Oxalic acid

Last updated
Oxalic acid
Oxalsaure2.svg
Oxalic acid molecule ball from xtal.png
Oxalic acid molecule spacefill from xtal.png
Oxalic acid dihydrate.jpg
Names
Preferred IUPAC name
Oxalic acid [1]
Systematic IUPAC name
Ethanedioic acid [1]
Other names
Wood bleach, Crab Acid
Identifiers
3D model (JSmol)
3DMet
385686
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.005.123 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 205-634-3
2208
KEGG
MeSH Oxalic+acid
PubChem CID
RTECS number
  • RO2450000
UNII
UN number 3261
  • InChI=1S/C6H6O6/c3-1(4)2(5)6/h(H,3,4)(H,5,6) Yes check.svgY
    Key: MUBZPKHOEPUJKR-UHFFFAOYSA-N Yes check.svgY
  • OC(=O)C(=O)O
Properties
C2H2O4
Molar mass 90.034 g·mol−1(anhydrous)
126.065 g·mol−1 (dihydrate)
AppearanceWhite crystals
Odor Odorless
Density 1.90 g·cm3 (anhydrous, at 17 °C) [2]
1.653 g·cm−3 (dihydrate)
Melting point 189 to 191 °C (372 to 376 °F; 462 to 464 K)
101.5 °C (214.7 °F; 374.6 K) dihydrate
46.9 g/L (5 °C), 57.2 (10 °C), 75.5 (15 °C), 95.5 (20 °C), 118 (25 °C), 139 (30 °C), 178 (35 °C), 217 (40 °C), 261 (45 °C), 315 (50 °C), 376 (55 °C), 426 (60 °C), 548 (65 °C) [3]
Solubility 237 g/L (15 °C) in ethanol
14 g/L (15 °C) in diethyl ether [4]
Vapor pressure <0.001 mmHg (20 °C) [5]
Acidity (pKa)1.25, 4.14 [6]
Conjugate base Hydrogenoxalate
−60.05·10−6 cm3/mol
Thermochemistry [7]
91.0 J·mol−1·K−1
109.8 J·mol−1·K−1
−829.9 kJ·mol−1
Pharmacology
QP53AG03 ( WHO )
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
Corrosive
GHS labelling:
GHS-pictogram-acid.svg GHS-pictogram-exclam.svg
H302+H312, H318, H402
P264, P270, P273, P280, P301+P312+P330, P302+P352+P312, P305+P351+P338+P310, P362+P364, P501
NFPA 704 (fire diamond)
3
1
0
Flash point 166 °C (331 °F; 439 K)
Lethal dose or concentration (LD, LC):
1000 mg/kg (dog, oral)
1400 mg/kg (rat)
7500 mg/kg (rat, oral) [8]
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 1 mg/m3 [5]
REL (Recommended)
TWA 1 mg/m3 ST 2 mg/m3 [5]
IDLH (Immediate danger)
500 mg/m3 [5]
Safety data sheet (SDS) External MSDS
Related compounds
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Oxalic acid is an organic acid with the IUPAC name ethanedioic acid and formula HO2C−CO2H. It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name comes from the fact that early investigators isolated oxalic acid from flowering plants of the genus Oxalis , commonly known as wood-sorrels. It occurs naturally in many foods. Excessive ingestion of oxalic acid or prolonged skin contact can be dangerous.

Contents

Oxalic acid has much greater acid strength than acetic acid. It is a reducing agent [9] and its conjugate base, known as oxalate (C2O2−4), is a chelating agent for metal cations. Typically, oxalic acid occurs as the dihydrate with the formula C2H2O4·2H2O.

