Oxalic acid is an important reagent in

lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises at least the 14 metallic chemical elements with atomic numbers 57–70, from lanthanum through ytterbium. In the periodic table, they fill the 4f orbitals. Lutetium (el ...

chemistry. Hydrated lanthanide oxalates form readily in very strongly acidic solutions as a densely

crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosc ...

, easily filtered form, largely free of contamination by nonlanthanide elements: : Thermal decomposition of these oxalates gives the

oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...

s, which is the most commonly marketed form of these elements.

Other

Oxalic acid and oxalates can be oxidized by

permanganate A permanganate () is a chemical compound with the manganate(VII) ion, , the conjugate base of permanganic acid. Because the manganese atom has a +7 oxidation state, the permanganate(VII) ion is a strong oxidising agent. The ion is a transition ...

ion in an

autocatalytic In chemistry, a chemical reaction is said to be autocatalytic if one of the reaction products is also a catalyst for the same reaction. Many forms of autocatalysis are recognized.Steinfeld J.I., Francisco J.S. and Hase W.L. ''Chemical Kinetics and ...

reaction. Oxalic acid vapor decomposes at and

formic acid Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid. It has the chemical formula HCOOH and structure . This acid is an important intermediate in chemical synthesis and occurs naturally, most notably in some an ...

().

Photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...

with 237–313 nm UV light also produces carbon monoxide () and water. Evaporation of a solution of

urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

and oxalic acid in 2:1 molar ratio yields a solid crystalline compound , consisting of stacked two-dimensional networks of the neutral molecules held together by

hydrogen bond In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

s with the oxygen atoms.

Occurrence

Biosynthesis

At least two pathways exist for the enzyme-mediated formation of oxalate. In one pathway,

oxaloacetate Oxaloacetic acid (also known as oxalacetic acid or OAA) is a crystalline organic compound with the chemical formula HO2CC(O)CH2CO2H. Oxaloacetic acid, in the form of its conjugate base oxaloacetate, is a metabolic intermediate in many processes ...

, a component of the

Krebs citric acid cycle The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle, or TCA cycle (tricarboxylic acid cycle)—is a series of chemical reaction, biochemical reactions that release the energy stored in nutrients through acetyl-Co ...

, is hydrolyzed to oxalate and acetic acid by the enzyme

oxaloacetase In enzymology, an oxaloacetase () is an enzyme that catalyzes the chemical reaction: :oxaloacetate + H2O \rightleftharpoons oxalate + acetate Thus, the two substrates of this enzyme are oxaloacetate and H2O, whereas its two products are oxalat ...

: : It also arises from the dehydrogenation of

, which is produced by the metabolism of

Occurrence in foods and plants

Early investigators isolated oxalic acid from

wood-sorrel ''Oxalis'' ( (British English) or (American 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 ...

(''Oxalis''). Members of the

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 ...

family and the

brassica ''Brassica'' () is a genus of plants in the cabbage and mustard family (Brassicaceae). The members of the genus are informally known as cruciferous vegetables, cabbages, mustard plants, or simply brassicas. Crops from this genus are sometim ...

s (

cabbage Cabbage, comprising several cultivars of '' Brassica oleracea'', is a leafy green, red (purple), or white (pale green) biennial plant grown as an annual vegetable crop for its dense-leaved heads. It is descended from the wild cabbage ( ''B.& ...

broccoli Broccoli (''Brassica oleracea'' var. ''italica'') is an edible green plant in the Brassicaceae, cabbage family (family Brassicaceae, genus ''Brassica'') whose large Pseudanthium, flowering head, plant stem, stalk and small associated leafy gre ...

brussels sprouts The Brussels sprout is a member of the Gemmifera cultivar group of cabbages (''Brassica oleracea''), grown for its edible buds. Etymology Though native to the Mediterranean region with other cabbage species, Brussels sprouts first appeared i ...

) are high in oxalates, as are

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 ''Ru ...

and

umbellifer Apiaceae () or Umbelliferae is a family of mostly aromatic flowering plants named after the type genus '' Apium,'' and commonly known as the celery, carrot, or parsley family, or simply as umbellifers. It is the 16th-largest family of flowering ...

s like

parsley Parsley, or garden parsley (''Petroselinum crispum''), is a species of flowering plant in the family Apiaceae that is native to Greece, Morocco and the former Yugoslavia. It has been introduced and naturalisation (biology), naturalized in Eur ...

