Oxalic Acid- Preparation, Physical and Chemical Properties, Uses, Practice problems, FAQs.

Why is it at all necessary to understand the chemistry of oxalic acid? Let us find out. Imagine a salad made with some leafy greens, tomatoes and almonds. Also some soy yoghurt, sweet potato-beet root smoothie with some raspberry toppings.

Now can you guess about a common link between the food items that I have just stuffed in? And your answer is- High oxalic acid/oxalate content.

Often we end up consuming around 200 - 300 mg of oxalates daily. Although people prone to kidney stone risks or kidney-related disorders are often recommended by doctors to lower their oxalate intake to 50-100 mg per day depending on the severity of the case. 

Hence it is important to understand the chemistry of this acid!

TABLE OF CONTENTS

• What is Oxalic Acid?
• Formula and Structure of Oxalic acid
• Laboratory Preparation of Oxalic Acid
• Commercial Preparation of Oxalic Acid
• Physical Properties of Oxalic Acid
• Chemical properties of Oxalic Acid
• Chemical Reactions of Oxalic Acid
• Use of Oxalic Acid
• Practice Problems
• Frequently Asked Questions-FAQs

What is Oxalic Acid?

Oxalic Acid is the simplest dicarboxylic acid with the IUPAC name Ethanedioic Acid and a molecular formula of HOOC - COOH. It is also known as Oxiric acid. It is a colourless crystalline acid which dissolves in water, forming a colourless solution.

Since 1745, salts of oxalic acid were extracted from wood sorrel by Dutch scientist Herman Boerhaave. However, later in the year 1773, François Pierre Savary isolated oxalic acids from these salts. Finally, oxalic acid was prepared synthetically in a laboratory in the year 1776.

Approximately 120,000 tonnes of oxalic acid are produced every year across the globe for usage.

Structure and Formula Oxalic Acid 

The formula of oxalic acid is H₂C₂O₄ and is a diprotic acid. Its conjugate base is called oxalate, (C₂O₄)^2-. 

Oxalic acid can also exist in the form of dihydrate H₂C₂O₄. 2H₂O It can exist as a dimer as well owing to hydrogen bond formation.

        Laboratory Preparation of Oxalic Acid

1. The most common method of preparing oxalic acid in the laboratory is by oxidation of certain carbohydrates such as sucrose by a strong oxidising agent nitric acid in presence of a catalyst like vanadium pentoxide.

In this reaction, the - CHOH. CHOH units present in the sucrose break up forming the di-carboxylic group of oxalic acid.

1. Another newly devised method is electrocatalysis, in which, with the help of a copper complex, carbon dioxide is reduced to oxalic acid. In this process, large quantities of carbon dioxide are utilized.
2. Ethylene glycol on oxidation with acidified potassium dichromate gives oxalic acid.
3. 
4. Upon hydrolysis of cyanogen with conc. HCl, oxalic acid is obtained.

Commercial Preparation of Oxalic Acid

• Oxalic acid is produced commercially by treating carbohydrates with nitric acid in the presence of a catalyst vanadium pentoxide, similar to that of laboratory preparation. The carbohydrate precursor that undergoes hydrolysis to form the compound of interest can be glycolic acid or even ethylene glycol. 
• Alternatively, a new method has been devised that involves carbonylation of alcohol to yield oxalic acid diester.
• 
• An important method to prepare oxalic acid is from sodium formate in the presence of an alkaline catalyst. This pyrolysis method leads to the formation of sodium oxalate, which can be later converted to oxalic acid.

Firstly, sodium formate is heated at 400' C to give the following product:

The sodium oxalate obtained is then dissolved in an aqueous solution of  Ca(OH)₂. Calcium oxalate precipitates out of the solution.

The calcium oxalate solid is with a calculated amount of dilute sulphuric acid in order to liberate oxalic acid. The reaction can be represented as follows:

Calcium sulphate precipitate is filtered out and oxalic acid is crystallized from the filtrate as a dihydrate.

Physical Properties of Oxalic Acid

• Oxalic acid is an odourless, white, prismatic crystalline solid with a molar mass of 126 g mol^-1. 
• The density of the compound is 1.90 g/cc in an anhydrous state and 1.653 g/cc in dihydrate state. 
• The melting point of oxalic acid ranges from 189.5˚C to 191˚C. The dihydrate acid becomes anhydrous when heated above 150˚C
• It is soluble in water, absolute alcohol as well as in ether.
• Oxalic acid can act as a poison. If ingested or inhaled, can cause severe irritation and tissue damage.
• Oxalic Acid absorbs moisture from the air and hence, is hygroscopic in nature.

Chemical Properties of Oxalic Acid

• Oxalic acids have strong acidic strength, being a dibasic acid it can donate two H⁺ions.
• Oxalic acid is a good reducing agent owing to its hydrogen donating nature.
• Oxalates, (C₂O₄)^2-, are good as chelating agents forming chelate complexes with metal cations.
• Equivalent weight of the stable Oxalic acid dihydrate is given by, 

Since It is a dibasic acid,  = 2

Equivalent mass of Oxalic acid dihydrate is given by = 

=

= 

Chemical Reactions of Oxalic Acid

Oxalic acid consists of two carboxyl groups. Hence it gives most of the reactions of the carboxylic acid group twice owing to two - COOH groups.

