Aluminium phosphide (AlP) is an inorganic compound widely recognized for its role as a highly effective fumigant and pesticide. It is a highly toxic inorganic compound with the chemical formula AlP, used as a wide band gap semiconductor and a fumigant. This colorless solid is generally sold as a grey-green-yellow powder due to the presence of impurities arising from hydrolysis and oxidation.
Chemically, aluminium phosphide is a binary compound of aluminium and phosphorus, with a molar mass of about 57.96 g/mol. It is non-volatile but highly reactive with water, leading to rapid phosphine release. The gas is flammable, poisonous, and can spontaneously ignite in air, making handling and storage critical.
Properties
It appears as a dark gray or yellow crystalline solid, usually formulated into tablets, pellets, or granules. When exposed to moisture or acidic conditions, AlP releases phosphine gas (PH₃), which is highly toxic to insects, rodents, and other pests. This property makes it valuable for protecting stored grains and other agricultural commodities from infestation.
- Chemical formula: AlP
- Molar mass: 57.9552 g/mol
- Appearance: Yellow or gray crystals
- Odor: garlic-like
- Density: 2.85 g/cm3
- Melting point: 2,530 °C (4,590 °F; 2,800 K)
- Solubility in water: reacts
- Band gap: 2.5 eV (indirect)
Preparation
AlP is synthesized by combination of the elements:
4Al + P4 → 4AlP
Caution must be taken to avoid exposing the AlP to any sources of moisture, as this generates toxic phosphine gas. Phosphine also poses fire hazards, as it is a dangerous pyrophoric compound, igniting easily in air.
Reaction
AlP crystals are dark grey to dark yellow in color and have a zincblende crystal structure with a lattice constant of 5.4510 Å at 300 K. They are thermodynamically stable up to 1,000 °C (1,830 °F).
Aluminium phosphide reacts with water or acids to release phosphine:
AlP + 3 H2O → Al(OH)3 + PH3
AlP + 3 H+ → Al3+ + PH3
This reaction is the basis of its toxicity.
Occurrences and Uses
Aluminium phosphide does not occur naturally due to its reactivity with air and moisture. It is a synthetic compound produced industrially by combining aluminium with phosphorus at high temperatures.
In practice, it is widely used as a pesticide and fumigant for stored grains and agricultural commodities, where its toxic phosphine gas kills pests. Because of its efficiency and low cost, AlP tablets or pellets are distributed commercially for storage protection. However, it is also a significant poisoning agent in accidental or intentional exposure cases.
Toxicity
AlP is cost-effective and penetrates deeply into stored products, making it one of the most efficient fumigants. However, its high toxicity also poses significant risks to humans and animals. Inhalation of phosphine can cause respiratory distress, cardiac problems, and even death, while ingestion of AlP pellets has been widely reported in cases of accidental or intentional poisoning.
