Oxalic Acid

Oxalic acid is an organic compound and a type of dicarboxylic acid. It is naturally found in many plants, leafy greens, vegetables and also in fruits, cocoa, nuts and seeds. It is odorless and colorless in solid-state, which can be dissolved in water. However, when it is purified, it changes into a white crystalline substance. Further, it is more acidic than oxalic acid.

Further, oxalic acid is also produced in the human body when glyoxylic acid or ascorbic acid is metabolized and flushed out from the body through the urine. Also, being highly corrosive, it is more toxic as compared to other substances. So, it should be properly disposed of after usage.

Oxalic Acid Chemical Formula

Oxalic acid is an alpha, omega-dicarboxylic acid with the chemical or molecular formula C₂H₂O₄ or (COOH) ₂ or HOOCCOOH and molecular weight 90.03 g/mol. It is also called ethanedioic acid and is a conjugate acid of an oxalate (1- ) and an oxalate. It is the simplest dicarboxylic acid, which is made of two carboxylic acid groups (COOH) directly joined or bonded to each other at the carbon atoms, as shown in the below image.

Oxalic acid in its anhydrous form or when it is dehydrated exists as two different polymorphs. Out of which, one has hydrogen bonding that helps develop the chain-like structure at the intermolecular level. Another polymorph also has hydrogen bonding but in this type, this bonding leads to the formation of a sheet-like structure at the intermolecular level.

Physical properties of oxalic acid

• In solid-state, it appears as white crystals.
• Oxalic acid's molar mass is 126 g/mole.
• Its density is 1.90 g/cm³
• Its melting point is around 190 degrees Celsius.

Chemical properties of oxalic acid

• Oxalic acid is a strong organic acid, which can remove carbonic acid and various other acids from their salts.
• It is commonly used in esterification reactions due to its acidic nature and hydrophilic nature. It is stronger than acetic acid, so it easily produces various acid chloride, amides, salts, ester derivatives, etc., in chemical reactions.
• It is a dibasic acid in nature, as it can donate two hydrogen ions (H⁺), which is also suggested by its condensed formula: COOH-COOH.
• When it is heated at 150 degrees Celsius, it undergoes decarboxylation which forms formic acid and releases carbon dioxide, as shown below:
  HOOC-COOH →HCOOH + CO₂
• When it is heated with concentrated sulphuric acid, it gets decomposed to form carbon dioxide, carbon monoxide and water. The reaction is shown below:
  HOOC-COOH + conc. H₂SO₄ → CO₂ + CO + H₂O
• When it reacts with glycerol, it forms formic acid, however, it depends on the experimental conditions. For example, if it reacts with glycerol at 530 K, allyl alcohol is formed.
• It is oxidized by acidified potassium permanganate to form carbon dioxide and water as shown below:
  HOOC-COOH + (O) → 2CO₂ + H₂O

Oxalic Acid Uses

Oxalic acid offers lots of uses. Some of its common and major uses are described below:

Cleansing agent: It is widely used for cleaning the toughest stains. It is present in various cleaning products, bleaches, and detergents. It is also suitable for polishing stones and treating old wood.

Industrial uses: In industries, it is used in the processing of minerals. It is also used to sterilize equipment and for bleaching purposes in the textile industry.

Medicinal uses: In the pharma industry, it is also used to purity and dilute some chemicals.

Reducing element: It is also used in developing photographic film.

Water treatment: It is also required in the treatment of wastewater as it can easily remove calcium deposits from water.

Rust: It is also used in a rust removal process wherein rust is removed from various iron products.

Dye: It is also required in a process that is used to form dyes.

Preparation of Oxalic Acid

Laboratory preparation of oxalic acid:

The preparation of oxalic acid is very easy. It is prepared by the oxidation of some carbohydrates by concentrated nitric acid. The steps are described below:

1. Add 10 gm of cane sugar into a 250 ml conical flask.
2. Now add 50 ml of concentrated nitric acid into the flask and heat on a boiling bath. It warms the mixture and sugar is dissolved.
3. Now take off the flask from the heat and place it on the table.
4. After 15 minutes, add the solution into an evaporating dish.
5. Let the acid solution evaporate by heating it in the boiling water bath until it is reduced to 10ml in volume.
6. Let the solution cool using an ice-water mixture.
7. Oxalic acid is formed in the form of crystals.
8. Collect the crystals, the yield would be nearly 3.5 gm of oxalic acid.

Industrial Preparation of Oxalic Acid

1. Sodium formate is heated at a temperature of 400 degrees Celsius to form sodium oxalate.
2. Now, sodium oxalate is added into the solution of water and calcium hydroxide. It leads to the formation of sodium hydroxide and calcium oxalate as a precipitate as shown below:
   Sodium oxalate + Aq. Ca(OH)₂ → Calcium oxalate + NaOH
3. The solution obtained is filtered and reacted with dilute sulphuric acid to release oxalic acid as shown in the below reaction:
   Calcium oxalate + H₂SO₄ → Oxalic acid + CaSO₄
   Calcium sulphate is filtered out as a precipitate and oxalic acid is crystallized from the filtrate as dihydrate.

Health Hazard

Oxalic acid is a strong acid with high toxicity. If it is ingested accidentally, it may cause severe symptoms like vomiting, diarrhea, renal damage, coma, shock, etc. It may also cause irritation in the eyes, skin, mucous membrane, and may also affect the kidney.

The reason for the toxic nature of oxalic acid is that it forms calcium oxalate when it reacts with the calcium in the tissues. It disturbs the calcium and potassium ratio and may cause the accumulation of oxalates in the tubules of kidneys and thus may lead to kidney damage.