In chemistry, phosphate refers to a polyatomic ion or radical consisting of one phosphorus atom and four oxygen. In the ionic form, it carries a -3 formal charge, and is denoted PO43-.

In a biochemical setting, a free phosphate ion in solution is called inorganic phosphate, to distinguish it from phosphates bound in the form of ATP, or perhaps in DNA or RNA. Inorganic phosphate is generally denoted Pi. Inorganic phosphate can be formed by the reactions of ATP, or ADP, with the formation of the corresponding ADP or AMP, and the release of phosphate ion. Similar reactions exist for the other nucleoside triphosphates and diphosphates.

In living systems, phosphate ions can also be created by the hydrolysis of a larger ion called pyrophosphate, which has the structure P2O74-, and is denoted PPi.

  • P2O74- + H2O → 2HPO42-

Energy stored by phosphate bonds in the form of ADP or ATP, or other nucleoside diphosphates or triphosphates, or the phosphagens in muscle tissues, is generally referred to as high energy phosphate.

In mineralogy and geology, it refers to a rock or ore containing phosphate ions.

In ecological terms, phosphate is often a limiting reagent in many environments--the availability of phosphate governs the rate of growth of many organisms. Introduction of non-naturally occurring levels of phosphate to those environments causes an ecological disequilibrium, leading to booms in the population of some organisms and subsequent busts in the populations of others deprived of other nutrients or essential elements by the rapid growth and consumption by the booming population.

Phosphates are a component of certain types of detergent, but because of boom-bust cycles tied to emission of phosphates into watersheds, phosphate detergent sale or usage is restricted in some areas.