The behavior of phosphoric acid in various pH conditions

Phosphoric acid (H₃PO₄) is a triprotic acid, meaning it can donate up to three hydrogen ions (H⁺) in aqueous solutions. This property allows phosphoric acid to exist in different forms depending on the pH of the solution. Understanding its behavior across various pH levels is essential in industries such as food processing, agriculture, pharmaceuticals, and water treatment.

1. Phosphoric Acid as a Triprotic Acid

Phosphoric acid undergoes stepwise dissociation, losing one proton at a time, with each dissociation step having a distinct equilibrium constant:

2. Phosphoric Acid Behavior at Different pH Levels

a) pH < 2: Predominantly H₃PO₄ (Undissociated Acid)

In highly acidic conditions (pH < 2), phosphoric acid remains mostly in its undissociated form (H₃PO₄).

This is common in strong acid solutions and industrial acid applications, such as metal treatment and etching.

b) pH 2 – 6: H₂PO₄⁻ (Dihydrogen Phosphate Dominates)

As the pH increases beyond 2, phosphoric acid begins to dissociate, and the dihydrogen phosphate ion (H₂PO₄⁻) becomes the dominant species.

This form is critical in biological systems and buffering solutions.

c) pH 6 – 9: HPO₄²⁻ (Hydrogen Phosphate Dominates)

In neutral to slightly basic conditions (pH 6-9), the second dissociation occurs, leading to the dominance of the hydrogen phosphate ion (HPO₄²⁻).

This is important in phosphate-buffered saline (PBS) solutions and biological systems where phosphate acts as a buffer.

d) pH > 10: PO₄³⁻ (Phosphate Ion Becomes Dominant)

At very high pH (pH > 12), phosphoric acid fully dissociates into the phosphate ion (PO₄³⁻).

This form is significant in alkaline industrial processes, wastewater treatment, and phosphate-based detergents.

3. Buffering Capacity of Phosphoric Acid

Due to its three dissociation steps, phosphoric acid functions as an effective buffer in three pH ranges:

Around pH 2.1: Buffers strong acids

Around pH 7.2: Key in biological and physiological buffering systems

Around pH 12.3: Buffers strong bases

4. Applications Based on pH Behavior

Agriculture: Phosphoric acid fertilizers adjust soil pH for plant growth.

Food Industry: Used to regulate acidity in soft drinks (pH 2-4).

Medical Field: Utilized in buffer solutions for pharmaceutical formulations.

Industrial Processes: Involved in metal cleaning and corrosion control under acidic conditions.

Conclusion

The behavior of phosphoric acid varies significantly with pH due to its triprotic nature. It serves as an essential component in buffering systems, industrial processes, and biological applications. Understanding its dissociation and dominant species at different pH levels is crucial for optimizing its use in various fields.