Oxalic Acid - Formula, Uses, Structure, Properties, FAQs

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What is Oxalic Acid?

Oxalic acid is a colourless, crystalline and poisonous organic compound which is a member of the family of carboxylic acids and is the simplest dicarboxylic acid. The IUPAC name of oxalic acid is ethanedioic acid and it is represented by a chemical formula H₂C₂O₄. The equivalent weight of oxalic acid or equivalent mass of oxalic acid (molecular weight of oxalic acid) or the molar mass of oxalic acid is is 90 g/mol and it is generally present in its crystalline hydrated form i.e., as oxalic acid dihydrate with chemical formula;

This Story also Contains

What is Oxalic Acid?
Oxalic Acid Structure
Preparation of Oxalic Acid
Calculation of equivalent weight of crystalline oxalic acid
Physical Properties of Oxalic Acid
Chemical Properties of Oxalic Acid
Uses of oxalic acid

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Oxalic Acid Structure

The molecular formula of oxalic acid is H₂C₂O₄ and structurally it can be represented as follows:

Oxalic Acid Structure

The oxalic acid exists in two crystalline structures, in one the hydrogen bonding which leads to the chain like structure of the molecule and in other, the hydrogen bonding pattern form a definite sheet like structure.

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Preparation of Oxalic Acid

Oxalic acid occurs as potassium hydrogen oxalate in the wood sorrel, tomatoes and rhubarb. The insoluble calcium oxalate is found in some stone deposits in gall bladder and kidney in the human body. The oxalic acid can be synthesised as per following methods:

1. Industrial manufacture of oxalic acid:

Firstly, sodium oxalate is formed by heating sodium formate to 400°C as per following reaction:

1. Industrial manufacture of oxalic acid

Then the sodium oxalate formed, is dissolved in the solution of water and calcium hydroxide. Solution of sodium oxide is formed along with the removal of calcium oxalate as a precipitate as per following reaction:

sodium oxalate formed

The solution is then filtered and treated with the calculated amount of dilute sulphuric acid to liberate oxalic acid. The reaction involved in the process is represented as follows:

Calcium sulphate

Calcium sulphate formed in the reaction is filtered out as a precipitate and oxalic acid is crystallised from the filtrate as dihydrate.

2. Laboratory method for preparation of oxalic acid:

In the laboratories, oxalic acid is prepared by oxidation of sucrose with concentrated nitric acid in the presence of a catalyst i.e., vanadium pentoxide. Basically, the -CHOH.CHOH units present in sucrose molecules split out and get oxidized to form oxalic acid. The reaction involved in the process is represented as follows:

oxalic acid

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Calculation of equivalent weight of crystalline oxalic acid

Oxalic acid is a dibasic acid which means it has a tendency to donate two hydrogen ions (H⁺), so the n-factor or valency factor of oxalic acid is 2. Therefore, the equivalent mass of oxalic acid can be calculated as per following formula:

Equivalent mass=molar mass / n-factor

As we know that the oxalic acid generally exist in its hydrated form and molar mass of oxalic acid dihydrate formula mass is 126gmol^-1. Substituting the values, the equivalent mass of oxalic acid will be as follows:

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Thus, equivalent weight of oxalic acid is 63g.

Physical Properties of Oxalic Acid

At its crystalline form, it exists as colourless prismatic crystals of oxalic acid dihydrate i.e., (COOH)₂.2H₂O. The oxalic acid dihydrate melts at 101.5ºC while the anhydrous oxalic acid melts at 189.5ºC. The hydrate acid becomes anhydrous when it is heated to a temperature of 150ºC. Oxalic acid is an active poison, depressing the central nervous system and causes malfunction of kidneys.

Chemical Properties of Oxalic Acid

The oxalic acid is composed of two carboxyl groups bonded via single bond in direct union. It undergoes all the usual reactions of COOH group twice. Also, the oxalic acid undergoes some peculiar reaction.

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1. Formation of mono and di-derivatives:

Oxalic acid is a much stronger acid as compared to acetic acid and have a tendency to readily form a series of salts, esters, acid halides and amides. The examples are shown as follows:

Formation of mono and di-derivatives

2. The action of heat on oxalic acid:

When oxalic acid is heated at 150ºC, decarboxylation takes place and formic acid is formed along with the removal of carbon dioxide. The reaction is represented as follows:

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3. Reaction of oxalic acid with sulphuric acid:

When oxalic acid is heated with concentrated sulphuric acid, it’s decomposed to form carbon dioxide and carbon monoxide along with the removal of water. The reaction takes place as follows:

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4. Reaction of oxalic acid with glycerol:

Oxalic acid react with glycerol to form formic acid depending on the experimental conditions. If oxalic acid reacts with glycerol at 530K, then allyl alcohol is formed as per following reaction:

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5. Action with KMnO₄:

Oxalic acid readily oxidized in the presence of acidified potassium permanganate to form carbon dioxide and water as per following reaction:

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Uses of oxalic acid

Some important uses of oxalic acid are discussed below:

1. It is used for removing ink stains as well as for bleaching straw for hats, since it has a tendency to reduce brown ferric compounds to soluble and colourless ferrous salts.

