Phosphate, a salt or an ester of orthophosphoric acid. Phosphates contain one or more phosphate groups, each consisting of one phosphorus atom linked to four oxygen atoms. Phosphates vary in physical and chemical properties. At ordinary temperatures, most are crystalline solids. Many are highly toxic.

Phosphates are classified as either inorganic or organic. Most inorganic phosphates occur naturally in small amounts in various rocks and soils. Phosphate rock is chiefly apatite (calcium orthophosphate), and is widely distributed in the earth's crust. Organic phosphates occur in, or can be produced from, animal and vegetable matter.

Certain phosphates are essential to life. Adenosine triphosphate (ATP) is found in all living cells and provides the energy needed for metabolism. Calcium orthophosphate is the chief constituent of bones and teeth.

Commercial inorganic phosphates are usually produced from phosphate rock or from orthophosphoric acid. A large percentage of the inorganic phosphates produced is used in fertilizers. The most important phosphate fertilizer is superphosphate, obtained by treating phosphate rock with sulfuric acid. Sodium phosphate and ammonium phosphate are also used in fertilizers.

Primarily because of their water-softening ability, phosphates are particularly useful in synthetic detergents. In many areas, however, large-scale use of phosphate detergents has led to water pollution. Phosphates that are carried in waste water are released into streams and lakes, where they serve as nutrients for water plants. The ecological balance is upset by the overgrowth of plant life. In an effort to prevent further harm, many local and state governments have banned or sharply restricted the sale of phosphate detergents.

Phosphates are used in protective treatments for metals and in foods and drugs. They are also used as plasticizers, gasoline additives, and insecticides.