The differences between phosphoric acid and phosphorous acid

Phosphoric acid (H₃PO₄) and phosphorous acid (H₃PO₃) are two distinct chemical compounds that share similarities in their chemical structure but differ significantly in their properties, reactivity, and applications. Both are important in industrial and chemical processes, yet they serve different roles and are used in various fields, ranging from agriculture to manufacturing. In this article, we will explore the differences between phosphoric acid and phosphorous acid in terms of their chemical structure, properties, reactivity, and industrial applications.

 

1. Chemical Structure and Composition

The most notable difference between phosphoric acid and phosphorous acid lies in their chemical structure and the number of oxygen atoms attached to the central phosphorus atom.

 

Phosphoric Acid (H₃PO₄): Phosphoric acid is composed of a phosphorus atom (P) bonded to four oxygen atoms (O), with three hydroxyl groups (OH) attached to the phosphorus. Its structure can be represented as:

 

H₃PO₄

H₃PO₄

In this structure, phosphorus is in the +5 oxidation state, and the molecule is tetrahedral, with the oxygen atoms arranged around the phosphorus atom.

 

Phosphorous Acid (H₃PO₃): Phosphorous acid, in contrast, contains only three oxygen atoms bonded to the phosphorus atom, with two hydroxyl groups attached. Its structure can be represented as:

 

H₃PO₃

H₃PO₃

In this compound, phosphorus is in the +3 oxidation state, and the molecule has a pyramidal geometry due to the presence of lone pairs on the phosphorus atom.

 

2. Oxidation States of Phosphorus

One of the key differences between these two acids is the oxidation state of the phosphorus atom.

 

In phosphoric acid, phosphorus has an oxidation state of +5. This is characteristic of compounds where the phosphorus atom is highly oxidized.

 

In phosphorous acid, phosphorus has an oxidation state of +3. This lower oxidation state makes phosphorous acid more reactive in certain chemical reactions, such as redox reactions, where it can act as a reducing agent.

 

3. Acidity and Reactivity

Both phosphoric acid and phosphorous acid are acidic, but they differ in terms of their acidity and reactivity.

 

Phosphoric Acid: Phosphoric acid is a triprotic acid, meaning it can donate three protons (H⁺ ions) in aqueous solutions. It is less reactive than phosphorous acid and is typically used in a variety of applications where a stable, less reactive acid is required. In aqueous solutions, phosphoric acid dissociates in a stepwise manner to form a weakly acidic solution.

 

Phosphorous Acid: Phosphorous acid, on the other hand, is a diprotic acid, meaning it can donate two protons. It is generally more reactive than phosphoric acid due to its lower oxidation state, making it a better reducing agent. In solution, phosphorous acid dissociates more readily than phosphoric acid.

Phosphorous acid also tends to be more acidic than phosphoric acid in terms of proton dissociation and its ability to react with bases and other substances.

 

4. Uses and Applications

The differences in chemical structure and reactivity lead to different uses for phosphoric acid and phosphorous acid in industry and manufacturing.

 

Phosphoric Acid:

 

Fertilizers: Phosphoric acid is widely used in the production of phosphate fertilizers, which are essential for agriculture. It is reacted with mineral sources of phosphorus, such as phosphate rock, to produce fertilizers like superphosphate.

 

Food and Beverage Industry: Phosphoric acid is used as an acidulant in soft drinks, particularly colas, to provide acidity and preserve flavor.

 

Water Treatment: Phosphoric acid is used in water treatment processes, particularly in the removal of scale and corrosion prevention in boilers and cooling systems.

 

Detergents and Cleaning Products: It is also used in the manufacture of detergents and cleaning agents due to its ability to break down organic materials.

 

Phosphorous Acid:

 

Reducing Agent: Phosphorous acid is commonly used as a reducing agent in various chemical processes, including in the production of certain polymers and in the reduction of organic compounds.

 

Fungicides and Pesticides: Phosphorous acid and its derivatives are used in the production of fungicides, particularly in the treatment of crops against plant diseases like downy mildew and Phytophthora.

 

Hydrolysis and Synthesis: Phosphorous acid is used in the production of phosphites, which are important intermediates in the synthesis of various chemicals, including flame retardants and stabilizers.

 

Electronics Industry: It is also used in the electronics industry for the production of semiconductors and in the preparation of certain metal phosphates.

 

5. Environmental Impact

While both acids are important in industrial applications, their environmental impact can differ. Phosphoric acid, being a more stable and less reactive compound, is generally less harmful to the environment when used in controlled quantities. In contrast, phosphorous acid's higher reactivity and its role as a reducing agent make it more prone to interacting with other chemicals in the environment, requiring careful handling and disposal.

 

6. Conclusion

In summary, while phosphoric acid and phosphorous acid share similarities in their chemical makeup, such as their phosphorus content, they differ significantly in terms of their oxidation states, chemical reactivity, and applications. Phosphoric acid is a more stable, less reactive compound commonly used in fertilizers, food products, and industrial processes, while phosphorous acid is a more reactive substance with applications as a reducing agent, in fungicides, and in the production of phosphites. Understanding these differences is essential for choosing the appropriate acid for specific industrial applications and ensuring their effective and safe use.