Oxalic Acid: What is the formula of oxalic acid? | Properties | Uses

Chemical Formula

• Oxamide – C₂H₄N₂O₂

• diethyl oxalate  – C₆H₁₀O₄
• glycolic acid – C₂H₄O₃

• Calcium oxalate – CaC₂O₄

• Sodium formate – HCOONa
• Magnesium Carbonate – MaCO₃ 

Oxalic Acid is the first member of the di-carboxylic acid group. Two carboxyl groups are present in one molecule. It is found in Rhubarb, sorrel and oxalic group plants as its potassium salt. In some plants and animals it is found as calcium oxalate.

In the laboratory, it is made by oxidizing sugar cane sugar. At the commercial level it is made by heating Sodium Formate. It is found in crystal and Amorphous forms.

One molecule of crystalline oxalic acid consists of 2 molecules of Kelsan water. The molecular formula for amorphous oxalic acid is H₂C₂O₄ and the following is the composition formula.

Physical Properties of Oxalic Acid

It is a colorless and odorless solid. The melting point of crystalline oxalic acid is 101.5°C and the melting point of anhydrous oxalic acid is 189.5°C. It is soluble in water and alcohol. And is indestructible in ether.

Chemical Properties of Oxalic Acid

Reaction with bases: oxalic acid is a dual incinerate acid. Hence it reacts with bases to form two types of salt. Example:

It reacts with ammonia to form ammonium oxalate, which upon heating gives oxamide.

Reaction with Ethanol: On heating anhydrous oxalic acid with ethanol, it forms mono ethyl oxalate and diethyl oxalate, respectively.

Reaction with phosphorus penta chloride: It reacts with phasphorus penta chloride to form oxalyl chloride.

Effect of heat: On heating crystalline oxalic acid to 100°C, its crystal water is separated. After heating to about 200°C, it forms formic acid.

H₂C₂O₄.2H₂O → H₂C₂O₄ → HCOOH + CO₂

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At 200°C formic acid partially decomposes to water and carbon mono oxide.

HCOOH → H₂O + CO

Therefore, heating of crystal oxalic acid at 200°C gives a mixture of formic acid, carbon mono oxide, carbon di oxide and water.

When heated to about 90°C with sulphuric acid, it decomposes and forms carbon di-oxide, carbon mono oxide and water.

H₂C₂O₄ → CO₂ + CO + H₂O

Oxidation: It is quickly oxidized to carbon d oxide and water under the influence of oxidants such as acidic potassium permanganate, potassium dichromate and lead peroxide.

H₂C₂O₄ + O → H₂O + 2CO₂

The following is the balanced equation of the reaction of oxalic acid to acidic KMnO₄.

2KMnO₄ + 3H₂SO₄ + 5H₂C₂O₄ → K₂SO₄ + 2MnSO₄ + 8H₂O + 10CO₂

Reduction: (i) It is reduced by zinc and sulphuric acid to form glycolic acid.

(ii) It is reduced by magnesium mercury (Mg-Hg) amalgam and H₂SO₄ to form Glyoxylic acid.

Reaction with Glycerol: when heated with glycerol at 100-110°C makes it formic acid and Allyl Alcohol at 260°C.

What is oxalic acid used for?

As reagents in the laboratory and in oxidation reduction measurements

Allyl alcohol, formic acid and other useful organic compounds

As mordant in printing and dyeing of clothes

To get rid of ink stains on clothes

Polish of metals, inks and dyes

Potassium ferrous oxalate is used as a developer in photography.

How do you test for oxalic acid?

Oxalic acid provides general tests of carboxylic acid.
Effervescences are obtained due to the formation of CO₂ gas when
oxalic acid is added to the NaHCO₃ solution. Specific tests are as
follows.

The addition of calcium chloride to a neutral solution
of oxalic acid gives a white precipitate of calcium oxalate, which is insoluble
in acetic acid but soluble in mineral acids.

The oxalic acid becomes colourless when the oxalic acid
solution is added to the hot and acidic potassium par magnate solution.

On heating it with concentrated sulphuric acid, carbon
monoxide gas is formed which burns with a blue flame.

The addition of silver nitrate to neutral solution of
oxalic acid gives a white precipitate, which is soluble in ammonium hydroxide
and does not give silver mirror when heated.