Oxalic acid

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Oxalic acid
IUPAC name

ethanedioic acid
Other names

oxalic acid
Identifiers
CAS number 144-62-7 YesY
PubChem 971
ChemSpider 946 YesY
UNII 9E7R5L6H31 YesY
EC number 205-634-3
UN number 3261
DrugBank DB03902
KEGG C00209 N
MeSH Oxalic+acid
ChEBI CHEBI:16995 YesY
ChEMBL CHEMBL146755 YesY
RTECS number RO2450000
ATCvet code QP53AG03
Beilstein Reference 385686
Gmelin Reference 2208
3DMet B00059
Jmol-3D images Image 1
Properties
Molecular formula C2H2O4
Molar mass 90.03 g mol−1
(anhydrous)
126.07 g mol-1 (dihydrate)
Appearance White crystals
Density 1.90 g cm-3 (anhydrous)
1.653g cm-3 (dihydrate)
Melting point

189-191 °C, 462-464 K, 372-376 °F (101.5 °C (214.7 °F) dihydrate)
Solubility in water 14.3 g/100ml (25 °C)
Solubility 23.7 g/100ml (15 °C) in ethanol
1.4 g/100ml (15 °C) in diethyl ether [1]
Acidity (pKa) 1.25, 4.14[2]
Hazards
MSDS External MSDS
Main hazards Toxic
NFPA 704
NFPA 704.svg
1
3
0
Flash point 166 °C (331 °F)
Related compounds
Related compounds oxalyl chloride
disodium oxalate
calcium oxalate
phenyl oxalate ester
 N (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Oxalic acid is an organic compound with the formula H2C2O4. It is a colorless crystalline solid that dissolves in water to give colorless solutions. It is classified as a dicarboxylic acid. In terms of acid strength, it is much stronger than acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate (C2O42−), is a chelating agent for metal cations. Typically, oxalic acid occurs as the dihydrate with the formula H2C2O4·2H2O. Oral consumption of oxalic acid or prolonged skin contact is dangerous.

Contents

Preparation[edit]

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.[3] 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 metric tons are produced annually.[4]

Laboratory methods[edit]

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.[5]

The hydrated solid can be dehydrated with heat or by azeotropic distillation.[6]

Of historical interest, Wöhler prepared oxalic acid by hydrolysis of cyanogen in 1824. This experiment may represent the first synthesis of a natural product.[4]

Structure[edit]

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.[7] Because the anhydrous material is both acidic and hydrophilic (water seeking), it is used in esterifications.

Reactions[edit]

Oxalic acid is a relatively strong acid, despite being a carboxylic acid:

C2O4H2 → C2O4H− + H+; pKa = 1.27
C2O4H− → C2O42− + 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. 52.5–53.5 °C).[8] It forms an acid chloride called oxalyl chloride.

Oxalate, the conjugate base of oxalic acid, is an excellent ligand for metal ions, e.g. the drug oxaliplatin.

Oxalic acid and oxalates can be oxidized by permanganate in an autocatalytic reaction.[9]

Occurrence[edit]

Biosynthesis[edit]

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:[10]

[O2CC(O)CH2CO2]2- + H2O → C2O42- + CH3CO2-

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

Occurrence in foods and plants[edit]

Calcium oxalate is the most common component of kidney stones. Early investigators isolated oxalic acid from wood-sorrel (Oxalis). Members of the spinach family are high in oxalates, as is sorrel.[11] Rhubarb leaves contain about 0.5% oxalic acid and jack-in-the-pulpit (Arisaema triphyllum) contains calcium oxalate crystals. Bacteria produce oxalates from oxidation of carbohydrates.[4]

Other[edit]

Oxidized bitumen or bitumen exposed to gamma rays also contains oxalic acid among its degradation products. Oxalic acid may increase the leaching of radionuclides conditioned in bitumen for radioactive waste disposal.[citation needed]

Biochemistry[edit]

The conjugate base of oxalic acid (oxalate) is a competitive inhibitor of the lactate dehydrogenase(LDH) enzyme.[12] 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 are 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,[13] thus is an interesting potential course of cancer treatment.

Applications[edit]

About 25% of produced oxalic acid is used as a mordant in dyeing processes. It is used in bleaches, especially for pulpwood. It is also used in baking powder.[4]

Cleaning[edit]

Oxalic acid's main applications include cleaning or bleaching, especially for the removal of rust (iron complexing agent), e.g. Bar Keepers Friend is an example of a household cleaner containing oxalic acid. Its utility in rust removal agents is due to its forming a stable, water soluble salt with ferric iron, ferrioxalate ion.

Extractive metallurgy[edit]

Oxalic acid is an important reagent in lanthanide chemistry. Hydrated lanthanide oxalates form readily in strongly acidic solutions in a densely crystalline, easily filtered form, largely free of contamination by nonlanthanide elements. Thermal decomposition of these oxalate gives the oxides, which is the most commonly marketed form of these elements.

Niche uses[edit]

Vaporized oxalic acid, or a 3.2% solution of oxalic acid in sugar syrup, is used by some beekeepers as a miticide against the parasitic varroa mite.

Oxalic acid is rubbed onto completed marble sculptures to seal the surface and introduce a shine.

Content in food items[edit]

This table was originally published in Agriculture Handbook No. 8-11, Vegetables and Vegetable Products, 1984.

