Phosphoric Acid: A Versatile Inorganic Acid in Modern Industry

Phosphoric acid, also known as orthophosphoric acid, is a widely used inorganic acid with applications ranging from fertilizer production to food processing and dental care. It is a triprotic acid, meaning it can donate three protons (H⁺), and it plays a vital role in both biological systems and industrial chemistry.

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What is Phosphoric Acid?

Phosphoric acid (H₃PO₄) is a colorless, odorless, and non-volatile liquid or crystalline solid depending on the concentration. It is commonly produced industrially by treating phosphate rock with sulfuric acid, yielding a solution that is then concentrated to desired strengths.

Chemical Formula: H₃PO₄

Molecular Weight: 98.00 g/mol

CAS Number: 7664-38-2

Other Names: Orthophosphoric acid, E338 (in food industry)

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Physical and Chemical Properties

Property	Value
Appearance	Colorless liquid or solid
Odor	Odorless
Density (85%)	~1.685 g/cm³
Melting Point	~42.35°C (pure)
Boiling Point	~213°C (decomposes)
Solubility	Miscible with water
Acidity (pKa values)	pKa1 ≈ 2.1, pKa2 ≈ 7.2, pKa3 ≈ 12.3

 

Phosphoric acid is a weak acid compared to mineral acids like hydrochloric or sulfuric acid but can release three protons, making it triprotic and useful for buffering applications.

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Types of Phosphoric Acid

1. Food Grade (75–85%) – Used in beverages, food additives, and dental products.

2. Technical Grade – Used in industrial cleaning, metal treatment, and detergents.

3. Fertilizer Grade – A key ingredient in the production of phosphate fertilizers.

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Applications of Phosphoric Acid

1. Fertilizer Industry

• The largest consumer of phosphoric acid

• Used to manufacture monoammonium phosphate (MAP) and diammonium phosphate (DAP) fertilizers

2. Food and Beverage Industry

• Serves as an acidity regulator (E338) in soft drinks, especially colas

• Used in processed cheeses, jams, and preservatives

3. Metal Treatment

• Involved in rust removal, pickling, and metal surface cleaning

• Converts rust (iron oxide) into stable phosphate coatings

4. Dental and Medical Uses

• Used in dental etchants before applying sealants or composites

• Present in some toothpastes and mouth rinses

5. Water Treatment

• Acts as a corrosion inhibitor for metal pipes

• Helps maintain pH balance in boiler water systems

6. Industrial Cleaners

• A component of acid-based cleaning products for removing mineral scale and buildup

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Safety and Handling

Phosphoric acid is corrosive in concentrated form, and proper handling is essential.

Health Hazards:

• Skin/Eye Contact: May cause burns or irritation

• Inhalation: Mists can irritate respiratory tract

• Ingestion: Harmful if swallowed in large quantities

Precautions:

• Wear gloves, safety goggles, and lab coat

• Use with adequate ventilation

• Store in plastic or lined containers, away from incompatible materials like strong bases and metals

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Environmental Impact

Phosphoric acid can contribute to eutrophication of water bodies if discharged improperly. However, when used responsibly, it is:

• Non-toxic to the environment in small, diluted quantities

• Biodegradable and neutralizable using lime or other bases

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Conclusion

Phosphoric acid is a multi-purpose, widely used acid that plays a crucial role in agriculture, food production, industrial cleaning, and healthcare. Its relative mildness, ease of handling, and biological compatibility make it one of the most important inorganic acids in the global chemical industry.

However, like all acids, it must be handled with proper precautions and environmental awareness.

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