Oxalic acid is an organic acid which is also called ethanedioic acid. It has the formula HO2C−CO2H and it is the simplest dicarboxylic acid.
It is a white crystalline solid that forms a colorless solution when mixed in water. Its name comes from the fact that early investigators isolated oxalic acid from flowering plants of the genus Oxalis which is commonly known as wood-sorrels. It occurs naturally in many foods but the excessive ingestion of oxalic acid or prolonged skin contact can be dangerous.
Oxalic acid has much greater acid strength when compared to acetic acid. It is a reducing agent with a conjugate base, known as oxalate (C2O2−4), which is a chelating agent for metal cations. Oxalic acid occurs as a dihydrate with the formula C2H2O4·2H2O.
Properties of Oxalic Acid
|Iupac Name||Ethanedioic Acid|
|Chemical Formula||The chemical formula for oxalic acid is C2H2O4.|
|Molar Mass||The molar mass for oxalic acid is 90.034 g·mol−1 (anhydrous)
126.065 g·mol−1 (dihydrate)
|Appearance||Oxalic acid has a white crystal appearance.|
|Odor||Oxalic acid is odorless|
|Density||Oxalic acid has a density of 1.90 g·cm3 (anhydrous, at 17 °C)
1.653 g·cm−3 (dihydrate)
|Melting Points||The melting points of oxalic acid are 189 to 191 °C (372 to 376 °F; 462 to 464 K)
101.5 °C (214.7 °F; 374.6 K) dihydrate
|Solubility in Water||The solubility of oxalic acid in water include the following: 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)|
|Solubility||The solubility of oxalic acid in ethanol is 237 g/L (15 °C)
The solubility of oxalic acid diethyl ether is 14 g/L (15 °C)
|Vapor Pressure||The vapor pressure of oxalic acid is <0.001 mmHg (20 °C).|
|Acidity (pKa)||1.25, 4.14|
|Conjugate Base||The conjugate base of oxalic acid is Hydrogenoxalate.|
|Heat Capacity||The heat capacity of oxalic acid is 91.0 J·mol−1·K−1.|
|Std enthalpy of
Preparation of Oxalic Acid
Oxalic acid is mainly prepared 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 for this preparation including glycolic acid and ethylene glycol. A newer method entails the oxidative carbonylation of alcohols to produce diesters of oxalic acid. These diesters are hydrolyzed to produce oxalic acid.
4 ROH + 4 CO + O2 → 2 (CO2R)2 + 2 H2O
Laboratory Methods Of Preparing Oxalic Acids
Although it can be easily gotten, oxalic acid can be prepared in the laboratory through the oxidation of sucrose using nitric acid in the presence of a small amount of vanadium pentoxide as a catalyst and the hydrated solid can be dehydrated with heat or by azeotropic distillation.
Uses of Oxalic Acid
- The main applications of oxalic acids include cleaning or bleaching, especially for the removal of rust. It is an ingredient in some tooth whitening products. It is also used as a mordant in dyeing processes. It is used in bleaches, especially for pulpwood, and for rust removal and other cleaning, in baking powder.
- Oxalic acid is used by some beekeepers as a miticide to fight against the parasitic varroa mite.
- Dilute solutions of oxalic acid can be used in the removal of iron from clays such as kaolinite to produce light-colored ceramics.
- Oxalic acid is used in the cleaning of minerals.
- It is sometimes used in the aluminum anodizing process, with or without sulfuric acid.
- It is used as a wood bleach, most often in its crystalline form to be mixed with water to its proper dilution for use.
- It is also used in electronic and semiconductor industries.
Hazardous Effects of Oxalic Acid on The Body
The exposure to oxalic acid can cause headache, dizziness, nausea and vomiting, convulsions, coma and even death and the prolonged or repeated contact can cause a skin rash, pain, redness, blisters, and slow healing ulcers.
Precautions To Take While Handling Oxalic Acid
- Always wear impenetrable gloves when handling any form of oxalic acid in the lab.
- Protective clothing should be worn to prevent contact of the liquid or powder oxalic acid with the skin.
- Goggles and respirators should be used when vaporizing oxalic acid crystals in the lab.
Is oxalic acid poisonous to humans?
Yes, oxalic acid is a poisonous compound and if in contact with the tissues, it can cause severe damage, such as burning or ulcers, on contact.
What can you not mix with oxalic acid?
Oxalic acid is hygroscopic and sensitive to heat and it may react violently with furfuryl alcohol, silver, sodium, perchlorate, sodium hypochlorite, strong oxidizers, sodium chlorite, acid chlorides, metals and alkali metals.
Where oxalic acid is found?
Oxalic acid is a crystalline solid which forms a colourless solution in water. It is found in foods like spinach, cabbage, broccoli, parsley, tomato etc.