Zinc phosphide is the inorganic chemical compound. It is a grey solid, although
commercial samples are often dark or even black. It is used as a rodenticide.
Zn3P2 is a semiconductor with a direct band gap of 1.5 eV. and may have
applications in photovoltaic cells. A second zinc phosphide is known, with the
stoichiometry ZnP2. Synthesis and reactions
Zinc phosphide can be prepared by the reaction of zinc with phosphorus;
however, for critical applications, additional processing to remove arsenic
compounds may be needed. 3 Zn + 2 P → Zn3P2
Another method of preparation include reacting tri-n-octylphosphine with
dimethylzinc. Zinc phosphide reacts with water to
produce phosphine and zinc hydroxide2): Zn3P2 + 6 H2O → 2 PH3 + 3 Zn(OH)2
Structure Zn3P2 has two forms, a room temperature
tetragonal form which converts to a cubic form at around 845 °C. In the
room temperature form there are discrete P atoms, zinc atoms are tetrahedrally
coordinated and phosphorus six coordinate, with zinc atoms at 6 of the
vertices of a distorted cube. ZnP2 has two forms a lower temperature red
tetragonal form and a black monoclinic form. In both of these there are chains
of P atoms, helical in the tetragonal, semi-spiral in the monoclinic.
Rodenticide Metal phosphides have been used as
rodenticides. A mixture of food and zinc phosphide is left where the rodents can
eat it. The acid in the digestive system of the rodent reacts with the phosphide
to generate the toxic phosphine gas. This method of vermin control has
possible use in places where rodents are immune to other common poisons. Other
pesticides similar to zinc phosphide are aluminium phosphide and calcium
phosphide. Zinc phosphide is typically added to
rodent baits in amount of around 0.75-2%. Such baits have strong, pungent
garlic-like odor characteristic for phosphine liberated by hydrolysis. The
odor attracts rodents, but has a repulsive effect on other animals;
However, birds, notably wild turkeys, are not sensitive to the smell. The
baits have to contain sufficient amount of zinc phosphide in sufficiently
attractive food in order to kill rodents in a single serving; a sublethal dose
may cause aversion towards zinc-phosphide baits encountered by
surviving rodents in the future. Rodenticide-grade zinc phosphide usually
comes as a black powder containing 75% of zinc phosphide and 25% of antimony
potassium tartrate, an emetic to cause vomiting if the material is accidentally
ingested by humans or domestic animals. However, it is still effective against
rats, mice, guinea pigs and rabbits, none of which have a vomiting reflex.
Zinc phosphide use in New Zealand The New Zealand Environmental Protection
Authority has approved the import and manufacture of Microencapsulated Zinc
Phosphide for the ground control of possums. The application was made by
Pest Tech Limited, with support from Connovation Ltd, Lincoln University and
the Animal Health Board. It will be used as an additional vertebrate poison in
certain situations. Unlike 1080 poison, it cannot be used for aerial
application. Safety
Zinc phosphide is highly toxic. References
External links Zinc Phosphide Pesticide Information
Profile - Extension Toxicology Network EPA Proposed Risk Mitigation Decision
for Nine Rodenticides Zinc phosphide in the Pesticide
Properties DataBase Zinc phosphide properties and use in
Michigan MD0173 - Pesticides in the military