Vegetable Oxalic acid (g/100 g)
Amaranth 7000109000000000000 1.09
Asparagus 6999130000000000000 0.13
Beans, snap 6999360000000000000 0.36
Beet leaves 6999610000000000000 0.61
Broccoli 6999190000000000000 0.19
Brussels sprouts 6999360000000000000 0.36
Cabbage 6999100000000000000 0.10
Carrot 6999500000000000000 0.50
Cassava 7000126000000000000 1.26
Cauliflower 6999150000000000000 0.15
Celery 6999190000000000000 0.19
Chicory 6999210000000000000 0.21
Chives 7000148000000000000 1.48
Collards 6999450000000000000 0.45
Coriander 6998100000000000000 0.01
Corn, sweet 6998100000000000000 0.01
Cucumbers 6998200000000000000 0.02
Eggplant 6999190000000000000 0.19
Endive 6999110000000000000 0.11
Garlic 6999360000000000000 0.36
Kale 6998200000000000000 0.02
Lettuce 6999330000000000000 0.33
Okra 6998500000000000000 0.05
Onion 6998500000000000000 0.05
Parsley 7000170000000000000 1.70
Parsnip 6998400000000000000 0.04
Pea 6998500000000000000 0.05
Bell pepper 6998400000000000000 0.04
Potato 6998500000000000000 0.05
Purslane 7000131000000000000 1.31
Radish 6999480000000000000 0.48
Rutabaga 6998300000000000000 0.03
Spinach 6999970000000000000 0.97
Squash 6998200000000000000 0.02
Sweet potato 6999240000000000000 0.24
Tomato 6998500000000000000 0.05
Turnip 6999210000000000000 0.21
Turnip greens 6998500000000000000 0.05
Watercress 6999310000000000000 0.31

[14]

Toxicity and safety[edit]

Oxalic acid has toxic effects through contact and if ingested; manufacturers provide details in Material Safety Data Sheets (MSDS). It is not identified as mutagenic or carcinogenic; there is a possible risk of congenital malformation in the foetus; may be harmful if inhaled, and extremely destructive to tissue of mucous membranes and upper respiratory tract; harmful if swallowed and causes burns; harmful, destructive of tissue and causes burns if absorbed through skin or in contact with the eyes. Symptoms and effects include burning sensation, cough, wheezing, laryngitis, shortness of breath, spasm, inflammation and oedema of the larynx, inflammation and oedema of the bronchi, pneumonitis, pulmonary oedema.[15]

In humans, ingested oxalic acid has an oral LDLo (lowest published lethal dose) of 600 mg/kg.[16][dead link] It has been reported that the lethal oral dose is 15 to 30 grams.[17]

The toxicity of oxalic acid is due to kidney failure, which arises because it causes precipitation of solid calcium oxalate,[18] the main component of kidney stones. Oxalic acid can also cause joint pain due to the formation of similar precipitates in the joints. Ingestion of ethylene glycol results in oxalic acid as a metabolite which can also cause acute kidney failure.

References[edit]

  1. ^ Radiant Agro Chem. "Oxalic Acid MSDS". 
  2. ^ Bjerrum, J., et al. Stability Constants, Chemical Society, London, 1958.
  3. ^ Yonemitsu Eiichi, Isshiki Tomiya, Suzuki Tsuyoshi, Yashima Yukio, Process for the production of oxalic acid, US 3678107 
  4. ^ a b c d Wilhelm Riemenschneider, Minoru Tanifuji "Oxalic acid" in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. doi: 10.1002/14356007.a18_247.
  5. ^ Practical Organic Chemistry by Julius B. Cohen, 1930 ed. preparation #42
  6. ^ Clarke H. T.;. Davis, A. W. (1941), "Oxalic acid (anhydrous)", Org. Synth.: 421 ; Coll. Vol. 1 
  7. ^ Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.
  8. ^ Bowden, E. (1943), "Methyl oxalate", Org. Synth.: 414 ; Coll. Vol. 2 
  9. ^ Kovacs K.A., Grof P., Burai L., Riedel M. (2004). "Revising the mechanism of the permanganate/oxalate reaction". J. Phys. Chem. A 108 (50): 11026–11031. doi:10.1021/jp047061u. 
  10. ^ Martin V. Dutton, Christine S. Evans "Oxalate production by fungi: its role in pathogenicity and ecology in the soil environment" Canadian Journal of Microbiology, 1996, vol. 42, p. 881-895. doi:10.1139/m96-114.
  11. ^ Rombauer, Rombauer Becker, and Becker (1931/1997). Joy of Cooking, p.415. ISBN 0-684-81870-1.
  12. ^ Novoa, William; Alfred Winer, Andrew Glaid, George Schwert (1958). "Lactic Dehydrogenase V. inhibiton by Oxamate and Oxalate". Journal of Biological Chemistry. 
  13. ^ Le, Anne; Charles Cooper, Arvin Gouw, Ramani Dinavahi, Anirban Maitra, Lorraine Deck, Robert Royer, David Vander Jagt, Gregg Semenza, Chi Dang (14). "Inhibition of lactate dehydrogenase A induces oxidative stress and inhibits tumor progression". Proceedings of the National Academy of Sciences. 
  14. ^ http://www.nal.usda.gov/fnic/foodcomp/Data/Other/oxalic.html
  15. ^ Sigma-Aldrich: Oxalic acid dihydrate Material Safety Data Sheet (MSDS)
  16. ^ Safety Officer in Physical Chemistry (August 13, 2005). "Safety (MSDS) data for oxalic acid dihydrate". Oxford University. Retrieved December 30, 2009. 
  17. ^ CDC: Documentation for Immediately Dangerous To Life or Health Concentrations (IDLHs), Oxalic acid, May 1994
  18. ^ EMEA Committee for veterinary medicinal products, oxalic acid summary report, December 2003

External links[edit]

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