Oxalate [C₂O₄⁻²] – Structure, Molecular Mass, Properties & Uses

When you were kids, your elders must have pressured you to eat green leafy vegetables. On asking the reason, they told you these are good for your growth and health. They must tell you many other benefits of eating green vegetables.

But do you know those luxuriant green vegetables are a good source of oxalic acid in addition to being lofty in fibre? You consume many other food items that contain oxalic acid and many other forms of oxalates. But the question is, what are oxalates? And what is oxalic acid?

Let’s dive into this article to know more about oxalates and oxalic acid.

What Is Oxalate?

Oxalate is a colourless dianion. Its IUPAC name is ethanedioate. In some food items, they occur naturally. It is the conjugate base of oxalic acid, also known as oxalic acid dianion. Oxalate is often operated for derivatives like salts of oxalic acid. For instance, dimethyl oxalate or sodium oxalate. One can obtain it by deprotonation of the carboxyl groups of C2H2O4, i.e. oxalic acids.

Under normal conditions, oxalate ions are non-toxic. But, it may be toxic in an adverse environment.

Oxalate Structure

The molecular formula of oxalate ion is C₂O₄⁻². It exists in a planar conformation. There are two parts to what makes oxalate ion planar. Oxalate structure is -O₂C-CO₂-, or two carboxylates are bonded.

A carboxylate, by itself, is planar because the carbonyl carbon is sp² hybridised. There is a double bond (=) between one of the oxygens and the carbon.

In the oxalate structure, there are two carboxylates, each is planar, but the dihedral angle between them does not have to be 0° or 180° if the whole molecule is flat. What causes the angle to be roughly 0° or 180° (i.e., flat) is that it maximises the orbital interaction between the C=O bonds on either end.

The structure of oxalate ion is as follows:

Uses of Oxalate Ion

Natural oxalate acids are detrimental to the human body. But, some artificial oxalate acids are useful to humanity in many ways. Some of these uses are

• Oxalates of barium and strontium act as reducing agents in chemistry and pyrotechnics laboratories.
• Escitalopram oxalate is useful in the treatment of anxiety and depression.
• The oxalate acid salt of calcium is used in producing ceramic materials.
• Ferric oxalate is used in platinum printmaking and many other purposes.
• Oxalate ions act as an excellent ligand for metal ions.
• Potassium hydrogen oxalate, i.e., the salt of sorrel, is used as a chemical reagent due to its occurrence in the sorrel plant. It is also utilised for the removal of ink stains and in photography. 
• Oxalates are used as sources of the metals they contain.
• Cobalt oxalate is utilised in the manufacturing of cobalt catalysts.

Oxalic Acid and Its Molecular Formula

Oxalic acid is an organic acid. It is present in spinach, tomatoes, and other vegetables. In the body, it is synthesised from oxaloacetate, which is broken down by ‘oxaloacetase’, an enzyme in the body. Oxalic acid inhibits lactate dehydrogenase enzyme, which prevents tumour growth. It is used as bleach.

Oxalic acid is an organic compound having the molecular formula C₂H₂O₄. It is a colourless crystalline solid that forms a colourless solution in water. The IUPAC name of oxalic acid is Ethanedioic Acid.

Oxalic Acid Structure

The oxalic acid structure is given below:

As per the valency of a carbon atom, it forms 3𝜎-bonds and 1𝜋-bond with carbon and oxygen atoms. Two 𝜎-bonds are formed between C-C bonds and -C-O-H bonds, and a double bond is formed between C=O atoms.

Oxalic acid structure exists in two forms, i.e., anhydrous and dihydrate. In anhydrous form, it exists in a chain-like structure and, as a result, forms a sheet-like pattern. While in dihydrate form, it forms a crystalline structure.

Preparation of Oxalic Acid

There are different ways of preparing oxalic acid. These are given below:

1. By nitric acid (HNO₃):

Oxalic acid is prepared by oxidation of definite carbohydrates, for instance, sucrose by a powerful oxidising agent, nitric acid in the presence of a catalyst such as vanadium pentoxide (V₂O₅).

C₁₂H₂₂O₁₁ + 18[O] → (COOH)₂ + H₂O

2. By electrocatalysis:

In this method, oxalic acid undergoes electrocatalysis. With the aid of a copper complex, carbon dioxide (CO₂) is reduced to oxalic acid. In this procedure, large quantities of carbon dioxide are utilised.

3. With the help of ethylene glycol:

On oxidation, ethylene glycol with acidified potassium dichromate and sulphuric acid gives oxalic acid. The chemical reaction is as follows:

(CH₂OH)₂ + 4[O] → (COOH)₂ + 2H₂O

4. By cyanogen:

Upon hydrolysis of cyanogen with concentrated hydrochloric acid, oxalic acid is obtained. The chemical reaction is as follows:

(CN)₂ + 4H₂O + 2HCl → (COOH)₂ + 2NH₄Cl

5. By carbonylation of alcohol:

In this process, oxalic acid diester is obtained by carbonylation of alcohol.

