Oxalic Acid

Oxalic Acid is a toxic crystalline acid with a sour taste that can be found in natural sources such as rhubarb leaves, wood sorrel, and other plants. It is commonly known as dicarboxylic acid as it is two nothing but two carboxylic groups joined by a single bond. IUPAC’s name for oxalic acid is Ethanedioic Acid and it is a white crystalline solid in appearance which forms a colourless solution in water. Also, anhydrous oxalic acid exists as two polymorphs due to hydrogen bonding, one is a chain-like structure and the other is a sheet-like structure.

Oxalic Acid Formula

Oxalic acid is a strong dicarboxylic acid in nature which is much more acidic in nature than acetic acid which contains Carbon(C), Hydrogen(H), and Oxygen(O) in the ratio of 1:1:2. It is the simplest dicarboxylic acid containing only two carboxylic acids directly bonded to each other at carbon atoms, i.e. (COOH) groups. Hence, the formula of oxalic acid is HOOC-COOH, which can also be represented as (COOH)₂ or C₂H₂O₄.

Molar Mass of Oxalic Acid

Molar mass is the total mass of the molecule present in a one-mole substance and is equivalent to the atomic weight of the substance. Carbon, Hydrogen, and Oxygen have molar masses of 12 gms/mole, 1 gm/mole, and 16 gms/mole respectively, and one molecule of oxalic acid contains two atoms of Carbon, two atoms of Hydrogen, and four atoms of Oxygen. Thus the molar mass of oxalic acid is 2×12 + 2×1 + 4×16 i.e.,  90 grams per mole.

Note: Atomic weight of oxalic acid is 90 a.m.u as well.

Equivalent Weight of Oxalic Acid

The molecular weight or Atomic weight of hydrated Oxalic acid(C₂H₂O₄ .2H₂O) is 126 a.m.u (90 + 2×18) and the charge number or valency (number of hydrogen ions released by it when dissociated) is 2.

Equivalent weight is given by the formula,

Equivalent weight = Molecular weight/Valency or Charge number

So, the equivalent weight of oxalic acid is = 126/2 = 63

Thus, the equivalent weight of Oxalic acid is 63.

Oxalic Acid Structure

Oxalic acid is a dicarboxylic acid and its structure consist of two -COOH group attached together as shown in the diagram. 

Preparation of Oxalic Acid

Although oxalic acid is readily available in nature and can be extracted from different means, but also oxalic acid can be made in the laboratory by oxidizing sucrose with nitric acid in the presence of a small amount of vanadium pentoxide as a catalyst. The hydrated solid can be dehydrated by heat or azeotropic distillation.

Procedure

• Take the watch glass, wash it with distilled water, and let it dry.  
• Weigh the exact amount of clean and dry watch glass and record its weight in a notebook. 
• Correctly weigh 3.15 g of oxalic acid on a  watch glass and write down this weight in a notebook.  
• Using a  funnel, gently and carefully transfer oxalic acid to a clean, dry volumetric flask from the watch glass. 
• Wash the watch glass with distilled water and use a wash bottle to wash away the particles adhering to the watch glass with foam.
• For completing the experiment, the amount of water should not exceed 50 ml.
• Rinse the funnel with distilled water several times and use a wash bottle to flush the adherent particles into the volumetric flask.
• Add a small amount of water while washing the funnel. 
• The amount of distilled water used for this should not exceed 50 ml.
• Rinse the funnel with distilled water and rinse the solution to the funnel into a flask. 
• Rotate the volumetric flask until the oxalic acid dissolves. 
• Using a  wash bottle, fill the volumetric flask with sufficient distilled water just below the etched marks.
• Add the last ml of distilled water droplets to the volumetric flask until the reduced meniscus level touches the mark.
• Place a stopper in the opening of the flask and shake gently to even out the entire solution. 
• Now, we will get a solution of M/10 oxalic acid.

Laboratory Preparation of Oxalic Acid

In the laboratory, Oxalic Acid is easily produced by the oxidation of carbohydrates using any strong oxidizing agent. The reaction it follows is,

C₁₂H₂₂O₁₁ + 18[O] (HNO₃ and V₂O₅) —-> (COOH)₂ + H₂O

Commercial Preparation of Oxalic Acid

We produce Oxalic acid commercially by treating carbohydrates with nitric acid in the presence of a catalyst vanadium pentoxide, similar to that of laboratory preparation. Also, we can obtain oxalic acid by carbonylation of alcohol.

Properties of Oxalic Acid

The properties of Oxalic Acid can be divided into two parts,

• Physical Properties of Oxalic Acid
• Chemical Properties of Oxalic Acid

Physical Properties of Oxalic Acid

The physical properties of oxalic acid are,

• The molar mass of oxalic acid is 90 g/mol and for the hydrated oxalic acid molar mass is 126 g/mol. 
• Its Density is 1.90 g/cm³. 
• Its Melting points vary from 1890°C to 1910°C.
• It appears as a white crystal and smells odourless.
• It forms a colourless solution in water.
• Its heat capacity is 91.0 J·mol^-1·K^-1

Chemical Properties of Oxalic Acid

The chemical properties of oxalic acid are,

• For oxalic acid, the valence coefficient is 2. 
• It readily reacts with typical carboxylic acids to form acid chlorides and ester derivatives. 
• Oxalic acid is highly toxic and therefore harmful. Inhalation can cause serious damage to mucous membranes and the respiratory tract. Also, symptoms such as cough, and wheezing are one of the symptoms of this. It is also toxic if swallowed and can cause kidney failure.
• When oxalic acid reacts with Potassium Manganate(VII), get dissociates as:

2MnO₄^– + 5C₂H₂O₄ + 6H₃O⁺   →   2Mn²⁺  +  10CO₂ + 14H₂O

• The reaction of oxalic acid with concentrated sulphuric acid is given by,

 C₂H₂O₄ + H₂SO₄ (conc.)  →  CO + CO₂ + H₂O

Chemical Reactions of Oxalic Acid

Various chemical reactions of oxalic acid include,

Heat Treatment of Oxalic Acid

Heating Oxalic Acid at high temperatures of 150˚C results in the decarboxylation of oxalic acid and it results in the formation of Formic acid and carbon dioxide.

(COOH)₂ (heating)—-> HCOOH + CO₂

Reaction of Sulphuric Acid with Oxalic Acid

When oxalic acid is treated with conc. H₂SO₄ It produces carbon dioxide and carbon monoxide and water is removed.

(COOH)₂ + H₂SO₄(heating)—-> CO₂ + H₂O + CO

Action of KMnO₄ on Oxalic Acid 

Oxalic acid oxidized to carbon dioxide and water in the presence of acidified Potassium Permanganate solution.

(COOH)₂ + [O] (KMnO₄ and H₂SO₄)—-> CO₂ + H₂O + CO

Oxalic Acid Uses

Various uses of Oxalic Acid are discussed below,

• Oxalic acid encourages the dying process.
• As a strong acid, it is also useful as an extractor and reducing agent for lanthanide metals and as an analytical reagent.
• It can be used for polishing marble carvings and in bleach. 
• Oxalic acid is helpful in removing food and ink stains. 
• Also useful for developing movies and used in wastewater treatment to remove calcium deposits.

Health Hazard of Oxalic Acid

Oxalic Acid has very serious health implications for the human body. Some of the health implications of Oxalic acid are,

• It is toxic in nature and ingestion of oxalic acid cause vomiting, diarrhea, severe gastrointestinal disorder, etc.
• On contact with the eyes, it causes irritation and can damage the eyes.
• Overconsumption of oxalic acid causes kidney damage. 
• Inhaling fumes of oxalic acid can cause headaches, dizziness, nausea, etc.

Sample Questions on Oxalic Acid

Question 1: Write the formula for oxalic acid and determine its basicity.

Answer:

The formula for oxalic acid is (COOH)₂.2H₂O and its basicity is 2.

Question 2: How to convert oxalic acid solution M/20 to oxalic acid solution N/20?

Answer:

To make the conversion, we have to add an equal volume of water so that the solution is converted to N/20.

Question 3: What happens when KMnO4 reacts with oxalic acid?

Answer: 

The reaction between oxalic acid and potassium permanganate is endothermic as a redox reaction that occurs in the presence of sulphuric acid and heat. Potassium permanganate and sulphuric acid produce oxygen, which reacts with oxalic acid to form carbon dioxide and water.

Question 4: Which indicator do you use to titrate KMnO4 with oxalic acid?

Answer:

Potassium permanganate itself is purple and serves as its own indicator.

Question 5: Why do we heat oxalic acid solution containing sulphuric acid up to 50–60°C in the permanganate titration?

Answer:

When heated to 50-60 °C, the oxalic acid remains decomposed, which contributes to a better interaction between oxalate and potassium permanganate.

FAQs on Oxalic Acid

Q1: What is Oxalic Acid?

Answer:

Oxalic Acid is a poisonous crystalline acid with a sour taste with chemical formula (COOH)₂ or C₂H₂O₄.

Q2: What is the Formula of Oxalic Acid?

Answer:

Carboxlic acid is the simplest dicarboxylic acid containing only two carboxylic acids directly bonded to each other at carbon atoms, i.e. (COOH) groups. Hence, the formula of oxalic acid is HOOC-COOH or (COOH)₂ or C₂H₂O₄.

Q3: What are the Sources of Oxalic Acid?

Answer:

Oxalic acid occurs naturally in many plants, such as cocoa, leafy green vegetables, nuts, seeds, sweet potatoes, star fruits, etc.

Q4: How many Water Molecules are associated with Crystalline Oxalic Acid?

Answer:

The formula for crystalline oxalic acid is (COOH)₂.2H₂O or C₂H₂O₄.2H₂O, thus two water molecules are associated with crystalline oxalic acid.

Q5: What is formed when Oxalic Acid is Dehydrated?

Answer:

By heating oxalic acid with concentrated sulfuric acid, the water is removed, and other than water, carbon monoxide and carbon dioxide gas are produced.

C₂H₂O₄ + H₂SO₄ (conc.)  →  CO + CO₂ + H₂O