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 isolated a salt from wood sorrel. [10] By 1773, François Pierre Savary of Fribourg, Switzerland had isolated oxalic acid from its salt in sorrel. [11]

In 1776, Swedish chemists Carl Wilhelm Scheele and Torbern Olof Bergman [12] produced oxalic acid by reacting sugar with concentrated 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. [13]

In 1824, the German chemist Friedrich Wöhler obtained oxalic acid by reacting cyanogen with ammonia in aqueous solution. [14] This experiment may represent the first synthesis of a natural product. [15]

Preparation

Oxalic acid is mainly manufactured by the oxidation of carbohydrates or glucose using nitric acid or air in the presence of vanadium pentoxide. A variety of precursors can be used including glycolic acid and ethylene glycol. [16] A newer method entails oxidative carbonylation of alcohols to give the diesters of oxalic acid:

4 ROH + 4 CO + O2 → 2 (CO2R)2 + 2 H2O

These diesters are subsequently hydrolyzed to oxalic acid. Approximately 120,000 tonnes are produced annually. [15]

Historically oxalic acid was obtained exclusively by using caustics, such as sodium or potassium hydroxide, on sawdust, followed by acidification of the oxalate by mineral acids, such as sulfuric acid. [17] Oxalic acid can also be formed by the heating of sodium formate in the presence of an alkaline catalyst. [18]

Laboratory methods

Although it can be readily purchased, oxalic acid can be prepared in the laboratory by oxidizing sucrose using nitric acid in the presence of a small amount of vanadium pentoxide as a catalyst. [19]

The hydrated solid can be dehydrated with heat or by azeotropic distillation. [20]

Developed in the Netherlands, an electrocatalysis by a copper complex helps reduce carbon dioxide to oxalic acid; [21] this conversion uses carbon dioxide as a feedstock 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. [22] Because the anhydrous material is both acidic and hydrophilic (water seeking), it is used in esterifications.

Dihydrate

The dihydrate H
2
C
2
O
4
.2H
2
O
has space group C52hP21/n, with lattice parameters a = 611.9 pm, b = 360.7 pm, c = 1205.7 pm, β = 106°19', Z = 2. [23] The main inter-atomic distances are: C−C 153 pm, C−O1 129 pm, C−O2 119 pm. [24]

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), the a and c lattice parameters increase as the stress decreases from −1.17 GPa to −0.12 GPa and from −1.17 GPa to −0.51 GPa, respectively. [25]

Reactions

Acid-base properties

Oxalic acid's pKa values vary in the literature from 1.25–1.46 and 3.81–4.40. [26] [27] [28] The 100th ed of the CRC, released in 2019, has values of 1.25 and 3.81. [29] Oxalic acid is relatively strong compared to other carboxylic acids:

C2O4H2 ⇌ C2O4H + H+          pKa = 1.27
C2O4HC
2
O2−
4
+ H+
          pKa = 4.27

Oxalic acid undergoes many of the reactions characteristic of other carboxylic acids. It forms esters such as dimethyl oxalate (m.p. 52.5 to 53.5 °C, 126.5 to 128.3 °F). [30] It forms an acid chloride called oxalyl chloride.

Metal-binding properties

Transition metal oxalate complexes are numerous, e.g. the drug oxaliplatin. Oxalic acid has shown to reduce manganese dioxide MnO
2
in manganese ores to allow the leaching of the metal by sulfuric acid. [31]

Oxalic acid is an important reagent in lanthanide chemistry. Hydrated lanthanide oxalates form readily in very strongly acidic solutions as a densely crystalline, easily filtered form, largely free of contamination by nonlanthanide elements:

2 Ln3+ + 3 C2O4H2 → Ln2(C2O4)3 + 6 H+

Thermal decomposition of these oxalates gives the oxides, which is the most commonly marketed form of these elements. [32]

Other

Oxalic acid and oxalates can be oxidized by permanganate in an autocatalytic reaction. [33]

Oxalic acid vapor decomposes at 125–175 °C into carbon dioxide CO
2
and formic acid HCOOH. Photolysis with 237–313 nm UV light also produces carbon monoxide CO and water. [34]