. The leaves and stems of all species of the genus ''

Chenopodium ''Chenopodium'' is a genus of numerous species of perennial or annual herbaceous flowering plants known as the goosefoot, which occur almost anywhere in the world. It is placed in the family Amaranthaceae in the APG II system; older classifica ...

'' and related genera of the family

Amaranthaceae Amaranthaceae ( ) is a family of flowering plants commonly known as the amaranth family, in reference to its type genus '' Amaranthus''. It includes the former goosefoot family Chenopodiaceae and contains about 165 genera and 2,040 species, maki ...

, which includes

quinoa Quinoa (''Chenopodium quinoa''; , from Quechuan languages, Quechua ' or ') is a flowering plant in the Amaranthaceae, amaranth family. It is a herbaceous annual plant grown as a crop primarily for its edible seeds; the seeds are high in prote ...

, contain high levels of oxalic acid.

Rhubarb Rhubarb is the fleshy, edible stalks ( petioles) of species and hybrids (culinary rhubarb) of ''Rheum'' in the family Polygonaceae, which are cooked and used for food. The plant is a herbaceous perennial that grows from short, thick rhizomes. ...

leaves contain about 0.5% oxalic acid, and jack-in-the-pulpit (''

Arisaema triphyllum ''Arisaema triphyllum'', the Jack-in-the-pulpit, is a species of flowering plant in the arum family Araceae. It is a member of the ''Arisaema triphyllum'' complex, a group of four or five closely related taxa in eastern North America. The specif ...

'') contains

calcium oxalate Calcium oxalate (in archaic terminology, oxalate of lime) is a calcium salt of oxalic acid with the chemical formula or . It forms hydrates , where ''n'' varies from 1 to 3. Anhydrous and all hydrated forms are colorless or white. The monohydr ...

crystals. Similarly, the

Virginia creeper ''Parthenocissus quinquefolia'', commonly known as Virginia creeper, woodbine, five-leaved ivy, or five-finger, is a species of flowering vine in the grape family Vitaceae. The species is native to eastern and central North America, with its r ...

, a common decorative vine, produces oxalic acid in its berries as well as oxalate crystals in the sap, in the form of

raphide Raphides ( ; singular ''raphide'' or ''raphis'') are needle-shaped crystals of calcium oxalate monohydrate ( prismatic monoclinic crystals) or calcium carbonate as aragonite ( dipyramidal orthorhombic crystals), found in more than 200 familie ...

s. Bacteria produce oxalates from oxidation of

s. Plants of the genus ''

Fenestraria ''Fenestraria'' (known as babies' toes) is a (possibly monotypic) genus of succulent plants in the family Aizoaceae, native to the Namaqualand in Namibia. Description The only species currently recognised in this genus is ''Fenestraria rh ...

'' produce optical fibers made from crystalline oxalic acid to transmit light to subterranean photosynthetic sites.

Carambola Carambola, also known as star fruit, is the fruit of '' Averrhoa carambola'', a species of tree native to tropical Southeast Asia. The edible fruit has distinctive ridges running down its sides (usually 5–6). When cut in cross-section, it re ...

, also known as starfruit, also contains oxalic acid along with

caramboxin Caramboxin (CBX) is a toxin found in star fruit (''Averrhoa carambola'') and the related bilimbi fruit ('' Averrhoa bilimbi''). Individuals with some types of kidney disease are susceptible to adverse neurological effects including intoxication, ...

. Citrus juice contains small amounts of oxalic acid. The formation of naturally occurring calcium oxalate

patina Patina ( or ) is a thin layer that variously forms on the surface of copper, brass, bronze, and similar metals and metal alloys ( tarnish produced by oxidation or other chemical processes), or certain stones and wooden furniture (sheen prod ...

s on certain

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

and

marble Marble is a metamorphic rock consisting of carbonate minerals (most commonly calcite (CaCO3) or Dolomite (mineral), dolomite (CaMg(CO3)2) that have recrystallized under the influence of heat and pressure. It has a crystalline texture, and is ty ...

statues and monuments has been proposed to be caused by the chemical reaction of the carbonate stone with oxalic acid secreted by

lichen A lichen ( , ) is a hybrid colony (biology), colony of algae or cyanobacteria living symbiotically among hypha, filaments of multiple fungus species, along with yeasts and bacteria embedded in the cortex or "skin", in a mutualism (biology), m ...

or other

microorganisms A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...

