Phosphine (PH₃)

Preparation of phosphine

Phosphine is prepared by the reaction of calcium phosphide with water or dilute HCl.

Ca₃P₂ + 6H₂O → 3Ca(OH)₂ + 2PH₃

Ca₃P₂ + 6HCl → 3CaCl₂ + 2PH₃

In the laboratory, it is prepared by heating white phosphorus with concentrated NaOH solution in an inert atmosphere of CO₂.

P₄ + 3NaOH + 3H₂O → PH₃ + 3NaH₂PO₂ (sodium hypophosphite)

Pure PH₃ is non inflammable but becomes inflammable owing to the presence of P₂H₄ or P₄ vapours. To purify it from the impurities, it is absorbed in HI to form phosphonium iodide (PH₄I), which on treating with KOH gives phosphine.

PH₄I + KOH → KI + H₂O + PH₃

Properties of phosphine

a. It is a colourless gas with rotten fish like smell and is highly poisonous. It explodes in contact with traces of oxidising agents like HNO₃, Cl₂ and Br₂ vapours.

b. It is slightly soluble in water. The solution of PH₃ in water decomposes in presence of light giving red phosphorus and H₂. When absorbed in copper sulphate or mercuric chloride solution, the corresponding phosphides are obtained.

3CuSO₄ + 2PH₃ → Cu₃P₂ + 3H₂SO₄

3HgCl₂ + 2PH₃ → Hg₃P₂ + 6HCl

c. Phospine is weakly basic.

PH₃ + HBr → PH₄Br

Uses of phosphine

· It is used to prepare smoke screens in warfare.

· A mixture of CaC₂ and Ca₃P₂ is used in Holme’s signals. Containers containing calcium carbide and calcium phosphide are pierced and thrown in the sea when the gases evolved burn and serve as a signal.