2. Oxalic acid is used as a mordant in dyeing and calico printing.

3. It is used in the manufacture of inks and metal polishes.

4. It is widely used in the synthesis of allyl alcohol and formic acid in the laboratory.

5. It is also used in redox titrations.

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NCERT Chemistry Notes:

Frequently Asked Questions (FAQs)

1. Q1. Oxalic acid is a weak acid or a strong acid?

Oxalic acid is a weak acid also known as soft acid. It is weaker than hydronium ion i.e., H₃O⁺ but stronger than acetic acid, sulphuric acid, etc. due to the carboxyl group present in the molecule.

2. Q2. Which acid is present in spinach?

Oxalic acid present in spinach.

3. Q3. Is oxalic acid polar or nonpolar?

As oxalic acid is a dibasic acid i.e., it has a tendency to donate two hydrogen atoms. Thus, it is a polar molecule and capable of forming hydrogen bonds with water molecules. Since it is a polar molecule, it is soluble in all polar solvents.

4. Q4. Mention two points of advantages as well as disadvantages of oxalic acid.

Advantages-

a. Having adequate levels of oxalic acid in blood eliminates all abnormal cells effectively with no harmful side effect and thus, helps in curing cancer.

b. Oxalic acid can be reused and is very useful in beekeeping.

Disadvantages-

a. Oxalic acid can cause extensive damage to plants.

b. Oxalic acid in its crystalline form is poisonous.

5. Q5. Oxalic acid can be manufactured in the presence of nitric acid, by oxidation of:

Ethylene glycol

b. Sucrose

c. Ethanedioic acid

d. Propane

Ans. Correct answer is option (B).

6. How does the structure of oxalic acid contribute to its acidity?

The structure of oxalic acid contributes to its acidity due to the presence of two carboxyl groups (-COOH). These groups can easily donate protons (H+) in solution, making oxalic acid a strong organic acid. The proximity of the two carboxyl groups also enhances its acidity through inductive effects.

7. What are oxalates, and how are they related to oxalic acid?

Oxalates are the salt forms of oxalic acid. When oxalic acid loses its hydrogen ions, it forms oxalate ions (C2O4^2-). These oxalate ions can combine with various cations like calcium, potassium, or sodium to form oxalate salts. In nature and in the body, oxalic acid often exists in its oxalate form rather than as the free acid.

8. Why is oxalic acid considered a reducing agent?

Oxalic acid is considered a reducing agent because it can easily donate electrons to other substances in chemical reactions. This property is due to its ability to be oxidized to carbon dioxide (CO2). In redox reactions, oxalic acid can reduce other compounds while being oxidized itself.

9. How does oxalic acid interact with metal ions?

Oxalic acid readily forms complexes with many metal ions, acting as a chelating agent. It can bind to metal ions through its two carboxyl groups, forming stable metal oxalate complexes. This property is utilized in various applications, such as rust removal and metal cleaning, where oxalic acid can effectively dissolve metal oxides.

10. How does oxalic acid affect calcium absorption in the body?

Oxalic acid can interfere with calcium absorption in the body by forming insoluble calcium oxalate complexes in the digestive tract. This can reduce the bioavailability of calcium from foods consumed alongside oxalate-rich foods. However, the impact is generally small in a balanced diet and is more significant for individuals with specific health concerns or dietary restrictions.

11. What is the molecular formula of oxalic acid?

The molecular formula of oxalic acid is C2H2O4. This formula represents two carbon atoms, two hydrogen atoms, and four oxygen atoms in each molecule of oxalic acid.

12. Why is oxalic acid considered a dicarboxylic acid?

Oxalic acid is considered a dicarboxylic acid because it contains two carboxyl groups (-COOH) in its molecular structure. These two carboxyl groups are responsible for its acidic properties and ability to form salts and esters.

13. What is the IUPAC name for oxalic acid?

The IUPAC name for oxalic acid is ethanedioic acid. This name is derived from the two-carbon chain (ethane) and the presence of two carboxylic acid groups (dioic acid).

14. How does the structure of oxalic acid differ from other dicarboxylic acids?

Oxalic acid is the simplest dicarboxylic acid, with two carboxyl groups directly connected to each other. This structure differs from other dicarboxylic acids like malonic or succinic acid, which have one or more carbon atoms between the carboxyl groups. The unique structure of oxalic acid contributes to its higher acidity and distinct chemical properties.