Production by fungi

Many soil fungus species secrete oxalic acid, which results in greater solubility of metal cations and increased availability of certain soil nutrients, and can lead to the formation of calcium oxalate crystals. Some fungi such as ''

Aspergillus niger ''Aspergillus niger'' is a mold classified within the ''Nigri'' section of the ''Aspergillus'' genus. The ''Aspergillus'' genus consists of common molds found throughout the environment within soil and water, on vegetation, in fecal matter, on de ...

'' have been extensively studied for the industrial production of oxalic acid; however, those processes are not yet economically competitive with production from oil and gas.Jan S. Tkacz, Lene Lange (2012): ''Advances in Fungal Biotechnology for Industry, Agriculture, and Medicine''. 445 pages. ''

Cryphonectria parasitica The pathogenic fungus ''Cryphonectria parasitica'' (formerly ''Endothia parasitica'') is a member of the Ascomycota (sac fungi). This necrotrophic fungus is native to East Asia and South East Asia and was introduced into Europe and North America ...

'' may excrete oxalic acid containing solutions at the advancing edge of its chestnut

cambium A cambium (: cambiums or cambia), in plants, is a tissue layer that provides partially undifferentiated cells for plant growth. It is found in the area between xylem and phloem. A cambium can also be defined as a cellular plant tissue from whic ...

infection. The lower pH (<2.5) of more concentrated oxalic acid excretions may degrade cambium cell walls and have a toxic effect on chestnut cambium cells. Cambium cells that burst provide nutrients for a blight infection advance.

Biochemistry

The

of oxalic acid is the hydrogenoxalate anion, and its conjugate base (

) is a

competitive inhibitor Competitive inhibition is interruption of a chemical pathway owing to one chemical substance inhibiting the effect of another by competing with it for binding or bonding. Any metabolic or chemical messenger system can potentially be affected b ...

of the

lactate dehydrogenase Lactate dehydrogenase (LDH or LD) is an enzyme found in nearly all living cells. LDH catalyzes the conversion of pyruvic acid, pyruvate to lactic acid, lactate and back, as it converts NAD+ to NADH and back. A dehydrogenase is an enzyme that t ...

(LDH) enzyme. LDH catalyses the conversion of

pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell. Pyruvic ...

lactic acid Lactic acid is an organic acid. It has the molecular formula C3H6O3. It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as wel ...

(end product of the fermentation (anaerobic) process) oxidising the coenzyme

NADH Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an ade ...

to NAD⁺ and H⁺ concurrently. Restoring NAD⁺ levels is essential to the continuation of anaerobic energy metabolism through

glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...

. As cancer cells preferentially use anaerobic metabolism (see Warburg effect) inhibition of LDH has been shown to inhibit tumor formation and growth, thus is an interesting potential course of cancer treatment. Oxalic acid plays a key role in the interaction between pathogenic fungi and plants. Small amounts of oxalic acid enhances plant resistance to fungi, but higher amounts cause widespread programmed cell death of the plant and help with fungi infection. Plants normally produce it in small amounts, but some pathogenic fungi such as ''

Sclerotinia sclerotiorum ''Sclerotinia sclerotiorum'' is a plant pathogenic fungus and can cause a disease called white mold if conditions are conducive. ''S. sclerotiorum'' can also be known as cottony rot, watery soft rot, stem rot, drop, crown rot and blossom blight. ...

'' cause a toxic accumulation. Oxalate, besides being biosynthesised, may also be biodegraded. ''

Oxalobacter formigenes ''Oxalobacter formigenes'' is a Gram-negative bacteria, Gram negative oxalate-degrading Anaerobic organism, anaerobic bacterium that was first isolated from the gastrointestinal tract of a sheep in 1985. To date, the bacterium has been found to ...

'' is an important gut bacterium that helps animals (including humans) degrade oxalate.

