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Oxalic Acid

What is oxalic acid?

Oxalic acid or ethanedioic acid (chemical formula C2H2O4) is a poisonous, crystalline, dicarboxylic acid that presents in rhubarb, sorrel, and other plants of the oxalis group. It occurs as a dihydrate with molecular formula C2H2O4, 2H2O. Oxalic acid crystallizes from water as colourless crystals dihydrate with two molecules of water. The melting point of dihydrate is 101.5 °C but the meting point of anhydrous oxalic acid is 189.5 °C. It is soluble in water and ethanol but almost insoluble in ether. Oxidation of many organic compounds like sugar, starch with nitric acid is used for the production of oxalic acid. It is used widely as a cleaning or bleaching agent for the removal of rust or stains.

Oxalic acid formula, chemical structure and uses

Oxalic acid uses

Oxalic acid is an important household chemical that can be used mainly for cleaning or bleaching and removal of rust. It is also used for the manufacture of ink, polishing metal, removing stains from metal surfaces and clothes. In textiles industries, the antimony salt of the oxalic acid is used as mordants in printing and dyeing. For developing photographic film, we also used oxalic acid as a reducing agent.

Bleaching

It is a useful bleaching agent for wood and stone. When wood or stone turns gray, we used oxalic acid to brings back its natural coluor.

Oxalic acid for rust removal

It is useful for removing rust caused by iron, water, and tannic acid. It is used for cleaning or removing iron rust due to the formation of water-soluble iron salt with oxalic acid.

Removing Stains

It is very effective for removing stains caused by ink, different types of food. Oxalic acid removes the stain gently but does not affect the base surface. It is also used for removing strain from linen or cotton clothes.

Production of oxalic acid

Industrial production

It is prepared industrially by heating sodium formate rapidly to 360 °C.
2HCOONa → (COONa)2 + H2

The free acid from its sodium salt can be obtained by adding calcium hydroxide solution. Then calcium oxalate can be treated with the calculated amount of sulfuric acid to formed calcium sulfate and oxalic acid. Removing calcium sulfate from the solution and evaporating the filtrate to crystallization.

Production in laboratory

In the laboratory, oxalic acid can be prepared by oxidizing sucrose or molasses with concentrated nitric acid in presence of vanadium pentoxide as a chemical catalyst.
C12H22O11 + 18[O] → 6 (COOH)2 + 5H2O

Chemical properties

Oxalic acid is a poisonous acid that crystallizes from water to form colorless dihydrated crystals with the molecular formula C2H2O4, 2H2O. The hydrate loses water when heated at 100 to 105 °C. When heated above 200 °C, it decomposes to form carbon dioxide, carbon monoxide, formic acid, and water. Some important properties of the acid are given below the table,

Chemical properties
Chemical formula C2H2O4
Molar mass Anhydrous dihydrate
90.034 g mol-1 126.065 g·mol-1
Appearance white crystals
Odor odorless
Density Anhydrous dihydrate
1.90 g cm-3 1.653 g cm-3
Melting point 189 to 191 °C 101.5 °C
Solubility 90-100 g/L at 20 °C
Acidity (pka) 1.25, 4.14
Conjugate base hydrogenoxalate

Chemical Reaction

The anhydrous oxalic acid is easily obtained by heating hydrate with carbon tetrachloride. When heated with concentrated sulfuric acid, it decomposed to form carbon dioxide, carbon monoxide, and water.
(COOH)2 → CO + CO2 + H2O

It is readily oxidized by potassium permanganate (KMnO4) to form carbon dioxide but very slowly oxidized by concentrated nitric acid.
(COOH)2 + [O] → 2CO2 + H2O

Anhydrous oxalic acid reflux with ethanol to form di-ester like ethyl oxalate. The more general method for preparing di esters, to heating dicarboxylic acid with an alcohol, toluene, and concentrated sulfuric acid.
(COOH)2 + 2C2H5OH → (COOC2H5)2 + 2H2O

It is heated with ethylene glycol to form cyclic ethylene oxalate. This reaction is used to differentiate oxalic acid from other dicarboxylic acids. The other dicarboxylic acid reacts with glycol to form polyesters.

The reduction of acid by zinc and sulfuric acid formed glycolic acid (HOCH2COOH). Electrolytic reduction by lead electrode gives glycolic and glyoxalic acid. For better production of glyoxalic acid, we used the reagent like magnesium and sulfuric acid.

Toxicity of oxalic acid

The pure concentrated form of oxalic acid is very toxic or dangerous. It is corrosive and harmful if inhaled. It affects mostly the mucous membranes and upper respiratory system of our body. The acid causes eye irritation and burns our skin. Therefore, we always used latex gloves, protective glasses, and dust musk to prevent the toxic effect of oxalic acid.