• Mono and Di-derivatives of Oxalic Acid: Oxalic acid is a stronger acid as compared to acetic acid and readily produces a series of salts, namely- esters, acid halides and amides. Examples:

• Heat Treatment of Oxalic Acid: On being heated at 150˚C, decarboxylation of oxalic acid occurs and formic acid along with carbon dioxide is produced. 

• Reaction of Sulphuric acid with Oxalic Acid: Oxalic acid on being treated with conc. H₂SO₄ produces carbon dioxide, carbon monoxide along with removal of water.

• Reaction of Oxalic Acid with Glycerol: When glycerol is heated with oxalic acid at 383 K, formic acid is produced as one of the product. 

When glycerol is heated with oxalic acid at around 530K, allyl alcohol is produced.

• Action of KMnO₄ on Oxalic Acid: Oxalic acid gets easily oxidized to carbon dioxide and water in presence of acidified potassium permanganate solution.

• Esterification of Oxalic acid with alcohols: Due to presence of two carboxylic acid groups , it forms di-alkyl esters with alcohol. An esterification reaction of oxalic acid can be stated as given below.

• Reaction of Oxalic acid with Ammonia: Oxalic acid reacts with ammonia to form oxamide with the intermediate formation of ammonium oxalate.

• Acidic Nature of Oxalic acid: Being an organic acid, it is weak, but it is stronger than acetic acid. Some reactions of oxalic acid with bases and basic salts are used herein to represent its acidic nature.

Uses of Oxalic Acid

• Oxalic acid has the ability to form water-soluble complexes with ferrous ions present on the surface of the metals. Owing to this property of complex formation, oxalic acid is used as a cleaning agent to dissolve rust.
• Oxalic acid is used as a bleaching agent in order to bleach wood pulp. It is also used as a mordant to fix colours on the fabric in the dye industry.
• Lanthanides, elements belonging to the f-block of the periodic table, form a compound called hydrated lanthanide oxalate in the presence of oxalic acid. These oxalates can later be heated to form oxides of the elements; this process is useful in obtaining pure lanthanides from a mixture of other elements.
• Oxalic acids are used in the process of coating the surface of aluminium, also called aluminium anodizing. Oxalic acid is preferred over sulphuric acid, as oxalic acid layers are thinner and smoother.
• Because of its bleaching properties, oxalic acid is also used in teeth whitening products.
• Oxalic acid also has applications in the photography industry where it is used in the development of photographic films.
• Oxalic acid has great complex-forming abilities or chelation properties, due to which it is used in water treatment plants to remove calcium deposits present.

Practice Problems

Q1. What is the significance of oxalic acid in industries?

Answer: Oxalic acid is greatly used as an acid rinse in laundries, where it is effective in removing rust and ink stains owing to its reducing nature. Also because it converts most insoluble iron compounds into a soluble complex ion.

Q2.  Oxalic acid is made to react with conc. H₂SO₄ and the resultant mixture is made to pass through KOH solution. Which of the following will come out of the solution unchanged?

A. CO
B. CO₂
C. C₃O₂
D. HCOOH

Answer: Option A is correct. Oxalic acid on being treated with conc. H₂SO4 produces carbon dioxide, carbon monoxides along with the removal of water. KOH being a strong base, preferably absorbs CO₂.

Hence the gas which comes out finally is CO. So Option A is correct.

Q3.  IUPAC name of Oxalic acid is?

A. Ethanoic Acid
B. Ethanedioic Acid
C. Oxiric Acid
D. Butanedioic Acid

Answer: Option B is correct. Since the long chain has 2 carbon atoms and a functional group is - COOH present at both these two positions, So Ethanedioic acid is the answer.

Q.4 Oxalic acid can be obtained in the presence of nitric acid, by oxidation of :

A. Ethylene Glycol
B. Sucrose
C. Ethanedioic acid
D. Propane

Answer: On oxidation of carbohydrates by strong oxidizing agents like nitric acid, in presence of catalyst like V₂O₅, oxalic acid is produced. Hence Option B is the answer. 

Frequently Asked Questions (FAQs)

Question 1.  Is oxalic acid polar or non-polar?

Answer: Two carboxylic acid groups are present in oxalic acid (HOOC - COOH). These groups are polar as it has a tendency to donate 2 H⁺ ions and form the conjugate base. Also, it can form hydrogen bonds with molecules of water. Oxalic acid is therefore an ideal water-solute. In fact, it is polar and is thus soluble in all polar solvents.

Question 2. What are the natural sources of oxalic acid?

Answer: The most common sources are leafy greens like spinach, cabbage, broccoli, and brussels sprouts; tomatoes, soy milk, soy yoghurt, almonds, nuts, and raspberries etc.

Question 3. Is oxalic acid weak or strong?

Answer: Oxalic acid is an organic acid and hence a weak acid as compared to mineral acids. It is weaker than H₃O⁺ but stronger than acetic acid, nitrous acid and benzoic acid due to the two carboxyl group present in the molecule.

Question 4. Is oxalic acid anyhow linked with kidney stones?

Answer: It is estimated that approximately 80% of kidney stones, globally, are composed of calcium oxalate (Ca - Ox) mixed with calcium phosphate. A large number of people with potential risk of kidney stones are recommended by doctors to lower their oxalate intake levels to 50 - 100 mg per day.

Question 5. How can we eliminate oxalates from the body?

Answer: It is possible, by increasing the amount of calcium in the diet. A diet rich in calcium intake will help to reduce the amount of oxalate being absorbed by the human body. And as a result, stones are less likely to form.

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