Evaporation of a solution of urea and oxalic acid in 2:1 molar ratio yields a solid crystalline compound H
2
C
2
O
4
.[CO(NH
2
)
2
]
2
, consisting of stacked two-dimensional networks of the neutral molecules held together by hydrogen bonds with the oxygen atoms. [35]

Occurrence

Biosynthesis

At least two pathways exist for the enzyme-mediated formation of oxalate. In one pathway, oxaloacetate, a component of the Krebs citric acid cycle, is hydrolyzed to oxalate and acetic acid by the enzyme oxaloacetase: [36]

[O2CC(O)CH2CO2]2− + H2O → C
2
O2−
4
+ CH
3
CO
2
+ H+

It also arises from the dehydrogenation of glycolic acid, which is produced by the metabolism of ethylene glycol.

Occurrence in foods and plants

Stems of Oxalis triangularis contain oxalic acid. OxalisTriangularis.jpg
Stems of Oxalis triangularis contain oxalic acid.

Early investigators isolated oxalic acid from wood-sorrel (Oxalis). Members of the spinach family and the brassicas (cabbage, broccoli, brussels sprouts) are high in oxalates, as are sorrel and umbellifers like parsley. [37] The leaves and stems of all species of the genus Chenopodium and related genera of the family Amaranthaceae, which includes quinoa, contain high levels of oxalic acid, [38] . Rhubarb leaves contain about 0.5% oxalic acid, and jack-in-the-pulpit ( Arisaema triphyllum ) contains calcium oxalate crystals. Similarly, the 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 carbohydrates. [15]

Plants of the genus Fenestraria produce optical fibers made from crystalline oxalic acid to transmit light to subterranean photosynthetic sites. [39]

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. [40]

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. [41] [42]

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. [43] [44] Some fungi such as Aspergillus niger have been extensively studied for the industrial production of oxalic acid; [45] however, those processes are not yet economically competitive with production from oil and gas. [46]

Biochemistry

The conjugate base of oxalic acid is the hydrogenoxalate anion, and its conjugate base (oxalate) is a competitive inhibitor of the lactate dehydrogenase (LDH) enzyme. [47] LDH catalyses the conversion of pyruvate to lactic acid (end product of the fermentation (anaerobic) process) oxidising the coenzyme NADH to NAD+ and 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) inhibition of LDH has been shown to inhibit tumor formation and growth, [48] thus is an interesting potential course of cancer treatment.

Oxalic acid plays an 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 cause a toxic accumulation. [49]

Oxalate, besides being biosynthesised, may also be biodegraded. Oxalobacter formigenes is an important gut bacteria that helps animals (including humans) degrade oxalate. [50]

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, ferrioxalate ion. The cleaning product Zud contains oxalic acid. [51] 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 bleaches, especially for pulpwood, and for rust removal and other cleaning, in baking powder, [15] and as a third reagent in silica analysis instruments.

Niche uses

Honeybee coated with oxalate crystals Beecrystals.PNG
Honeybee coated with oxalate crystals

Oxalic acid is used by some beekeepers as a miticide against the parasitic varroa mite. [52] Thymovar combined with an oxalic acid treatment has proved effective against the varroa mite. [53]

Dilute solutions (0.05–0.15 M) of oxalic acid can be used to remove iron from clays such as kaolinite to produce light-colored ceramics. [54]

Oxalic acid is used to clean minerals. [55] [56]

Oxalic acid is sometimes used in the aluminum anodizing process, with or without sulfuric acid. [57] 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. [58]

Content in food items

[59] [ clarification needed ]

VegetableContent of oxalic acid
(%) a
Amaranth
Asparagus
Beans, snap
Beet leaves
Beetroot
Broccoli
Brussels sprouts
Cabbage
Carrot
Cassava
Cauliflower
Celery
Chicory
Chives
Collards
Coriander
Corn, sweet
Cucumber
Eggplant
Endive
Garlic
Kale
Lettuce
Okra
Onion
Parsley
Parsnip
Pea
Bell pepper
Potato
Purslane
Radish
Rhubarb leaves
Rutabaga
Spinach
Squash
Sweet potato
Swiss chard, green
Tomato
Turnip
Turnip greens
Watercress

