Phosphorus forms a number of oxoacids, such as hypophosphorus acid, orthophosphorus acid, pyrophosphorous acid, hypophosphoricacid, orthophosphoricacid, pyro phosphoric acid and metaphosphoricacid.
The oxoacids in which phosphorus exhibits lower oxidation state of +1or +3constitute the “phosphorus acid series.” While the ones in which phosphorus exhibits higher oxidation states of +4 or +5 constitute the “phosphoric acid series”.
Phosphorus acid series:
Preparation of hypophosphorus acid or phosphinicacid:
- It is prepared by the alkaline hydrolysis of white phosphorus.
- When white phosphorus is heated with an alkali solution like barium hydroxide, it forms hypophosphite. It is further treated with dilute sulphuric acid to recover the acid.
O → 3Ba(H2
White Barium Water Barium Phosphine
phosphorus hydroxide hypophosphite
(dil) → BaSO4
↓ + 2H3
Barium Sulphuric Barium Hypophosphorous
hypophosphite acid sulphate acid
Preparation of orthophosphorousacid or phosphonic acid or phosphorous acid:
- The hydrolysis of phosphorus trioxide or phosphorus trichloride forms phosphorous acid.
O → 4H3
Phosphorous water Orthophosphorous
O → H3
Phosphorus Water Orthophosphorous Hydrochloric acid
Preparation of pyrophosphorousacid:
- Orthophosphorousacid, when treated with phosphorus trichloride, gives pyrophosphorousacid.
Orthophosphorous Phosphorus Pyrophosphorous Hydrogen
acid trichloride acid chloride
Phosphoric acid series:
Preparation of Hypophosphoric acid:
Preparation of Orthophosphoric acid or phosphoric acid:
- It is obtained by the alkaline hydrolysis and oxidation of red phosphorus
- It is prepared by dissolving phosphorus pent oxide in water and then boiling the solution.
Phosphorous water Orthophosphoric acid
Preparation of pyrophosphoric acid:
- When phosphoric acid is heated gently to about 2200c, it loses water molecules to form pyro phosphoric acid.
Phosphoric Acid Pyrophosphoric acid water
Preparation of metaphosphoricacid acid:
- Pyrophosphoric acid or Orthophosphoric acid, when heated to red hot, forms metaphosphoricacid acid.
O7 Red Hot
pyrophosphoric acid Metaphosphoric acid water
Crystalline phosphorus acid, on being heated with bromine in a sealed tube, gives metaphosphoricacid acid.
+ Br2 Δ
Phosphorous Bromine Metaphosphoric Hydrobromic acid
Metaphosphoricacid acid exists as a polymer.
Cyclotrimetaphosphoric acid and polymetaphosphoricacid are some polymeric forms of metaphosphoricacid acid.