oxalic acid
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Oxalic acid is an
organic acid An organic acid is an with ic properties. The most common organic acids are the s, whose acidity is associated with their  –COOH. s, containing the group –SO2OH, are relatively stronger acids. Alcohols, with , can act as acids but t ...
with the IUPAC name ethanedioic acid and
formula In , a formula is a concise way of expressing information symbolically, as in a mathematical formula or a . The informal use of the term ''formula'' in science refers to the . The plural of ''formula'' can be either ''formulas'' (from the mos ...
. It is the simplest
dicarboxylic acidA dicarboxylic acid is an organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecul ...
. It is a white crystalline solid that forms a colorless solution in water. Its name comes from the fact that early investigators isolated oxalic acid from
flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greec ...

flowering plant
s of the genus ''
Oxalis ''Oxalis'' ( (American English) or (British English)) is a large genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Vi ...

Oxalis
'', commonly known as wood-sorrels. It occurs naturally in many foods, but excessive ingestion of oxalic acid or prolonged skin contact can be dangerous. Oxalic acid has much greater acid strength than
acetic acid Acetic acid , systematically named ethanoic acid , is a colourless liquid with the CH3COOH (also written as CH3CO2H, C2H4O2, or HC2H3O2). is no less than 4% acetic acid by volume, making acetic acid the main component of vinegar apart fr ...

acetic acid
. It is a
reducing agent A reducing agent (also called a reductant, reducer, or electron donor) is an element or compound that loses or "donates" an electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom ...
and its
conjugate base A conjugate acid, within the , is a chemical compound formed when an acid () to a —in other words, it is a base with a ion added to it, as in the reverse reaction it loses a hydrogen ion. On the other hand, a conjugate base is what is left ...
, known as
oxalate Oxalate (IUPAC: ethanedioate) is an anion An ion () is a particle, atom or molecule with a net electric charge, electrical charge. The charge of the electron is considered negative by convention. The negative charge of an ion is equal and ...
(), is a
chelating agent Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a Denticity, polydentate (multiple bonded) ligand and a single central metal atom. These l ...
for metal cations. Typically, oxalic acid occurs as the
dihydrate In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they under ...
with the formula .


History

The preparation of salts of oxalic acid (crab acid) from plants had been known, at least since 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. https://www.jstor.org/stable/20395297.) was a Dutch botanist ...

Herman Boerhaave
isolated a salt from
wood sorrel ''Oxalis'' ( (American English) or (British English)) is a large genus of flowering plants in the wood-sorrel family Oxalidaceae, comprising about 570 species. The genus occurs throughout most of the world, except for the Polar region, polar ar ...

wood sorrel
. 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 German and Swedish Pomerania Swedish Pomerania ( sv, Svenska Pommern; german: Schwedisch-Pommern) was a Dominion The word Dominion was used from 1907 to 1948 to refer to one of ...

Carl Wilhelm Scheele
and
Torbern Olof Bergman Torbern Olaf (Olof) Bergman (''KVO'') (20 March 17358 July 1784) was a Swedish chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist A scientist is a person who conducts ...

Torbern Olof Bergman
produced oxalic acid by reacting sugar with concentrated
nitric acid Nitric acid (), also known as ''aqua fortis'' (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latiu ...

nitric acid
; 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. In 1824, the German chemist
Friedrich Wöhler Friedrich Wöhler () FRS(For) HFRSE Fellowship of the Royal Society of Edinburgh (FRSE) is an award granted to individuals that the Royal Society of Edinburgh, Scotland's national academy of science and Literature, letters, judged to be "em ...

Friedrich Wöhler
obtained oxalic acid by reacting
cyanogen Cyanogen is the chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by chemica ...

cyanogen
with ammonia in aqueous solution. This experiment may represent the first synthesis of a
natural product A natural product is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together b ...
.


