Oxoacids of Phosphorus

Phosphorus forms several oxoacids, such as H₃PO₄ and H₃PO_3. In these oxoacids, the phosphorus atom is tetrahedrally surrounded by other atoms. Typically, these acids feature at least one P–OH bond and one P=O bond.

1.0Introduction

Oxoacids of phosphorus contain phosphorus, oxygen, and hydrogen and are formed when oxygen oxidises phosphorus. These acids are commonly found in natural sources such as minerals and plants. Animal waste is a significant source of phosphorus oxoacids, where oxygen oxidises phosphates to produce these compounds.

Phosphorus oxoacids are also present in fertilisers and insecticides. They play a crucial role in agriculture by enhancing crop yields and promoting plant health. In these acids, the phosphorus atom is tetrahedrally surrounded by other atoms, typically featuring at least one P=O bond and one P–OH bond.

In addition to P=O and P–OH bonds, phosphorus oxoacids may contain P–P or P–H bonds, especially when the phosphorus oxidation is less than +5.

2.0Phosphorus Acid, H₃PO₃

Phosphorous acid is a diprotic acid, which means it ionises two protons. It is made by hydrolysis of phosphorus trichloride with either steam or acid.

• PCl₃ + 3H₂O → HPO(OH)₂ + 3HCl

3.0Phosphoric Acid (H₃PO₄)

Phosphoric acid is a tricrotic acid, meaning it can donate three protons. It is a solid at room temperature and pressure and, when pure, is considered non-toxic. Phosphoric acid is typically produced by reacting sulfuric acid with tricalcium phosphate rock.

The reaction is as follows:

• Ca₅​(PO₄​)_3​X + 5H₂​SO_4​ + 10H_2​O → 3H₃​PO_{4 ​}+ 5CaSO₄​.2H₂​O + HX

X can be F, Cl, Br, or OH in this reaction.

4.0Metaphosphoric Acid (HPO₃)_n

Metaphosphoric acid forms by heating orthophosphoric acid at around 850 K. It is not a monomer but a cyclic trimer, cyclic tetramer, or polymer.

The formation reaction is:

• H₃​PO₄​ → HPO₃​ + H₂​O

5.0Hypophosphoric Acid (H₄P₂O₆)

Hypophosphoric acid is produced by the controlled oxidation of red phosphorus with sodium chlorite. Initially, the disodium salt of the acid is formed, which is then converted to hypophosphoric acid through a cation exchange process.

The reactions are as follows:

• Oxidation Reaction:

2P +2NaClO_2​ + 2H_2​O → Na₂​H₂​P₂​O_6​ + 2HCl

• Conversion to Hypophosphoric Acid: 

Na₂​H_2​P₂​O_6​ + 2H⁺(resin) → H₄​P₂​O_6​ + 2Na⁺ (resin)

Hypophosphoric acid is tetrabasic, meaning it has four replaceable hydrogen ions.

6.0Pyrophosphoric Acid (H₄P₂O₇)

Pyrophosphoric acid is prepared by heating orthophosphoric acid to approximately 250°C, forming a tetrabasic acid.

The reaction is:

• 2H₃​PO₄​→H₄​P₂​O₇​+H₂​O

7.0Orthophosphoric Acid (H₃PO₄)

Orthophosphoric acid is produced when phosphorus pentoxide (P₄O₁₀) is treated with water. This reaction yields a tribasic acid.

The reaction is:

• P_4​O₁₀​+6H₂​O→4H₃​PO₄​

8.0Acidic and Basic Strength

Acidic Strength:

The acidic strength of the oxoacids of phosphorus—H₃PO₄, H₃PO₃, and H₃PO₂—follows this order:

•    H₃PO₄  > H₃PO₃  > H₃PO₂ 
• In the first case, all hydrogen atoms are part of OH groups connected to oxygen, making it easy for them to be released as protons, thus resulting in the highest acidity. 
• In the second case, while two hydrogens are bonded to oxygen atoms, a third hydrogen is attached directly to phosphorus and cannot be released.
•  In the third case, only one hydrogen is bonded to oxygen, with the remaining two attached to phosphorus, which also cannot release protons. 

Therefore, acidity is determined by how readily the species releases H+ ions, leading to the order of acidity mentioned.

Basic Strength:

• The basicity of phosphorus oxoacids is determined by the number of ionisable H⁺ ions (protons) in the acid. While the structure of these acids may contain three hydrogen atoms, only those directly bonded to oxygen atoms as OH groups are readily ionisable. Therefore, the basicity of a phosphorus oxoacid corresponds to the number of these ionisable hydrogen atoms.
• For example, in H₃PO₄, there are three ionisable H⁺ ions, giving it a basicity of 3. In H₃PO₃, there are two such hydrogen atoms, so its basicity is 2.

9.0Uses of Oxoacids of Phosphorus

Phosphorus oxoacids have a variety of applications, including:

• Replacement for Sulfuric Acid: Phosphorus oxoacids substitute sulfuric acid by producing hydroiodic acid (HI) and hydrobromic acid (HBr).
• Souring Agent: Phosphoric acid produces soft drinks as a souring agent, providing a tangy taste.
• Production of Phosphate Salts: These acids produce sodium, ammonium, and potassium phosphate salts, which have various industrial and agricultural applications.
• Fertiliser Production: Phosphoric acid is a key ingredient in producing phosphatic fertilisers essential for promoting plant growth and increasing crop yields.