How is phosphine produced?

Phosphine can be produced by the reaction of white phosphorus with a strong base or by the hydrolysis of metal phosphides, such as calcium phosphide, in water: Ca3P2 + 6H2O → 3Ca(OH)2 + 2PH3 It can also be produced during the breakdown of organic matter containing phosphorus without oxygen.

Can phosphine occur naturally?

Yes, phosphine can occur naturally in small quantities. It can be released during the anaerobic decomposition of organic matter containing phosphorus, particularly in wetlands, sewage treatment plants, and landfills.

What are the environmental impacts of phosphine?

Phosphine is not persistent in the environment, as it rapidly oxidises in the presence of air. However, its use as a fumigant can pose risks to non-target organisms if not carefully managed. It does not contribute significantly to long-term environmental pollution.

Phosphine: Structure, Preparation and Properties

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JEE Chemistry

Phosphine

Phosphine (PH₃) is a compound that belongs to the group of organophosphorus compounds. It was discovered by Philippe Gengembre in 1783 and is found in various biological areas, including human tissues, blood, urine, and saliva.

1.0Introduction

Phosphine (PH₃), also known as phosphorus trihydride or phosphane, is a phosphorus hydride. It is highly toxic and can adversely affect the human respiratory system even at low concentrations. Its molecular structure is trigonal pyramidal, with one phosphorus atom covalently bonded to three hydrogen atoms.

Phosphine has a range of applications, including in semiconductors and flame retardants. It is also a hazardous, flammable gas produced in industrial and laboratory settings. Additionally, phosphine can be detected in human blood, urine, and saliva.

2.0Structure and Formula of Phosphine

Phosphine (PH₃) has a trigonal pyramidal structure, with the phosphorus atom at the centre. The lone pair of electrons on the phosphorus atom affects the molecule's symmetry.

The phosphorus atom undergoes sp³ hybridisation, forming three bonded pairs and one lone pair of electrons. The H–P–H bond angle is 93.5°, and the P–H bond length is 1.42 picometers (pm).

3.0Preparation of Phosphine

Hydrolysis of Metal Phosphides

Phosphine (PH₃) can be produced by the hydrolysis of metal phosphides, such as calcium phosphide, with water or hydrochloric acid (HCl):

With Water: Ca₃P₂+ 6H₂O → 3Ca(OH)₂+ 2PH₃
With HCl: Ca₃P₂+ 6HCl → 3CaCl₂ + 2PH₃

Laboratory preparation of Phosphine Gas

In the laboratory, phosphine is produced by heating white phosphorus with concentrated sodium hydroxide (NaOH) solution under an inert carbon dioxide atmosphere (CO₂).

2P₄ + 3NaOH + 3H₂O → PH₃+ 3NaH₂PO₂(Sodium hypophosphite)

When pure, phosphine is non-inflammable. However, it becomes inflammable due to the presence of impurities like P₂H₄ or P₄ vapours.
To purify phosphine, it is absorbed in hydroiodic acid (HI) to form phosphonium iodide (PH₄I). This compound, when treated with potassium hydroxide (KOH), releases purified phosphine:

PH₄I + KOH → KI + H₂O + PH₃

From Phosphorous Acid

Pure phosphine can also be prepared through the thermal decomposition of phosphorous acid (H₃PO₃):

4H₃PO₃ → 3H₃PO₄ + PH₃

4.0Physical Properties of Phosphine

Appearance: Phosphine (PH₃) is a colourless gas.
Toxicity: It is hazardous and toxic.
Flammability: In its pure form, phosphine is non-flammable but can become combustible due to impurities like P₄ vapours. It can ignite spontaneously upon contact with oxidising agents such as nitric acid (HNO₃), chlorine (Cl₂), or bromine (Br₂) vapours, even in trace amounts.
Odour: Phosphine has a distinctive odour similar to that of rotten fish.
Solubility: It is only slightly soluble in water.
Boiling Point: Phosphine has a boiling point of -87.7°C.

5.0Chemical Properties of Phosphine

Lewis Base: Phosphine (PH₃) acts as a Lewis base. It reacts with acids to form phosphonium compounds:

PH₃ + HI → PH₄I
PH₃ + HBr → PH₄Br
PH₃ + HCl → PH₄Cl

Combustibility: Phosphine burns to produce phosphorus pentoxide and water:

2PH₃ + 4O₂ → P₂O₅ + 3H₂O

Reaction with Chlorine: Phosphine reacts with chlorine to form phosphorus pentachloride and hydrochloric acid:

PH₃ + 4Cl₂ → PCl₅ + 3HCl

Explosiveness: Phosphine is usually stable but can explode with flame when heated, releasing phosphoric acid. It also explodes dramatically upon contact with oxidising substances such as HNO₃, Cl₂, and Br₂.
Decomposition in Water: In the presence of light, phosphine decomposes in water to produce red phosphorus and water:

PH₃ (in H₂O) → P (red) + H₂O

Reactions with Metal Salts: When absorbed in copper sulfate or mercuric chloride solutions, phosphine forms copper phosphide and mercuric phosphide:

3CuSO₄ + 2PH₃ → Cu₃P₂ + 3H₂SO₄
3HgCl₂ + 2PH₃ → Hg₃P₂ + 6HCl

6.0Uses of Phosphine

Organophosphorus Compounds: Phosphine is a key precursor in synthesising various organophosphorus compounds.
Semiconductor Industry: It is used as a dopant for creating n-type semiconductors.
Insecticide: Phosphine is employed as an insecticide to protect stored products.
Flame Retardant Production: It serves as an intermediate in synthesising flame retardants.
Catalyst: Phosphine acts as a catalyst in condensation reactions.
Battery Production: It plays a role in the manufacturing of batteries.
Holme's Signal: Due to its rapid ignition properties, phosphine is used in Holme’s signals for maritime distress.
Smoke Screens: These are utilised to create smoke screens.
Fumigation: Phosphine is used in fumigants to deter bugs and rodents.

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