15. How does oxalic acid interact with proteins?

Oxalic acid can interact with proteins in several ways:

16. How does the melting point of oxalic acid compare to other organic acids?

Oxalic acid has a relatively high melting point (about 189-191°C) compared to many other organic acids. This high melting point is due to strong hydrogen bonding between oxalic acid molecules in the solid state, resulting from the presence of two carboxyl groups per molecule.

17. What is the difference between oxalic acid and its anhydrous form?

Oxalic acid typically exists in its dihydrate form (C2H2O4 · 2H2O), which contains two water molecules per oxalic acid molecule. The anhydrous form (C2H2O4) has no water of crystallization. The dihydrate form is more common and stable at room temperature, while the anhydrous form is obtained by careful dehydration and is more reactive.

18. How does oxalic acid compare to other common household acids in terms of strength?

Oxalic acid is stronger than many common household acids like acetic acid (vinegar) or citric acid, but weaker than mineral acids like hydrochloric or sulfuric acid. Its strength is due to its ability to dissociate both of its protons relatively easily. This makes it effective for cleaning and descaling but also means it requires careful handling to avoid skin irritation or other harmful effects.

19. How does oxalic acid affect soil chemistry?

Oxalic acid can significantly impact soil chemistry:

20. What is the role of oxalic acid in the formation of certain minerals?

Oxalic acid plays a significant role in the formation of certain minerals, particularly oxalate minerals. Some examples include:

21. How is oxalic acid synthesized industrially?

Industrial synthesis of oxalic acid typically involves the oxidation of carbohydrates or hydrocarbons. Common methods include:

22. What are the differences between synthetic and naturally occurring oxalic acid?

The main differences between synthetic and naturally occurring oxalic acid are:

23. What is the relationship between oxalic acid and photosynthesis in plants?

Oxalic acid is related to photosynthesis in plants in several ways:

24. What are the environmental impacts of oxalic acid use and disposal?

The environmental impacts of oxalic acid include:

25. What is the role of oxalic acid in the weathering of rocks and minerals?

Oxalic acid plays a significant role in rock and mineral weathering:

26. What are some common uses of oxalic acid in industry?

Oxalic acid has various industrial applications, including:

27. How does oxalic acid contribute to the taste of certain foods?

Oxalic acid contributes to the tart or sour taste in many foods, particularly in plants like rhubarb and sorrel. Its acidic nature gives these foods their characteristic sharp flavor. However, cooking can often reduce the oxalic acid content and thus alter the taste of these foods.

28. What is the role of oxalic acid in the formation of kidney stones?

Oxalic acid plays a significant role in the formation of certain types of kidney stones. When oxalic acid combines with calcium in the body, it forms calcium oxalate crystals. If these crystals accumulate in the kidneys or urinary tract, they can grow into painful kidney stones. This is why individuals prone to kidney stones are often advised to limit their intake of oxalate-rich foods.

29. How does oxalic acid form in the human body?

In the human body, oxalic acid is primarily formed as a metabolic end-product of vitamin C (ascorbic acid) and certain amino acids. It can also be absorbed from dietary sources. The body typically excretes oxalic acid through urine, but excessive accumulation can lead to health issues like kidney stones.

30. What is the relationship between oxalic acid and vitamin C metabolism?

Oxalic acid is one of the end products of vitamin C (ascorbic acid) metabolism in the body. When vitamin C is broken down, a portion of it can be converted to oxalic acid. This relationship is important to consider for individuals prone to kidney stones, as high vitamin C intake might potentially increase oxalate levels in the body.

31. How does oxalic acid occur naturally?

Oxalic acid occurs naturally in many plants, particularly in leafy greens like spinach, rhubarb, and sorrel. It is also found in some fruits and vegetables. In plants, oxalic acid often exists as oxalate salts, which can form crystals in plant tissues.

32. What role does oxalic acid play in plant defense mechanisms?

In plants, oxalic acid serves as a defense mechanism against herbivores and pathogens. The high concentration of oxalates in some plants can make them unpalatable or toxic to animals. Additionally, oxalic acid can help plants tolerate heavy metals by forming complexes with them, reducing their toxicity to the plant.

33. What are the health risks associated with oxalic acid exposure?

Health risks associated with oxalic acid exposure include:

34. How does oxalic acid affect the nutritional value of foods?

Oxalic acid can affect the nutritional value of foods in several ways:

35. What is the significance of oxalic acid in the study of plant evolution?

Oxalic acid is significant in plant evolution studies for several reasons:

36. What is the significance of oxalic acid in forensic science?

In forensic science, oxalic acid is significant for several reasons:

37. What is the role of oxalic acid in the paper and pulp industry?

In the paper and pulp industry, oxalic acid serves several purposes:

38. How does oxalic acid contribute to the preservation of wood?

Oxalic acid contributes to wood preservation in several ways: