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

1. Hydrolysis of Metal Phosphides

Phosphine (PH_3​) can be produced by the hydrolysis of metal phosphides, such as calcium phosphide, with water or hydrochloric acid (HCl):

• With Water: Ca_3​P_2​ + 6H₂​O → 3Ca(OH)_2​ + 2PH_3​
• With HCl: Ca₃​P_{2 ​}+ 6HCl → 3CaCl₂​ + 2PH_3​

2. 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 + 3H₂​O → PH₃ ​+ 3NaH₂​PO_2​ (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 (PH₄​I). This compound, when treated with potassium hydroxide (KOH), releases purified phosphine:

PH₄​I + KOH → KI + H₂​O + PH_3​

3. From Phosphorous Acid 

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

4H_3​PO₃​ → 3H₃​PO₄​ + 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

1. 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

2. Combustibility: Phosphine burns to produce phosphorus pentoxide and water:

• 2PH₃ + 4O₂ → P₂O₅ + 3H₂O

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

• PH₃ + 4Cl₂ → PCl₅ + 3HCl

4. 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.
5. Decomposition in Water: In the presence of light, phosphine decomposes in water to produce red phosphorus and water:

• PH₃ (in H₂O) → P (red) + H₂O

6. 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.