Preparation

Oxalic acid is mainly manufactured by the oxidation of
carbohydrate is a disaccharide found in animal milk. It consists of a molecule of D-galactose and a molecule of D-glucose bonded by beta-1-4 glycosidic linkage. A carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) ato ...
s or
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
using
nitric acid Nitric acid (), also known as ''aqua fortis'' (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latiu ...

nitric acid
or air in the presence of
vanadium pentoxide Vanadium(V) oxide (''vanadia'') is the inorganic compound In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition ...

vanadium pentoxide
. A variety of precursors can be used including
glycolic acid Glycolic acid (hydroxyacetic acid, or hydroacetic acid); chemical formula C2H4O3 (also written as HOCH2CO2H), is the smallest α-hydroxy acid (AHA). This colorless, odorless, and hygroscopic crystalline solid is highly soluble in water. It is used ...

glycolic acid
and
ethylene glycol Ethylene glycol (IUPAC name In chemical nomenclatureA chemical nomenclature is a set of rules to generate systematic names for chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecula ...

ethylene glycol
. A newer method entails oxidative
carbonylation Carbonylation refers to reaction Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry *Chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic tra ...
of
alcohol In , alcohol is an that carries at least one (−OH) bound to a atom. The term alcohol originally referred to the primary alcohol (ethyl alcohol), which is and is the main alcohol present in s. An important class of alcohols, of which ...

alcohol
s to give the diesters of oxalic acid: These diesters are subsequently hydrolyzed to oxalic acid. Approximately 120,000
tonne The tonne ( or ; symbol: t) is a metric unit of mass equal to 1,000 kilogram The kilogram (also kilogramme) is the base unit of mass in the International System of Units (SI), the current metric system, having the unit symbol kg. I ...
s are produced annually.Wilhelm Riemenschneider, Minoru Tanifuji "Oxalic acid" in ''Ullmann's Encyclopedia of Industrial Chemistry'', 2002, Wiley-VCH, Weinheim. . Historically oxalic acid was obtained exclusively by using caustics, such as sodium or
potassium hydroxide Potassium hydroxide is an inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. However, the distinction is not clearly defi ...

potassium hydroxide
, on sawdust. Pyrolysis 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 ...

sodium formate
(ultimately prepared from carbon monoxide), leads to the formation of
sodium oxalate Sodium oxalate, or disodium oxalate, is the sodium Sodium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance ...

sodium oxalate
, easily converted to oxalic acid.


Laboratory methods

Although it can be readily purchased, oxalic acid can be prepared in the laboratory by
oxidizing (mild reducing agent) are added to powdered potassium permanganate Potassium permanganate is an inorganic compound with the chemical formula KMnO4 and composed of potassium ion, K+ and permanganate, . It is a purplish-black crystalline salt, th ...
sucrose Sucrose is a type of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated sugar, or regular sugar, refers to sucrose, a disaccharide composed of glucose a ...

sucrose
using
nitric acid Nitric acid (), also known as ''aqua fortis'' (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latiu ...

nitric acid
in the presence of a small amount of
vanadium pentoxide Vanadium(V) oxide (''vanadia'') is the inorganic compound In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition ...

vanadium pentoxide
as a
catalyst that utilizes a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide at room temperature. It can also remove formaldehyde from the air. Catalysis () is the process of increasing the reaction rate, rate of a ...

catalyst
.''Practical Organic Chemistry'' by Julius B. Cohen, 1930 ed. preparation #42 The hydrated solid can be dehydrated with heat or by
azeotropic distillation In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they under ...
. Developed in the Netherlands, an electrocatalysis by a
copper Copper is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

copper
complex helps to oxalic acid; this conversion to generate oxalic acid.


Structure


Anhydrous

Anhydrous oxalic acid exists as two polymorphs; in one the hydrogen-bonding 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 File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atoms ...
(water seeking), it is used in
esterification An ester is a chemical compound derived from an acid (organic or inorganic) in which at least one –OH hydroxyl group is replaced by an –O– alkyl ( alkoxy) group, as in the substitution reaction of a carboxylic acid and an alcohol. Glycer ...

esterification
s.


