Phosphine is a colourless, flammable, and toxic gas with an odour of garlic or decaying fish.

Phosphine is pyrophoric in high concentrations and may form explosive mixtures with air – it ignites spontaneously on contact with air.

The gas is shipped as a liquefied, compressed gas. Phosphine is rarely found in nature. Small amounts can be formed during the breakdown of organic matter, although it is rapidly degraded.

Phosphine is released into the air via emissions from various manufacturing processes and from the use of metal (magnesium, aluminium and zinc) phosphide fumigants and pesticides, which release phosphine on contact with water or acid.

Production
Most often produced when metallic phosphides such as aluminium, calcium, or zinc phosphides react with water or acid, phosphine can also be produced during the generation of acetylene gas.

The gas may be prepared in a variety of ways. Industrially it is produced by the reaction of white phosphorus with sodium hydroxide, producing sodium hypophosphite and sodium phosphite as a by-product.

Alternatively, the acidcatalyzed disproportioning of white phosphorus may be used, which yields phosphoric acid and phosphine.

Both routes have industrial significance, with the acid route as the preferred method if further reaction of the phosphine to substituted phosphines is needed.

This latter step requires purification and pressurizing. It can also be made (as described above) by the hydrolysis of a metal phosphide such as aluminium phosphide or calcium phosphide.

Applications
The main uses of phosphine are as a chemical dopant in the manufacture of semi-conductors for the electronics industry, and in the fumigation of stored agricultural products such as cereal grains and tobacco.

Phosphine is also used as a condensation catalyst and in the manufacture of some polymers, though it can also be used in the production of some chemicals and metal alloys and is an unintentional by-product in the production of the drug methamphetamine.

Phosphine is used in silicon processing as a phosphorus (N-type dopant) source in chemical vapor deposition (CVD) and ion implantation. It is also used in gallium arsenide as an ion implantation gas in the manufacture of light emitting diodes (LEDs).

Aluminium phosphide (Celphos, Phostoxin, Quick Phos) and zinc phosphide are solids used as grain fumigants and as a rodenticide, respectively. When phosphides are ingested or exposed to moisture, they release phosphine gas.

Phosphine gas may also be released however, when acetylene is made by the action of water on calcium carbide which is contaminated with calcium phosphide, as is commonly the case.

Phosphine is highly toxic to organisms undergoing oxidative respiration, but is non toxic to organisms kept under low oxygen (<1%) or that can anaerobically respire. Because of these characteristics, phosphine is widely used as a fumigant of metabolically dormant stored products such as grain.

Health and safety
Symptoms of phosphine intoxication are primarily related to the cardiovascular and pulmonary systems and may include restlessness, irritability, drowsiness, tremors, vertigo, diplopia, ataxia, cough, dyspnea, retrosternal discomfort, abdominal pain, and vomiting.

Phosphine interferes with enzymes and protein synthesis, primarily in the mitochondria of heart and lung cells. Metabolic changes in heart muscle cause action disturbances that alter transmembrane potentials. Ultimately, cardiac arrest, peripheral vascular collapse and pulmonary edema can occur.

Exposure to even small amounts of phosphine can cause headache, dizziness, nausea, vomiting, diarrhoea, drowsiness, cough, and chest tightness. More serious exposure can cause shock, convulsions, coma, irregular heartbeat, and liver and kidney damage.

Generally, the more serious the exposure - the more severe the symptoms.