Citric Acid

1.0What is Citric Acid?

Citric Acid Definition

Citric acid is a weak organic acid found naturally in citrus fruits like lemons, oranges, and limes.
Chemically, it is a tricarboxylic acid, meaning it contains three carboxyl (-COOH) groups, which give it acidic properties.

It is widely used in food, medicine, and industry for its sour taste, preservative properties, and chelating ability(binding with metal ions).
In chemical terms, it belongs to the class of carboxylic acids and plays an essential role in metabolism through the citric acid cycle (Krebs cycle) in living organisms.

Natural Occurrence and Discovery

Citric acid was first isolated in 1784 by Carl Wilhelm Scheele from lemon juice.
It occurs naturally in almost all aerobic organisms as an intermediate in the Krebs cycle, where it helps in energy production from carbohydrates, fats, and proteins.

Why Citric Acid is Important in Science and Daily Life

Citric acid plays a vital role in biochemistry (Krebs cycle), food industry, cosmetics, pharmaceuticals, and industrial cleaning. Its ability to adjust pH, bind metal ions, remove stains, and inhibit microbial growth makes it one of the most widely used natural additives.

2.0Chemical Composition and Formula

• Molecular Formula: C₆H₈O₇
• Molar Mass: 192.12 g/mol
• Citric acid consists of six carbon atoms, eight hydrogen atoms, and seven oxygen atoms.
• It has three carboxylic acid groups (-COOH) and one hydroxyl group (-OH) attached to a hydrocarbon backbone.
• The structural formula of citric acid can be represented as:
  HOOC–CH₂–C(OH)(COOH)–CH₂–COOH
• This structure explains its acidic behavior and ability to form hydrogen bonds and complexes with metal ions.

Citric acid IUPAC Name and Classification

• IUPAC Name: 2-hydroxy-1,2,3-propane-tricarboxylic acid
• Classification: Tricarboxylic acid (organic acid)
  Citric acid is both a weak acid and a natural preservative, making it highly useful in chemical and biological systems.

Explanation of Structure:

• It has three -COOH (carboxylic acid) groups, which are acidic.
• One -OH (hydroxyl) group makes it polar and highly soluble in water.
• The central carbon atom connects to both the hydroxyl and one carboxyl group, giving citric acid its unique acidic properties.

Because of this molecular structure, citric acid acts as a weak acid and an excellent chelating agent — it can bind to metal ions and neutralize them, which is why it’s widely used in cleaning and preservation.

3.0Physical and Chemical Properties of Citric Acid

Physical Properties

Citric acid is non-volatile and hygroscopic, meaning it can absorb moisture from the air.

Chemical Properties

1. Acidic Nature:
   It releases H⁺ ions when dissolved in water, showing its weakly acidic behaviour.
   $C_6{H}_8{O}_7\rightleftharpoons 3H^{+}+C_6{H}_5{O}_7^{3-}$
2. Reacts with Bases:
   Forms salts known as citrates when neutralized by bases.
   Example: $C_6{H}_8{O}_7+3NaOH\to N{a}_3{C}_6{H}_5{O}_7+3H_{2}O$
3. Decomposition:
   On heating, citric acid decomposes to form carbon dioxide (CO₂) and water (H₂O).

4.0Natural Sources of Citric Acid

Citrus Fruits and Vegetables

Citric acid occurs naturally in:

• Lemons
• Limes
• Oranges
• Grapefruits
• Tomatoes
• Pineapples

Lemon juice contains about 5–8% citric acid by weight.

Other Biological Sources

Citric acid is also found in small amounts in berries, kiwi, strawberries, and even human tissues, where it plays a role in metabolism.

Industrial Production: Fermentation Process

Most commercial citric acid today is produced industrially through fermentation using the mold Aspergillus niger. The process involves:

1. Using sugar solutions (from molasses or corn syrup) as a carbon source.
2. Fermenting under controlled conditions.
3. Extracting and crystallizing citric acid.

This method is cost-effective, sustainable, and ensures large-scale production.

5.0Environmental Impact and Biodegradability

Eco-Friendly Nature of Citric Acid: Citric acid is biodegradable, non-toxic, and environmentally safe. It breaks down easily without leaving harmful residues, making it a sustainable alternative in green cleaning and eco-friendly products.

Role in Green Chemistry: In modern industries, citric acid plays a role in green chemistry, serving as a natural catalyst and chelating agent that reduces the need for synthetic chemicals.

