Phosphate Arsenates

In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .

The phosphate or orthophosphate ion is derived from phosphoric acid by the removal of three protons . Removal of one or two protons gives the dihydrogen phosphate ion and the hydrogen phosphate ion ion, respectively. These names are also used for salts of those anions, such as ammonium dihydrogen phosphate and trisodium phosphate.

File:3-phosphoric-acid-3D-balls.png,
Phosphoric
acid
File:2-dihydrogenphosphate-3D-balls.png,
Dihydrogen
phosphate
File:1-hydrogenphosphate-3D-balls.png,
Hydrogen
phosphate
File:0-phosphate-3D-balls.png,
Phosphate

In organic chemistry, phosphate or orthophosphate is an organophosphate, an ester of orthophosphoric acid of the form where one or more hydrogen atoms are replaced by organic groups. An example is trimethyl phosphate, . The term also refers to the trivalent functional group in such esters.

Orthophosphates are especially important among the various phosphates because of their key roles in biochemistry, biogeochemistry, and ecology, and their economic importance for agriculture and industry. The addition and removal of phosphate groups (phosphorylation and dephosphorylation) are key steps in cell metabolism.

Orthophosphates can condense to form pyrophosphates.

Chemical properties

The phosphate ion has a molar mass of 94.97 g/mol, and consists of a central phosphorus atom surrounded by four oxygen atoms in a tetrahedral arrangement. It is the conjugate base of the hydrogen phosphate ion , which in turn is the conjugate base of the dihydrogen phosphate ion , which in turn is the conjugate base of orthophosphoric acid, .

Many phosphates are soluble in water at standard temperature and pressure. The sodium, potassium, rubidium, caesium, and ammonium phosphates are all water-soluble. Most other phosphates are only slightly soluble or are insoluble in water. As a rule, the hydrogen and dihydrogen phosphates are slightly more soluble than the corresponding phosphates.

Equilibria in solution

In water solution, orthophosphoric acid and its three derived anions coexist according to the dissociation and recombination equilibria below

Values are at 25°C and 0 ionic strength.

The p''K''_''a'' values are the pH values where the concentration of each species is equal to that of its conjugate bases. At pH 1 or lower, the phosphoric acid is practically undissociated. Around pH 4.7 (mid-way between the first two p''K''_''a'' values) the dihydrogen phosphate ion, , is practically the only species present. Around pH 9.8 (mid-way between the second and third p''K''_''a'' values) the monohydrogen phosphate ion, , is the only species present. At pH 13 or higher, the acid is completely dissociated as the phosphate ion, .

This means that salts of the mono- and di-phosphate ions can be selectively crystallised from aqueous solution by setting the pH value to either 4.7 or 9.8.

In effect, , and behave as separate weak acids because the successive p''K''_''a'' differ by more than 4.

Phosphate can form many polymeric ions such as pyrophosphate, , and triphosphate, . The various metaphosphate ions (which are usually long linear polymers) have an empirical formula of and are found in many compounds.

Biochemistry of phosphates

In biological systems, phosphorus can be found as free phosphate anions in solution (inorganic phosphate) or bound to organic molecules as various organophosphates.

Inorganic phosphate is generally denoted P_i and at physiological (homeostatic) pH primarily consists of a mixture of and ions. At a neutral pH, as in the cytosol (pH = 7.0), the concentrations of the orthophoshoric acid and its three anions have the ratios
: / ≈ 7.5 × 10⁴
: / ≈ 0.62
: / ≈ 2.14 × 10⁻⁶
Thus, only and ions are present in significant amounts in the cytosol (62% , 38% ). In extracellular fluid (pH = 7.4), this proportion is inverted (61% , 39% ).

Inorganic phosphate can be present also as of pyrophosphate anions , which can give orthophosphate by hydrolysis:

: + H₂O 2

Organic phosphates are commonly found in the form of esters as nucleotides (e.g. AMP

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