Oxalic acid - (COOH)2

What is Oxalic acid?

Oxalic acid is a dicarboxylic acid with a chemical formula C2H2O4. It is also known as Ethanedioic acid or Oxiric acid.

This organic compound is found in many vegetables and plants. It is the simplest dicarboxylic acid with condensed formula HOOC-COOH and has an acidic strength greater than acetic acid. Excess consumption of oxalic acid can be dangerous. It is produced by the oxidation of carbohydrates. It can also be prepared in the laboratory by the oxidation of sucrose in the presence of nitric acid and a catalyst like vanadium pentoxide.

Oxalic acid has a structure with two polymorphs and it appears as a white crystalline solid which becomes a colourless solution when dissolved in water. It is a reducing agent and is used as a chelating agent with oxalate as its conjugate base.

Oxalic acid Formula

Oxalic acid is a dicarboxylic acid with the chemical formula C2H2O4. Oxalic acid occurs in the cell sap of Oxalis and Rumex species of plants as the potassium and calcium salt.

Oxalic acid is one of the most powerful of the organic acids and expels carbonic acid and many other acids from their salts. Oxalic acid is produced by the action of either hydrate of potash or of nitric acid upon most organic compounds of natural occurrence. It is also called diprotic acid.

Oxalic acid Structure – C2H2O4

Oxalic acid Structure

Oxalic acid Structure

Preparation of Oxalic acid – C2H2O4

Oxalic acid can be easily prepared by oxidation of certain carbohydrates like sucrose by concentrates nitric acid. During oxidation, the carbon atoms are split off in pairs giving oxalic acid.


  1. Take 10gm of cane sugar in a 250ml conical flask.
  2. Add 50ml of concentrated nitric acid and heat on a boiling water bath.
  3. The mixture becomes warm and most of the sugar gets dissolved.
  4. Remove the flask and keep on the wooden surface. After 15min pour the hot solution in an evaporating dish, washing the flask with 10ml of concentrated nitric acid and adding to the evaporating dish.
  5. Evaporate the acid solution by heating on the boiling water bath until the volume is reduced to about 10ml.
  6. Cool the solution in the ice water mixture.
  7. Oxalic acid crystallizes out rapidly.
  8. Collect the product on the Buchner funnel at the pump.
  9. Dry the product by pressing between the filter paper and then in the air.
  10. The yield of the product is about 3.5gm of oxalic acid.

Properties of Oxalic acid – C2H2O4

C2H2O4 Oxalic acid
Molecular Weight/ Molar Mass 90.03 g/mol
Density 1.90 g cm−3
Appears White crystals
Melting Point 189 to 191 °C

C2H2O4 Uses (Oxalic acid)

  • It is used in the dyeing process as a mordant
  • It is used in removing rust
  • In lanthanide chemistry, it is used as an important reagent
  • It is applied on marble sculptures to make it shine
  • It is used in the manufacture of dye
  • It is used in bleaches
  • It is used in removing food and ink stains
  • It is used in developing photographic film
  • It is used in wastewater treatment to remove the calcium deposit.

Health Hazard

Oxalic acid is a strong poison. The toxic symptoms from ingestion include vomiting, diarrhea, and severe gastrointestinal disorder, renal damage, shock, convulsions, and coma. Death may result from cardiovascular collapse. Oxalic acid is an irritant of the eyes, mucous membranes, and skin. Inhalation or ingestion may result in kidney damage.

Oxalic acid is a strong poison. The toxic symptoms include renal damage, shock, convulsions. The toxicity arises as oxalic acid reacts with the calcium in the tissues to form calcium oxalate, thereby upsetting the calcium potassium ratio. The deposition of oxalates in the kidneys tubules may result in kidney damage.

Frequently Asked Questions

Is oxalic acid a strong acid?

As an organic acid, oxalic acid is a weak acid. Oxalic acid is known to be a soft acid. It’s weaker than (water) H3O+ atom. But it is better than acetic acid, sulphuric acid, nitrous acid, benzoic acid, and so on.

What is the pH of oxalic acid?

The constant acid dissociation of oxalic acid is 5.60 between 10-2 and 5.42 between 10-5. As oxalic acid is a polyprotic acid, there are two values and it has a chemical formula HOOC-COOH.

What is the world’s strongest acid?

Carborane superacids can be considered the strongest solar acid in the world since fluoroantimonic acid is a combination of hydrofluoric acid and pentafluoride antimony. The pH value of Carborane is -18.

What is the basicity of oxalic acid?

Acid weight equivalent = (acid molecular weight)/Basicity. Oxalic acid basicity is = 2. Oxalic acid (H2C2O4) molecular weight = 90.

What is the Valency of oxalic acid?

The valence factor is the no. of acid and base produced H+ or OH-ion. The acid releases 2 H+ ion, hence the valence factor is 2 in the case of oxalic acid. While the no. of e exchanged by a molecule is called that compound’s valence factor for that reaction in the case of the redox reaction.

Also Read:

Preparation of Standard Solution of Oxalic Acid

Learn more about the properties, production and the structure of C2H2O4 from the expert faculties at BYJU’S.

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