Oxoacids Of Phosphorus -Preparation, Properties

Oxoacids are acids that contain the element oxygen in compound. Phosphorus forms a number of oxoacids like H₄P₂O₅, H₃PO₄, H₃PO₃, HPO₃, etc.

In oxoacids, other atoms surrounds phosphorus tetrahedrally. All these acids contain at least one P=O bond and one P–OH bond. The oxoacids in which phosphorus has lower oxidation state (less than +5) contain, in addition to P=O and P–OH bonds, either P–P (e.g., in H4P2O6) or P–H (e.g., in H3PO2) bonds but not both. These acids in +3 oxidation state of phosphorus tend to disproportionate to higher and lower oxidation states. 

For example, orthophophorous acid (or phosphorous acid) on heating disproportionates to give orthophosphoric acid (or phosphoric acid) and phosphine.

4H₃PO₃ → 3H₃PO₄ + PH₃

The acids which contain P–H bond have strong reducing properties. Thus, hypo-phosphorous acid is a good reducing agent as it contains two P–H bonds and reduces, for example, AgNO3 to metallic silver.

4 AgNO₃ + 2H₂O + H3PO₂ → 4Ag + 4HNO₃ + H₃PO₄

These P–H bonds are not ionizable to give H+ and do not play any role in basicity. Only those H atoms which attach with oxygen in P–OH form are ionizable and cause the basicity. Thus, H₃PO₃  and H₃PO₄ are dibasic and tribasic, respectively as the structure of H₃PO₃ has two P–OH bonds and H₃PO₄ has three.

Hypo Phosphorus Acid (H₃PO₂) or Phosphinic Acid:

• It is a powerful reducing agent with molecular formula H₃PO₂.

• Generally we write H₃PO₂, but a more descriptive representation is HOP(O)H₂, which highlights its monoprotic character. 

• Oxidation state of phosphorus- +1
• One P-OH bond 
• Two P-H bond
• One P=O bond

• Salts derived from this acid are called hypophosphites.

Preparation

• You can prepare it by the oxidation of phosphine by iodine in the presence of figured amount of water. It is a monobasic acid.

PH₃ + 2I₂ + 2H₂O → H₃PO₂ + 4 HI

• Hypophosphite salts of the alkali and alkaline earth metals result from the reaction of white phosphorus with hot aqueous solution of the appropriate hydroxide.

P₄ + 4 OH⁻ + 4 H₂O → 4 H₂PO^2- + 2 H₂

• The salt is then treated with a strong, non-oxidizing acid to give the free hypo-phosphorous acid.

H₂PO^2- + H⁺ → H₃PO₂

Phosphorus Acid or ortho-phosphorous (H₃PO₃) or Phosphonic Acid:

• Phosphorous acid, molecular formula is H₃PO₃. This acid is diprotic (not triprotic) phosphorous acid. It is an intermediate in the preparation of other phosphorus compounds.

• Oxidatative state of phosphorus- +3
• Two P-OH bond
• One P-H bond
• One P=O bond

Preparation

1. On an industrial scale, the acid is prepared by hydrolysis of phosphorus trichloride with water or steam.

PCl₃ + 3 H₂O → HPO(OH)₂ + 3 HCl

Potassium phosphite is also a precursor to phosphorous acid:

K₂HPO₃ + 2 HCl → 2 KCl + H₃PO₃

2. It formed by hydrolysis of phosphorous trioxide (P₄O₆). It a is dibasic acid.

P4O6 + 6H2O → 4H3PO3

Pyrophosphorus Acid (H₄P₂O₅)

Another, one of the most important among oxoacids of phosphorus is pyrophosphorus acid.

Preparation 

PCl₃ +H₃PO₃ →

• Oxidatative state of phosphorus- +3
• Two P-OH bond
• Two P-H bond
• Two P=O bond

Hypophosphoric Acid (H₄P₂O₆):

Hypo-phosphoric acid is a mineral acid with the formula H₄P₂O₆. In the solid state it is present as the dihydrate, H4P2O6·2H2O. In hypophosphoric acid the phosphorus atoms are identical and joined directly with a P−P bond.

Preparation

• It is formed by controlled oxidation of red phosphorus with sodium chlorite. Hypophosphoric acid is tetrabasic in nature.

2P + 2NaClO₂ + 2H₂O → Na₂H₂P₂O₆ + 2HCl

Na₂H₂P₂O₆ + 2H-resin → H₄P₂O₆ + 2Na-resin

• Oxidatative state of phosphorus- +4
• Four P-OH bond
• Two P=O bond
• One P-P bond

Orthophosphoric Acid (H3PO4)

• Phosphoric acid, also known as orthophosphoric acid or phosphoric(V) acid, is a weak acid. Its molecular formula is H₃PO₄.

• The pure compound is a colorless solid.

• Oxidatative state of phosphorus- +5
• Three P-OH bond
• One P=O bond

Preparation

1. Phosphoric acid is produced industrially by a wet process. A phosphate-containing mineral such as calcium hydroxyapatite is treated with sulfuric acid.
2. It is formed by treating P4O10 with bubbled water. It is a tribasic acid.

P₄O₁₀ + 6H₂O → 4H₃PO₄

Pyrophosphoric Acid (H4P2O7)

• Pyrophosphoric acid, also known as diphosphoric acid, is the inorganic compound with the formula H₄P₂O₇ or, more descriptively, [(HO)₂P(O)]₂O. It is colorless and odorless and is soluble in water, diethyl ether, and ethyl alcohol. The anhydrous acid crystallizes in two polymorphs, which melt at 54.3 °C and 71.5 °C.

Preparation

• It is formed by heating orthophosphonic acid at about 250 °C. It is a tetrabasic acid.

2H₃PO₄ → H₄P₂O₇ + H₂O

• Oxidatative state if phosphorus- +5
• Four P-OH bond
• Two P=O bond
• One P-O-P bond

Meta Phosphoric Acid (HPO3)n

Preparation

• It is formed by warming orthophosphoric acid to around 850 K. It exists as cyclic trimer, cyclic tetramer or polymer.

H₃PO₄ → HPO₃ + H₂O

• Oxidatative state if phosphorus- +5
• Three P-OH bond
• Three P=O bond
• Three P-O-P bond

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