Dihydrate

The dihydrate .2 has space group ''C''52''h''–''P''21/''n'', with
lattice parameter Image:UnitCell.png, upright=1.3, Unit cell definition using parallelopiped with lengths ''a'', ''b'', ''c'' and angles between the sides given by ''α'', ''β'', ''γ'' The lattice constant, or lattice parameter, refers to the physical dimension of ...
s ''a'' = 611.9 pm, ''b'' = 360.7 pm, ''c'' = 1205.7 pm, ''β'' = 106°19', ''Z'' = 2.T. M. Sabine, G. W. Cox and B. M. Craven (1969): "A neutron diffraction study of lphaoxalic acid dihydrate" ''Acta Crystallographica Section B'', volume B25, pages 2437-2441. The main inter-atomic distances are: C-C 153 pm, C-O1 129 pm, C-O2 119 pm.F. R. Ahmed and D. W. J. Cruickshank (1953): "A refinement of the crystal structure analyses of oxalic acid dihydrate". ''Acta Crystallographica'' volume 6, pages 385-392. Theoretical studies indicate that oxalic acid dihydrate is one of very few crystalline substances that exhibit negative area compressibility. Namely, when subjected to isotropic tension stress (negative
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

pressure
), the ''a'' and ''c''
lattice parameter Image:UnitCell.png, upright=1.3, Unit cell definition using parallelopiped with lengths ''a'', ''b'', ''c'' and angles between the sides given by ''α'', ''β'', ''γ'' The lattice constant, or lattice parameter, refers to the physical dimension of ...
s increase as the stress decreases from −1.17 
GPa The pascal (symbol: Pa) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, ...
to −0.12 GPa and from −1.17 GPa to −0.51 GPa, respectively.Francisco Colmenero (2019): "Negative area compressibility in oxalic acid dihydrate". ''Materials Letters'', volume 245, pages 25-28.


Reactions


Acid-base properties

Oxalic acid's p''K''a values vary in the literature from 1.25–1.46 and 3.81–4.40. The 100th ed of the CRC, released in 2019 has values of 1.25 and 3.81. Oxalic acid is relatively strong for a
carboxylic acid A carboxylic 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 acid ...
: Oxalic acid undergoes many of the reactions characteristic of other carboxylic acids. It forms esters such as
dimethyl oxalate Dimethyl oxalate is the organic compound with the formula (CO2CH3)2. It is the methyl group, 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 e ...
( ). It forms an acid chloride called
oxalyl chloride Oxalyl chloride is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by ch ...

oxalyl chloride
.


Metal-binding properties

Transition metal oxalate complexes are numerous, e.g. the drug
oxaliplatin Oxaliplatin, sold under the brand name Eloxatin, is a cancer medication used to treat colorectal cancer Colorectal cancer (CRC), also known as bowel cancer, colon cancer, or rectal cancer, is the development of cancer from the Colon (anatomy), ...

oxaliplatin
. Oxalic acid has 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 Manganese is a chemical element Image:Simple Periodic Table Chart- ...

manganese dioxide
in
manganese Manganese is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

manganese
ores to allow the leaching of the metal by
sulfuric acid Sulfuric acid ( and the ) or sulphuric acid (), known in antiquity as oil of vitriol, is a composed of the elements , and , with the . It is a colorless, odorless and liquid that is with water. Pure sulfuric acid does not exist naturally ...

sulfuric acid
. Oxalic acid is an important reagent in
lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an elem ...
chemistry. Hydrated lanthanide oxalates form readily in very strongly acidic solutions as a densely
crystalline A crystal or crystalline solid is a solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together and contain the least amount of kinet ...

crystalline
, easily filtered form, largely free of contamination by nonlanthanide elements: :2Ln3+ + 3C2O4H2 → Ln2(C2O4)3 + 6H+ Thermal decomposition of these oxalates gives the
oxide of rutile Rutile is a mineral composed primarily of titanium dioxide (TiO2), and is the most common natural form of TiO2. Other rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest re ...
s, which is the most commonly marketed form of these elements.


Other

Oxalic acid and oxalates can be oxidized by
permanganate A permanganate is the general name for a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, elemen ...

permanganate
in an
autocatalytic A single chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes ...
reaction. Oxalic acid vapor decomposes at 125-175 into
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is ...

carbon dioxide
and
formic acid Formic acid, systematically named methanoic acid, is the simplest carboxylic acid, and has the chemical formula HCOOH. It is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. The word "formic" comes ...

formic acid
HCOOH.
Photolysis Photodissociation, photolysis, or photodecomposition is a chemical reaction in which a chemical compound is broken down by photons. It is defined as the interaction of one or more photons with one target molecule. Photodissociation is not limited ...
with 237-313 nm light also produces
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom. It is the simplest molecule of the oxocarbon family. In ...