Toxicity

Oxalic acid has an oral LDLo (lowest published lethal dose) of 600 mg/kg. [63] It has been reported that the lethal oral dose is 15 to 30 grams. [64] The toxicity of oxalic acid is due to kidney failure caused by precipitation of solid calcium oxalate. [65]

Oxalate is known to cause mitochondrial dysfunction. [66]

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

Kidney stones

Most kidney stones, 76%, are composed of the calcium salt of oxalic acid. [67]

Other effects

Oxalic acid can cause joint pain by formation of precipitates in the joints.

Calcium hydroxide decreases urinary oxalate in both humans and rats. [68]

Notes

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

Related Research Articles

<span class="mw-page-title-main">Rhubarb</span> Species of herbaceous perennial plant with fleshy, sour edible stalks

Rhubarb is the fleshy, edible stalks (petioles) of species and hybrids of Rheum in the family Polygonaceae, which are cooked and used for food. The whole plant – a herbaceous perennial growing from short, thick rhizomes – is also called rhubarb. Historically, different plants have been called "rhubarb" in English. The large, triangular leaves contain high levels of oxalic acid and anthrone glycosides, making them inedible. The small flowers are grouped in large compound leafy greenish-white to rose-red inflorescences.

<span class="mw-page-title-main">Calcium oxalate</span> Calcium compound

Calcium oxalate (in archaic terminology, oxalate of lime) is a calcium salt of oxalic acid with the chemical formula CaC2O4. It forms hydrates CaC2O4·nH2O, where n varies from 1 to 3. Anhydrous and all hydrated forms are colorless or white. The monohydrate CaC2O4·H2O occurs naturally as the mineral whewellite, forming envelope-shaped crystals, known in plants as raphides. The two rarer hydrates are dihydrate CaC2O4·2H2O, which occurs naturally as the mineral weddellite, and trihydrate CaC2O4·3H2O, which occurs naturally as the mineral caoxite, are also recognized. Some foods have high quantities of calcium oxalates and can produce sores and numbing on ingestion and may even be fatal. Tribes with diets that depend highly on fruits and vegetables high in calcium oxalate, such as in Micronesia, reduce the level of it by boiling and cooking them. They are a constituent in 76% of human kidney stones. Calcium oxalate is also found in beerstone, a scale that forms on containers used in breweries.

<span class="mw-page-title-main">Oxalate</span> Any derivative of oxalic acid; chemical compound containing oxalate moiety

Oxalate (IUPAC: ethanedioate) is an anion with the formula C2O42−. This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (Na2C2O4), and several esters such as dimethyl oxalate (C2O4(CH3)2). It is a conjugate base of oxalic acid. At neutral pH in aqueous solution, oxalic acid converts completely to oxalate.

<i>Oxalis</i> Genus of flowering plants

Oxalis 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 diversity is particularly rich in tropical Brazil, Mexico, and South Africa.

<span class="mw-page-title-main">Sodium oxalate</span> Chemical compound

Sodium oxalate, or disodium oxalate, is the sodium salt of oxalic acid with the formula Na2C2O4. It is a white, crystalline, odorless solid, that decomposes above 290 °C.

<span class="mw-page-title-main">Glyoxylic acid</span> Chemical compound

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.

<span class="mw-page-title-main">Dicalcium phosphate</span> Chemical compound

Dicalcium phosphate is the calcium phosphate with the formula CaHPO4 and its dihydrate. The "di" prefix in the common name arises because the formation of the HPO42– anion involves the removal of two protons from phosphoric acid, H3PO4. It is also known as dibasic calcium phosphate or calcium monohydrogen phosphate. Dicalcium phosphate is used as a food additive, it is found in some toothpastes as a polishing agent and is a biomaterial.