6.0Production of Citric Acid (Industrial Process)

Fermentation Process

Industrially, citric acid is mainly produced using the fermentation method with microorganisms such as Aspergillus niger (a fungus).
Steps include:

1. Fermentation Medium Preparation:
   Sucrose or glucose solution acts as a carbon source.
2. Inoculation:
   The fungus Aspergillus niger is added to the medium.
3. Fermentation:
   Under controlled conditions of pH, temperature (around 30°C), and aeration, the microorganism converts sugar into citric acid.

$C_6{H}_{12}{O}_6\to C_6{H}_8{O}_7+H_{2}O$

4. Separation and Purification:
   The acid is separated, filtered, and crystallized to obtain pure citric acid crystals.

Raw Materials Used

• Sugar sources: Molasses, glucose, or sucrose.
• Microorganism: Aspergillus niger.
• Nutrients: Ammonium salts, phosphates, and trace metals.

Purification and Crystallization

After fermentation:

• The citric acid solution is filtered to remove fungal biomass.
• It is treated with lime (Ca(OH)₂) to form calcium citrate, which is then acidified with sulfuric acid (H₂SO₄) to regenerate pure citric acid.
• Finally, it is crystallized and dried for commercial use.

7.0Chemical Reactions of Citric Acid

Reaction with Bases (Neutralization)

Citric acid reacts with strong bases to form citrate salts:

$C_6{H}_8{O}_7+3NaOH\to N{a}_3{C}_6{H}_5{O}_7+3H_{2}O$

These citrates are used in pharmaceuticals and food preservation.

Reaction with Carbonates

When citric acid reacts with carbonates or bicarbonates, it produces carbon dioxide gas:

$C_6{H}_8{O}_7+3NaHC{O}_3\to N{a}_3{C}_6{H}_5{O}_7+3C{O}_2+3H_{2}O$

This reaction is used in effervescent tablets and baking powders.

Ester Formation

With alcohols, citric acid forms esters (citric esters), which are used in flavoring agents and plasticizers.

8.0Citric Acid Uses

In Food and Beverages

• Acts as a flavoring agent and preservative in soft drinks, candies, and fruit juices.
• Prevents discoloration and spoilage by controlling pH.
• Used in baking powders to release CO₂.
• Enhances the sour taste in foods.

In Medicine and Pharmaceuticals

• Used to adjust pH in medicines and syrups.
• Forms buffer solutions in biochemistry.
• Combined with sodium or potassium to treat kidney stones and urinary disorders.

In Cleaning and Industrial Applications

• Citric acid acts as a chelating agent, binding metal ions like calcium and magnesium.
• Used in descaling and stain removal products to clean metal, glass, and ceramics.
• Helps in environmentally friendly cleaning due to its biodegradability.

In Biochemistry and Laboratory Use

• Plays a key role in the citric acid cycle (Krebs cycle)—a vital energy-producing process in cells.
• Used as a buffer in laboratory experiments.
• Regulates enzyme activity in biochemical studies.

9.0Role of Citric Acid in Cellular Respiration (Krebs Cycle)

Formation of Citric Acid in Cells

Inside living cells, citric acid is formed in the mitochondria when acetyl-CoA combines with oxaloacetic acid:

Acetyl-CoA + Oxaloacetic acid → Citric acid + CoA

Function in Energy Production

This citric acid enters a cyclic pathway known as the Krebs Cycle or Citric Acid Cycle, which:

• Produces ATP (Adenosine Triphosphate) – the energy currency of the cell.
• Releases CO₂ and H₂O as by-products.
  This cycle is fundamental for metabolism in plants, animals, and microorganisms.

10.0Role of Citric Acid in the Human Body

The Citric Acid Cycle (Krebs Cycle)

In biology, citric acid is a key intermediate in the Krebs cycle, also known as the tricarboxylic acid cycle (TCA cycle). This cycle occurs in the mitochondria of cells and is responsible for:

• Oxidizing carbohydrates, fats, and proteins.
• Generating ATP (adenosine triphosphate), the main energy currency of the cell.

Importance in Metabolism and Energy Production

Without citric acid, cells wouldn’t efficiently convert nutrients into energy. It acts as a metabolic hub, linking multiple biochemical pathways and maintaining energy balance.

11.0Hazards and Precautions

Health and Safety Aspects

• Citric acid is generally safe and non-toxic in small amounts.
• However, concentrated solutions may cause skin or eye irritation.
• Inhalation of citric acid dust should be avoided.

Storage and Handling Guidelines

• Store in a cool, dry place away from moisture.
• Use gloves and goggles while handling concentrated forms.
• Keep tightly sealed to prevent clumping due to humidity.