carbon monoxide
CO and water.James Higgins, Xuefeng Zhou, Ruifeng Liu, and Thomas T.-S. Huang (1997): "Theoretical Study of Thermal Decomposition Mechanism of Oxalic Acid" ''Journal of Physical Chemistry A'', volume 101, issue 14, pages 2702–2708. Evaporation of a solution of
urea Urea, also known as carbamide, is an organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the prop ...

urea
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 A hydrogen bond (or H-bond) is a primarily Electrostatics, electrostatic force of attraction between a hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of ...

hydrogen bond
s with the oxygen atoms.S. Harkema, J. W. Bats, A. M. Weyenberg and D. Feil (1972) "The crystal structure of urea oxalic acid (2:1)". ''Acta Crystallographica Section B'', volume B28, pages 1646-1648.


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 , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemi ...

oxaloacetate
, a component of the
Krebs citric acid cycle The citric acid cycle (CAC) – also known as the TCA cycle (tricarboxylic acid cycle) or the Krebs cycle – is a series of chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of ...
, is hydrolyzed to oxalate and acetic acid by the enzyme oxaloacetase: It also arises from the dehydrogenation of
glycolic acid Glycolic acid (hydroxyacetic acid, or hydroacetic acid); chemical formula C2H4O3 (also written as HOCH2CO2H), is the smallest α-hydroxy acid (AHA). This colorless, odorless, and hygroscopic crystalline solid is highly soluble in water. It is used ...

glycolic acid
, which is produced by the metabolism of
ethylene glycol Ethylene glycol (IUPAC name In chemical nomenclatureA chemical nomenclature is a set of rules to generate systematic names for chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecula ...

ethylene glycol
.


Occurrence in foods and plants

Early investigators isolated oxalic acid from
wood-sorrel ''Oxalis'' ( (American English) or (British English)) is a large genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie ...

wood-sorrel
(''Oxalis''). Members of the
spinach Spinach (''Spinacia oleracea'') is a leafy LEAFY (abbreviated LFY) is a plant gene that causes groups of undifferentiated Cell (biology), cells called meristems to develop into flowers instead of leaves with associated shoots. ''LEAFY'' is invo ...

spinach
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, or mustard plants. Crops from this genus are sometimes called ''cole cro ...
s (
cabbage Cabbage, comprising several cultivars of ''Brassica oleracea ''Brassica oleracea'' is a plant species that includes many common foods as cultivars, including cabbage, broccoli, cauliflower, kale, Brussels sprouts, collard greens, Savoy cabba ...

cabbage
,
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 green ...

broccoli
,
brussels sprouts The brussels sprout is a member of the Gemmifera Group of cabbage Cabbage (comprising several cultivars of ''Brassica oleracea'') is a leafy green, red (purple), or white (pale green) biennial plant grown as an Annual plant, annual vegetab ...
) are high in oxalates, as are
sorrel left, Polish_cuisine.html"_;"title="Sorrel_soup_with_egg_and_croutons,_part_of_Polish_cuisine">Sorrel_soup_with_egg_and_croutons,_part_of_Polish_cuisine_ Sorrel_(''Rumex_acetosa''),_also_called_common_sorrel_or_garden_sorrel,_is_a_perennial ...
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 plan ...
s like parsley. Rhubarb leaves contain about 0.5% oxalic acid, and jack-in-the-pulpit (''Arisaema triphyllum'') contains calcium oxalate crystals. Similarly, the Parthenocissus quinquefolia, Virginia creeper, a common decorative vine, produces oxalic acid in its berries as well as oxalate crystals in the sap, in the form of raphides. Bacteria produce oxalates from oxidation of
carbohydrate is a disaccharide found in animal milk. It consists of a molecule of D-galactose and a molecule of D-glucose bonded by beta-1-4 glycosidic linkage. A carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) ato ...
s. Plants of the genus ''Fenestraria'' produce optical fibers made from crystalline oxalic acid to transmit light to subterranean photosynthetic sites. Carambola, also known as starfruit, also contains oxalic acid along with caramboxin. Citrus juice contains small amounts of oxalic acid. Citrus fruits produced in organic agriculture contain less oxalic acid than those produced in conventional agriculture. The formation of naturally occurring calcium oxalate patinas on certain limestone and marble statues and monuments has been proposed to be caused by the chemical reaction of the carbonate stone with oxalic acid secreted by lichen or other microorganisms.


