Oxalic acid

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Oxalic acid
Oxalic acid dihydrate.jpg
Oxalic acid dihydrate sample
Oxalic acid structure.png
Structure of oxalic acid
IUPAC name
Oxalic acid
Systematic IUPAC name
Ethanedioic acid
Other names
1,2-Ethanedioic acid
Wood bleach
Jmol-3D images Image
Molar mass 90.035 (anhydrous)
126.066 (dihydrate)
Appearance White hygroscopic crystals
Odor Odorless
Density 1.90 g/cm3 (anhydrous)
1.653 g/cm3 (dihydrate)
Melting point 189.5 °C (373.1 °F; 462.6 K) (sublimes)
Boiling point Sublimes
14.3 g/100 ml
Solubility Poorly soluble in acetone, POCl3
Insoluble in benzene, chloroform, petroleum ether, toluene, xylene
Solubility in diethyl ether 1.4 g/100 ml (15 °C)
Solubility in ethanol 23.7 g/100 ml (15 °C)
Solubility in formic acid 9.74 g/100 ml (16.8 °C)[1]
Solubility in methanol 27.01 g/100 ml (25 °C)[2]
Vapor pressure 0.001 mmHg (20 °C)
Acidity (pKa) 1.25
-827.5 kJ/mol
Safety data sheet Sigma-Aldrich (anhydrous)
Sigma-Aldrich (dihydrate)
Flash point 166 °C
Related compounds
Related compounds
Malonic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Oxalic acid is an organic compound with the chemical formula H2C2O4 or (COOH)2. Along with formic acid, it is one of the most reactive organic acids, but unlike formic acid, pure oxalic acid is solid, non-volatile and can be handled much safer.



Oxalic acid is very strong for an organic acid (pHa1 = 4.14; pHa2 = 1.25). It will react with manganese dioxide to form manganese oxalate and carbon dioxide.

MnO2 + 2 H2C2O4 → MnC2O4 + 2 CO2 + 2H2O

Oxalic acid is particularly notable for a number of insoluble salts it forms with common metals, such as magnesium and calcium. Thus, it can be used to displace other acids from these metals' salts, which allows for synthesis of several rare acids.

Heating oxalic acid in conc. sulfuric acid will yield carbon dioxide and carbon monoxide.

H2C2O4 → CO2 + CO + H2O


Oxalic acid is a colorless hygroscopic crystalline solid, with a snow-like aspect. It is soluble in water (143 g/L at 25˚C), more soluble in ethanol (240 g/L) but poorly soluble in ether (18 g/L). It has a weak smell and is irritating to skin. It has a melting point of 102 °C and sublimes between 149 - 160 °C.


Oxalic acid is sometimes available as wood bleach or as beehive disinfecting powder, from beekeeping suppliers. It can also be bought online, such as from Amazon. "Bar Keepers Friend" is a very cheap multi-surface cleaner containing oxalic acid dihydrate, available at hardware stores.

Its name comes from the fact that early investigators isolated oxalic acid from flowering plants of the genus Oxalis, commonly known as wood-sorrels. Oxalic acid can also be found in other plants, such as chives (1.48 g/100 g plant, on average), rhubarb (the leaf has the highest concentration, 0.5 % w/w), spinach, Fenestraria genus, Virginia creeper, etc.


Oxalic acid can be made from the oxidation of sucrose, glucose, or ethylene glycol using nitric acid or air in the presence of vanadium pentoxide at high temperatures.

Below is a synthesis of oxalic acid found online, using nitric acid and sucrose:

10 g of sugar are added in a flat bottom flask and 50 ml of concentrated nitric acid and heat the flask in a water bath. The reaction will yield nitrogen oxide fumes, so it's best performed outside or in a fume hood. Stop the heating and remove the flask from the water bath. When the reaction subsides, add the hot solution into an evaporating basin. Wash out the flask with 10 ml of conc. nitric acid and evaporate the solution on the water bath until it has a volume of 10 ml. Add 20 ml of water to the solution and evaporate again to 10 ml. Cool the solution in a cooling bath to crystallize the oxalic acid. Dry the crystals.[3]




Oxalic acid is corrosive to human tissues, so proper protection must be worn. It is not volatile under normal conditions, so its vapors aren't usually a hazard. Oxalates are the notorious cause of kidney stones, so ingestion of both acid and salt should be avoided.


Should be stored in closed bottles.


Oxalic acid and oxalates can be destroyed with hydrogen peroxide.[4]


  1. Aschan; Chemiker-Zeitung, Chemische Apparatur; vol. 37; (1913); p. 1117
  2. Gumtya; Lahiri; Zeitschrift fur Physikalische Chemie; vol. 217; nb. 11; (2003); p. 1341 - 1359
  3. http://www.scribd.com/doc/45869039/preparation-of-Oxalic-acid
  4. http://web.ornl.gov/info/reports/1981/3445605762877.pdf

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