Applications

Oxalic acid's main applications include cleaning or bleaching, especially for the removal of rust (iron complexing agent). Its utility in rust removal agents is due to its forming a stable, water-soluble salt with ferric iron,

ion. Oxalic acid is an ingredient in some tooth whitening products. About 25% of produced oxalic acid is used as a

mordant A mordant or dye fixative is a substance used to set (i.e., bind) dyes on fabrics. It does this by forming a coordination complex with the dye, which then attaches to the fabric (or tissue). It may be used for dyeing fabrics or for intensifying ...

in dyeing processes. It is also used in bleaches, especially for

pulpwood Pulpwood can be defined as timber that is ground and processed into a fibrous pulp. It is a versatile natural resource commonly used for Papermaking, paper-making but also made into low-grade wood and used for chips, energy, pellets, and engineered ...

, cork, straw, cane, feathers, and for rust removal and other cleaning, in baking powder, and as a third reagent in silica analysis instruments.

Niche uses

Oxalic acid is used by some

beekeeper A beekeeper is a person who keeps honey bees, a profession known as beekeeping. The term beekeeper refers to a person who keeps honey bees in beehives, boxes, or other receptacles. The beekeeper does not control the creatures. The beekeeper ow ...

s as a

miticide Acaricides are pesticides that kill members of the arachnid subclass ''Acari'', which includes ticks and mites. Acaricides are used both in medicine and agriculture, although the desired selective toxicity differs between the two fields. Termino ...

against the parasitic

varroa mite ''Varroa destructor'', the Varroa mite, is an external parasitic mite that attacks and feeds on honey bees and is one of the most damaging honey bee pests in the world. A significant mite infestation leads to the death of a honey bee colony, us ...

. Dilute solutions (0.05–0.15 M) of oxalic acid can be used to remove iron from clays such as

kaolinite Kaolinite ( ; also called kaolin) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral sheet of alumina () ...

to produce light-colored

ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...

s. Oxalic acid can be used to clean minerals like many other acids. Two such examples are quartz crystals and pyrite. Oxalic acid is sometimes used in the aluminum

anodizing Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called ''anodizing'' because the part to be treated forms the anode electrode of an electr ...

process, with or without sulfuric acid. Compared to sulfuric-acid anodizing, the coatings obtained are thinner and exhibit lower surface roughness. Oxalic acid is also widely used as a wood bleach, most often in its crystalline form to be mixed with water to its proper dilution for use.

Semiconductor industry

Oxalic acid is also used in electronic and semiconductor industries. In 2006 it was reported being used in electrochemical–mechanical planarization of copper layers in the semiconductor devices fabrication process.

Proposed uses

Reduction of

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

to oxalic acid by various methods, such as

electrocatalysis An electrocatalyst is a catalyst that participates in electrochemical reactions. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or, most commonly, may be the electrode surface itself. An electrocatalyst ca ...

using a

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

complex, is under study as a proposed chemical intermediate for

carbon capture and utilization Carbon capture and storage (CCS) is a process by which carbon dioxide (CO2) from industrial installations is separated before it is released into the atmosphere, then transported to a long-term storage location.IPCC, 2021Annex VII: Glossary at ...

Content in food items

Toxicity

Oxalic acid has an oral LD_Lo (lowest published lethal dose) of 600 mg/kg. It has been reported that the lethal oral dose is 15 to 30 grams. The toxicity of oxalic acid is due to kidney failure caused by precipitation of solid

. Oxalate is known to cause

mitochondrial dysfunction Mitochondrial disease is a group of disorders caused by mitochondrial dysfunction. Mitochondria are the organelles that generate energy for the cell and are found in every cell of the human body except red blood cells. They convert the energy of ...

. Ingestion of

results in oxalic acid as a metabolite which can also cause acute kidney failure.

Kidney stones

Most

kidney stone Kidney stone disease (known as nephrolithiasis, renal calculus disease, or urolithiasis) is a crystallopathy and occurs when there are too many minerals in the urine and not enough liquid or hydration. This imbalance causes tiny pieces of cr ...

s, 76%, are composed of

Notes

Unless otherwise cited, all measurements are based on raw vegetable weights with original moisture content.

References

External links

Oxalic acid MS Spectrum
*

* * ttps://web.archive.org/web/20110902070929/http://www.ohf.org/docs/Oxalate2008.pdf Oxalosis & Hyperoxaluria Foundation (OHF) The Oxalate Content of Food 2008 (PDF)
Oxalosis & Hyperoxaluria Foundation (OHF) Diet Information

Calculator: Water and solute activities in aqueous oxalic acid
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