Production by fungi

Many soil fungus species secrete oxalic acid, resulting in greater solubility of metal cations, increased availability of certain soil nutrients, and can lead to the formation of calcium oxalate crystals. Some fungi such as ''Aspergillus niger'' have been extensively studied for the industrial production of oxalic acid;Hermann Strasser, Wolfgang Burgstaller, Franz Schinner(1994): "High-yield production of oxalic acid for metal leaching processes by ''Aspergillus niger''". ''FEMS Microbiology Letters'', volume 119, issue 3, pages 365–370. 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.


Biochemistry

The
conjugate base A conjugate acid, within the , is a chemical compound formed when an acid () to a —in other words, it is a base with a ion added to it, as in the reverse reaction it loses a hydrogen ion. On the other hand, a conjugate base is what is left ...
of oxalic acid is the hydrogenoxalate anion, and its conjugate base (
oxalate Oxalate (IUPAC: ethanedioate) is an anion An ion () is a particle, atom or molecule with a net electric charge, electrical charge. The charge of the electron is considered negative by convention. The negative charge of an ion is equal and ...
) is a competitive inhibitor of the lactate dehydrogenase (LDH) enzyme. LDH catalyses the conversion of pyruvate to lactic acid (end product of the fermentation (anaerobic) process) oxidising the coenzyme NADH to NAD+, NAD+ and Hydron (chemistry), H+ concurrently. Restoring NAD+ levels is essential to the continuation of anaerobic energy metabolism through glycolysis. As cancer cells preferentially use anaerobic metabolism (see Warburg effect (oncology), Warburg effect) inhibition of LDH has been shown to inhibit tumor formation and growth, thus is an interesting potential course of cancer treatment.


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, Sodium ferrioxalate, ferrioxalate ion. The cleaning product Zud contains oxalic acid. Oxalic acid is an ingredient in some tooth whitening products. About 25% of produced oxalic acid will be used as a mordant in dyeing processes. It is also used in Bleach (chemical), bleaches, especially for pulpwood, 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 beekeepers as a miticide against the parasitic Varroa destructor, varroa mite. Dilute solutions (0.05–0.15 Molar mass, M) of oxalic acid can be used to remove iron from clays such as kaolinite to produce light-colored ceramics.Sung Oh Lee, Tam Tran, Byoung Hi Jung, Seong Jun Kim, and Myong Jun Kim (2007): "Dissolution of iron oxide using oxalic acid". ''Hydrometallurgy'', volume 87, issues 3–4. pages 91-99. Oxalic acid is used to clean minerals. Oxalic acid is sometimes used in the aluminum anodizing 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
wood bleach
most often in its crystalline form to be mixed with water to its proper dilution for use.


Content in food items


Toxicity

Oxalic acid has an oral Lowest published lethal dose, LDLo (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 calcium oxalate. Oxalate is known to cause mitochondrial dysfunction. Ingestion of
ethylene glycol Ethylene glycol (IUPAC name In chemical nomenclatureA chemical nomenclature is a set of rules to generate systematic names for chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecula ...

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


Kidney stones

The most kidney stones, 76%, are composed of the calcium oxalate, calcium salt of oxalic acid. Oxalic acid can also cause joint pain by formation of similar precipitates in the joints. Calcium hydroxide decreases urinary oxalate in both humans and rats. Ingesting both calcium containing foods, such as milk, with food high in oxalic acid, cause the formation of calcium oxalate in the stomach, which is not absorbed into the body.


Notes

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


References


External links


Oxalic acid MS Spectrum
*

*[https://web.archive.org/web/20081016043139/http://www.rhubarbinfo.com/rhubarb-poison.html About rhubarb poisoning (The Rhubarb Compendium)]
Oxalosis & Hyperoxaluria Foundation (OHF) The Oxalate Content of Food 2008 (PDF)Calculator: Water and solute activities in aqueous oxalic acid
{{Authority control Oxalates Household chemicals Dicarboxylic acids Chelating agents Beekeeping Western honey bee medications Nephrotoxins Conjugated ketones