<span class="mw-page-title-main">Potassium hydrogenoxalate</span> Chemical compound, salt of sorrel

Potassium hydrogenoxalate is a salt with formula KHC2O4 or K+·HO2C-CO2. It is one of the most common salts of the hydrogenoxalate anion, and can be obtained by reacting potassium hydroxide with oxalic acid in 1:1 mole ratio.

<span class="mw-page-title-main">Ammonium oxalate</span> Chemical compound

Ammonium oxalate, C2H8N2O4 – more commonly written as (NH4)2C2O4 – is an oxalate salt with ammonium (sometimes as a monohydrate). It is a colorless (white) salt under standard conditions and is odorless and non-volatile. It is the ammonium salt of oxalic acid, and occurs in many plants and vegetables.

<span class="mw-page-title-main">Magnesium oxalate</span> Magnesium compound

Magnesium oxalate is an organic compound comprising a magnesium cation with a 2+ charge bonded to an oxalate anion. It has the chemical formula MgC2O4. Magnesium oxalate is a white solid that comes in two forms: an anhydrous form and a dihydrate form where two water molecules are complexed with the structure. Both forms are practically insoluble in water and are insoluble in organic solutions.

Chromium(II) oxalate is an inorganic compound with the chemical formula CrC2O4.

<span class="mw-page-title-main">Sodium hydrogenoxalate</span> Partly deprotonated oxalic acid

Sodium hydrogenoxalate is salt of formula NaHC
2
O
4
, consisting of sodium cations Na+
and hydrogenoxalate anions HC
2
O
4
or -. The anion can be described as the result of removing one hydrogen ion H+
from oxalic acid H
2
C
2
O
4
, or adding one to the oxalate anion C
2
O2−
4
.

The nickel organic acid salts are organic acid salts of nickel. In many of these the ionised organic acid acts as a ligand.

<span class="mw-page-title-main">Beryllium oxalate</span> Chemical compound

Beryllium oxalate is an inorganic compound, a salt of beryllium metal and oxalic acid with the chemical formula C
2
BeO
4
. It forms colorless crystals, dissolves in water, and also forms crystalline hydrates. The compound is used to prepare ultra-pure beryllium oxide by thermal decomposition.

Praseodymium(III) oxalate is an inorganic compound, a salt of praseodymium metal and oxalic acid with the chemical formula C6O12Pr2. The compound forms light green crystals, insoluble in water, also forms crystalline hydrates.

<span class="mw-page-title-main">Copper oxalate</span> Chemical compound

Copper oxalate is an inorganic compound, a salt of copper metal and oxalic acid with the chemical formula CuC
2
O
4
. The compound is practically insoluble in water, alcohol, ether, and acetic acid but soluble in ammonium hydroxide. Copper oxalate forms a hydrate, which forms acid-blue crystals.

<span class="mw-page-title-main">Yttrium oxalate</span> Chemical compound

Yttrium oxalate is an inorganic compound, a salt of yttrium and oxalic acid with the chemical formula Y2(C2O4)3. The compound does not dissolve in water and forms crystalline hydrates—colorless crystals.

Manganese oxalate is a chemical compound, a salt of manganese and oxalic acid with the chemical formula MnC
2
O
4
. The compound creates light pink crystals, does not dissolve in water, and forms crystalline hydrates. It occurs naturally as the mineral Lindbergite.

<span class="mw-page-title-main">Tin(II) oxalate</span> Chemical compound

Tin(II) oxalate is an inorganic compound, a salt of tin and oxalic acid with the chemical formula SnC
2
O
4
. The compound looks like colorless crystals, does not dissolve in water, and forms crystalline hydrates.

Samarium(III) oxalate is an inorganic compound, a salt of samarium and oxalic acid with the formula Sm2(C2O4)3. The compound does not dissolve in water, forms a crystalline hydrate with yellow crystals.