6. From sodium formate:

In this process, oxalic acid is prepared from sodium formate (OHCO⁻Na⁺) in the presence of an alkaline catalyst. This method further leads to the formation of sodium oxalate (C₂O₄Na₂), which is later converted to oxalic acid. The chemical reactions that take place are as follows:

OHCO⁻Na⁺ + 400°C heat → C₂O₄Na₂ + H₂

C₂O₄Na₂ + aq. Ca(OH)₂ → [C₂O₄]⁻²Ca⁺² + NaOH

[C₂O₄]⁻²Ca⁺² + H₂SO₄ → (COOH)₂ + CaSO₄↓

Properties of Oxalic Acid

The physical and chemical properties of oxalic acid are given below:

1. Physical Properties:

Some physical properties of oxalic acid are given below:

• It is the smallest double carboxylic acid.
• The chemical formula of oxalic acid is C₂H₂O₄ or (COOH)₂.
• It is an odourless and white crystalline solid.
• It exists in two forms: anhydrous and dihydrate.
• The molar mass of anhydrous oxalic acid is 90.034 g/mol, and the dihydrate form is 126.065 g/mol.
• The density of anhydrous oxalic acid is 1.90 g/cm³, and the dihydrate form is 1.653 g/cm³.
• The melting point of oxalic acid is 189-191°C.
• The solubility of oxalic acid in water is 118 g/L at room temperature.
• It is commonly found in the hydrated form (COOH)₂⋅2H₂O.
• It is a water-soluble carboxylic acid, and as it releases H⁺ ions in water, it forms an acidic solution.

2. Chemical Properties:

Some chemical properties of oxalic acid are given below:

• It is the simplest and strongest dicarboxylic acid. It reacts like carboxylic acid and forms ester derivatives and acid chloride.
• It can react with phosphorus pentachloride (PCl₅) to generate phosphorus oxychloride.
• Anhydrous oxalic acid is hydrophilic and attracts water.
• It has strong acidic strength and can donate two protons easily.
• On heating, oxalic acid undergoes the removal of carbon dioxide to produce formic acid.

(COOH)₂ + Heat → HCOOH + CO₂

• If heated in the presence of H₂SO₄, oxalic acid gives carbon monoxide, carbon dioxide, and water.

(COOH)₂ + Heat (in the presence of H₂SO₄) → CO + CO₂ + H₂O

• It is used for quantitative analysis of potassium permanganate.
• Oxidation of oxalic acid with KMnO₄ or K₂Cr₂O₇ results in carbon dioxide and water formation.

(COOH)₂ + [O] → 2CO₂ + H₂O

• It works as an excellent chelating ligand for many metal cations.

Use of Oxalic Acid

Various use of oxalic acid is given below:

• About 25% use of oxalic acid is in the dyeing and printing industry.
• It is used in the bleaching of pulpwood.
• It acts as a reducing agent.
• The key use of oxalic acid cleaning or bleaching, especially for removing rust found on plumbing pipes, etc.
• In lanthanide chemistry, it is one of the important reagents.
• Some beekeepers use oxalic acid.
• It is used as a laboratory, analytical, dye intermediates, and chromatography analysis reagent.
• Oxalic acid is a bleaching agent for wood. It removes black spots germed by solvated iron that ties to the wood when water penetrates it.
• It is sometimes used in the aluminium anodising process.
• It is also used in the refining of rare metals.
• It is used in the manufacturing of drugs such as antibiotics.

Conclusion

Incomplete oxidation of carbohydrates in plants leads to the synthesis of oxalate compounds. While reacting with the oxalates, many metal ions form non-dissolvable precipitates. They are also able to form coordination compounds. Calcium oxalate is a common salt of this kind, and oxalic acid is the most general acid formed by the protonation of oxalate ions. The main constituent of the most usual oxalate may be in the human body in the form of kidney stones.

Frequently Asked Questions

1) Are there any health hazards caused by oxalate ions?

 A) Oxalates may vary in the form of kidney stones obstructing the kidney tubules. Approximately 80% of kidney stones are only due to calcium oxalate salt.

Oxalate ions in your body combine with diatomic metallic cations such as iron and calcium and produce crystals of the corresponding oxalates. These crystals are excreted from urine.

2) What are the natural sources of oxalate?

 A) Oxalate naturally occurs in many foods, including vegetables and fruits, nuts, seeds, legumes, grains, chocolate, and tea. Some foods with more oxalate levels include beets, spinach, rhubarb, peanuts, sweet potatoes and chocolate.

3) What do oxalates in the body cause the diseases?

 A) Oxalate is an ionic form of oxalic acid, a toxic acid formed in the body from various food sources. The absorption of these food items is in the digestive tract. These oxalate crystals wedge the absorption and utilisation of calcium in your body. As a result, they cause diseases such as Osteomalacia and Rickets. These diseases are related to weakening bones.