Phosphate

In water solution, orthophosphoric acid and its three derived anions coexist according to the dissociation and recombination equilibria below[3]

Equilibrium	Dissociation constant Ka[4]	pKa
H3PO4 ⇌ H2PO−4 + H+	${\displaystyle K_{a1}={\frac {[{\ce {H+}}][{\ce {H2PO4-}}]}{[{\ce {H3PO4}}]}}\approx 7.5\times 10^{-3}}$	pKa1 = 2.14
H2PO−4 ⇌ HPO2−4 + H+	${\displaystyle K_{a2}={\frac {[{\ce {H+}}][{\ce {HPO4^2-}}]}{[{\ce {H2PO4-}}]}}\approx 6.2\times 10^{-8}}$	pKa2 = 7.20
HPO2−4 ⇌ PO3−4 + H+	${\displaystyle K_{a3}={\frac {[{\ce {H+}}][{\ce {PO4^3-}}]}{[{\ce {HPO4^2-}}]}}\approx 2.14\times 10^{-13}}$	pKa3 = 12.37

Values are at 25 °C and 0 ionic strength.

The pK_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 pK_a values) the dihydrogen phosphate ion, [H
₂PO
₄]⁻
, is practically the only species present. Around pH 9.8 (mid-way between the second and third pK_a values) the monohydrogen phosphate ion, [HPO
₄]²⁻
, is the only species present. At pH 13 or higher, the acid is completely dissociated as the phosphate ion, (PO
₄)³⁻
.

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, H
₃PO
₄, H
₂(PO
₄)⁻
and H(PO
₄)²⁻
behave as separate weak acids because the successive pK_a differ by more than 4.

Phosphate can form many polymeric ions such as pyrophosphate, (P
₂O
₇)⁴⁻
, and triphosphate, (P
₃O
₁₀)⁵⁻
. The various metaphosphate ions (which are usually long linear polymers) have an empirical formula of (PO
₃)⁻
and are found in many compounds.

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 [HPO
₄]²⁻
and [H
₂PO
₄]⁻
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

Thus, only [H
₂PO
₄]⁻
and [HPO
₄]²⁻
ions are present in significant amounts in the cytosol (62% [H
₂PO
₄]⁻
, 38% [HPO
₄]²⁻
). In extracellular fluid (pH = 7.4), this proportion is inverted (61% [HPO
₄]²⁻
, 39% [H
₂PO
₄]⁻
).

Inorganic phosphate can also be present as pyrophosphate anions [P
₂O
₇]⁴⁻
, which give orthophosphate by hydrolysis:

[P
₂O
₇]⁴⁻
+ H₂O ⇌ 2 [HPO
₄]²⁻

Organic phosphates are commonly found in the form of esters as nucleotides (e.g. AMP, ADP, and ATP) and in DNA and RNA. Free orthophosphate anions can be released by the hydrolysis of the phosphoanhydride bonds in ATP or ADP. These phosphorylation and dephosphorylation reactions are the immediate storage and source of energy for many metabolic processes. ATP and ADP are often referred to as high-energy phosphates, as are the phosphagens in muscle tissue. Similar reactions exist for the other nucleoside diphosphates and triphosphates.

An important occurrence of phosphates in biological systems is as the structural material of bone and teeth. These structures are made of crystalline calcium phosphate in the form of hydroxyapatite. The hard dense enamel of mammalian teeth may contain fluoroapatite, a hydroxy calcium phosphate where some of the hydroxyl groups have been replaced by fluoride ions.

Phosphates are medicinal salts of phosphorus. Some phosphates, which help cure many urinary tract infections, are used to make urine more acidic. To avoid the development of calcium stones in the urinary tract, some phosphates are used.[5] For patients who are unable to get enough phosphorus in their daily diet, phosphates are used as dietary supplements, usually because of certain disorders or diseases.[5] Injectable phosphates can only be handled by qualified health care providers.[5]

Plants take up phosphorus through several pathways: the arbuscular mycorrhizal pathway and the direct uptake pathway.

Phosphates are the naturally occurring form of the element phosphorus, found in many phosphate minerals. In mineralogy and geology, phosphate refers to a rock or ore containing phosphate ions. Inorganic phosphates are mined to obtain phosphorus for use in agriculture and industry.[2]

The largest global producer and exporter of phosphates is Morocco. Within North America, the largest deposits lie in the Bone Valley region of central Florida, the Soda Springs region of southeastern Idaho, and the coast of North Carolina. Smaller deposits are located in Montana, Tennessee, Georgia, and South Carolina. The small island nation of Nauru and its neighbor Banaba Island, which used to have massive phosphate deposits of the best quality, have been mined excessively. Rock phosphate can also be found in Egypt, Israel, Palestine, Western Sahara, Navassa Island, Tunisia, Togo, and Jordan, countries that have large phosphate-mining industries.

Phosphorite mines are primarily found in:

• North America: United States, especially Florida, with lesser deposits in North Carolina, Idaho, and Tennessee
• Africa: Morocco, Algeria, Egypt, Niger, Senegal, Togo, Tunisia, Mauritania
• Middle East: Saudi Arabia, Jordan, Israel, Syria, Iran and Iraq, at the town of Akashat, near the Jordanian border.
• Central Asia: Kazakhstan
• Oceania: Australia, Makatea, Nauru, and Banaba Island

In 2007, at the current rate of consumption, the supply of phosphorus was estimated to run out in 345 years.[8] However, some scientists thought that a "peak phosphorus" would occur in 30 years and Dana Cordell from Institute for Sustainable Futures said that at "current rates, reserves will be depleted in the next 50 to 100 years".[9] Reserves refer to the amount assumed recoverable at current market prices. In 2012 the USGS estimated world reserves at 71 billion tons, while 0.19 billion tons were mined globally in 2011.[10] Phosphorus comprises 0.1% by mass of the average rock[11] (while, for perspective, its typical concentration in vegetation is 0.03% to 0.2%),[12] and consequently there are quadrillions of tons of phosphorus in Earth's 3×10¹⁹-ton crust,[13] albeit at predominantly lower concentration than the deposits counted as reserves, which are inventoried and cheaper to extract. If it is assumed that the phosphate minerals in phosphate rock are mainly hydroxyapatite and fluoroapatite, phosphate minerals contain roughly 18.5% phosphorus by weight. If phosphate rock contains around 20% of these minerals, the average phosphate rock has roughly 3.7% phosphorus by weight.

Some phosphate rock deposits, such as Mulberry in Florida,[14] are notable for their inclusion of significant quantities of radioactive uranium isotopes. This is a concern because radioactivity can be released into surface waters[15] from application of the resulting phosphate fertilizer.

In December 2012, Cominco Resources announced an updated JORC compliant resource of their Hinda project in Congo-Brazzaville of 531 million tons, making it the largest measured and indicated phosphate deposit in the world.[16]

Around 2018, Norway discovered phosphate deposits almost equal to those in the rest of Earth combined.[17][18]

In July 2022 China announced quotas on phosphate exportation.[19]

The largest importers in millions of metric tons of phosphate are Brazil 3.2, India 2.9 and the USA 1.6.[20]

The three principal phosphate producer countries (China, Morocco and the United States) account for about 70% of world production.