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In inorganic chemistry, a phosphate is a salt of phosphoric acid. Phosphates are also important in biochemistry.

Chemical properties

The phosphate ion is a polyatomic anion with the empirical formula PO43− and a molecular mass of 94.97 daltons; it consists of one central phosphorus atom surrounded by four identical oxygen atoms in a tetrahedral arrangement. The phosphate ion carries a negative three formal charge and is the conjugate base of the hydrogenphosphate ion, HPO42−, which is the conjugate base of H2PO4, the dihydrogen phosphate ion, which in turn is the conjugate base of H3PO4, phosphoric acid.

A phosphate salt forms when a positively charged ion attaches to the negatively charged oxygen atoms of the ion, forming an ionic compound. Many phosphates are insoluble in water at standard temperature and pressure, except for the alkali metal salts.

In dilute aqueous solution, phosphate exists in four forms. In strongly basic conditions, the phosphate ion predominates, while in weakly basic conditions, the hydrogen phosphate ion is prevalent. In weakly acid conditions, the dihydrogen phosphate ion is most common. In strongly acid conditions, aqueous phosphoric acid is the main form.

Phosphate can form many polymeric ions, diphosphate (also pyrophosphate), P2O74−, triphosphate, P3O105−, et cetera. The various metaphosphate ions have an empirical formula of PO3 and are found in many compounds.

In living systems

Phosphates are the naturally occurring form of the element phosphorus, found in many phosphate minerals. Elemental phosphorus and phosphides are not found (rare phosphide minerals may be found in meteorites).

In biological systems, phosphorus is found as a free phosphate ion in solution and is called inorganic phosphate, to distinguish it from phosphates bound in various phosphate esters. Inorganic phosphate is generally denoted Pi and can be created by the hydrolysis of pyrophosphate, which is denoted PPi:

P2O74− + H2O → 2HPO42−

However, phosphates are most commonly found in in the form of adenosine phosphates, (AMP, ADP and ATP) and in DNA and RNA and can be released by the hydrolysis of ATP or ADP. Similar reactions exist for the other nucleoside diphosphates and triphosphates. Phosphoanhydride bonds in ADP and ATP, or other nucleoside diphosphates and triphosphates, contain high amounts of energy which give them their vital role in all living organisms. They are generally referred to as high energy phosphate, as are the phosphagens in muscle tissue. Compunds such as substituted phosphines, have uses in organic chemistry but do not seem to have any natural counterparts.

Phosphate rock, alongside a penny to demonstrate size

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 imbalance, 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 often used in laundry detergent as a water softener, but, because of boom-bust cycles tied to emission of phosphates into watersheds, phosphate detergent sale or usage is restricted in some areas.

In agriculture phosphate refers to one of the three primary plant nutrients, and it is a component of fertilizers. Rock phosphate is quarried from phosphate beds in sedimentary rocks. In former times it was simply crushed and used as is, but the crude form is now used only in organic farming. Normally it is chemically treated to make superphosphate, triple superphosphate, or ammonium phosphates, which have higher concentration of phosphate and are also more soluble, therefore more quickly usable by plants.

Fertilizer grades normally have three numbers; the first is the available nitrogen, the second is the available phosphate (expressed on a P2O5 basis), and the third is the available potash (expressed on a K2O basis). Thus a 10-10-10 fertilizer would contain ten percent of each, with the remainder being filler.

The largest rock phosphate deposits in North America lie in the Bone Valley region of central Florida, United States, the Soda Springs region of Idaho, and the coast of North Carolina. Smaller deposits are located in Tennessee and Georgia. The small island nation of Nauru, which used to have massive phosphate deposits of the best quality, has been mined excessively. Rock phosphate can also be found on Navassa Island. Morocco, Tunisia, Israel and Jordan have large phosphate mining industries as well.

Surface runoff of phosphates from excessively fertilized farmland can be a cause of phosphate pollution in surface waters, leading to eutrophication (algal bloom) and consequent oxygen deficit, leading to (anoxia) for fish and other aquatic life in the same manner as phosphate-based detergents.

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