References

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  2. Record in the GESTIS Substance Database of the Institute for Occupational Safety and Health
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  10. See:
    • Herman Boerhaave, Elementa Chemiae (Basil, Switzerland: Johann Rudolph Im-hoff, 1745), volume 2, pp. 35-38. (in Latin) From p. 35: "Processus VII. Sal nativum plantarum paratus de succo illarum recens presso. Hic Acetosae." (Procedure 7. A natural salt of plants prepared from their freshly pressed juice. This [salt obtained] from sorrel.)
    • Henry Enfield Roscoe and Carl Schorlemmer, ed.s, A Treatise on Chemistry (New York, New York: D. Appleton and Co., 1890), volume 3, part 2, p. 105.
    • See also Wikipedia's articles "Oxalis acetosella" and "Potassium hydrogen oxalate".
  11. See:
    • François Pierre Savary, Dissertatio Inauguralis De Sale Essentiali Acetosellæ [Inaugural dissertation on the essential salt of wood sorrel] (Jean François Le Roux, 1773). (in Latin) Savary noticed that when he distilled sorrel salt (potassium hydrogen oxalate), crystals would sublimate onto the receiver. From p. 17: "Unum adhuc circa liquorem acidum, quem sal acetosellae tam sincerissimum a nobis paratum quam venale destillatione fundit phoenomenon erit notandum, nimirum quod aliquid ejus sub forma sicca crystallina lateribus excipuli accrescat, ..." (One more [thing] will be noted regarding the acid liquid, which furnished for us sorrel salt as pure as commercial distillations, [it] produces a phenomenon, that evidently something in dry, crystalline form grows on the sides of the receiver, ...) These were crystals of oxalic acid.
    • Leopold Gmelin with Henry Watts, trans., Hand-book of Chemistry (London, England: Cavendish Society, 1855), volume 9, p. 111.
  12. See:
    • Torbern Bergman with Johan Afzelius (1776) Dissertatio chemica de acido sacchari [Chemical dissertation on sugar acid] (Uppsala, Sweden: Edman, 1776).
    • Torbern Bergman, Opuscula Physica et Chemica, (Leipzig (Lipsia), (Germany): I.G. Müller, 1776), volume 1, "VIII. De acido Sacchari," pp. 238-263.
  13. Carl Wilhelm Scheele (1784) "Om Rhabarber-jordens bestånds-delar, samt sått at tilreda Acetosell-syran" (On rhubarb-earth's constituents, as well as ways of preparing sorrel-acid), Kungliga Vetenskapsakademiens Nya Handlingar [New Proceedings of the Royal Academy of Science], 2nd series, 5 : 183-187. (in Swedish) From p. 187: "Således finnes just samma syra som vi genom konst af socker med tilhjelp af salpeter-syra tilreda, redan förut af naturen tilredd uti o̊rten Acetosella." (Thus it is concluded [that] the very same acid as we prepare artificially by means of sugar with the help of nitric acid, [was] previously prepared naturally in the herb acetosella [i.e., sorrel].)
  14. See:
    • F. Wöhler (1824) "Om några föreningar af Cyan" (On some compounds of cyanide), Kungliga Vetenskapsakademiens Handlingar [Proceedings of the Royal Academy of Science], pp. 328-333. (in Swedish)
    • Reprinted in German as: F. Wöhler (1825) "Ueber Cyan-Verbindungen" (On cyanide compounds), Annalen der Physik und Chemie, 2nd series, 3 : 177-182.
  15. 1 2 3 4 Wilhelm Riemenschneider, Minoru Tanifuji "Oxalic acid" in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. doi : 10.1002/14356007.a18_247.
  16. Eiichi, Yonemitsu; Tomiya, Isshiki; Tsuyoshi, Suzuki and Yukio, Yashima "Process for the production of oxalic acid", U.S. Patent 3,678,107 , priority date March 15, 1969
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