Phosphate

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Phosphate
Phosphat-Ion.svg
Aromatic ball and stick model of phosphate Phosphate-3D-balls.png
Aromatic ball and stick model of phosphate
Space-filling model of phosphate Phosphate-3D-vdW.png
Space-filling model of phosphate
Names
IUPAC name
Phosphate [1]
Other names
Orthophosphate
Tetraoxophosphate(V)
Tetraoxidophosphate(V)
Identifiers
3D model (JSmol)
3903772
ChEBI
ChemSpider
1997
MeSH Phosphates
PubChem CID
UNII
  • InChI=1S/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4)/p-3 Yes check.svgY
    Key: NBIIXXVUZAFLBC-UHFFFAOYSA-K Yes check.svgY
  • [O-]P([O-])([O-])=O
  • [O-]P(=O)([O-])[O-]
  • O=P([O-])([O-])[O-]
  • [O-] [P+]([O-])([O-])[O-]
Properties
PO3−
4
Molar mass 94.9714 g mol−1
Conjugate acid Monohydrogen phosphate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

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, a.k.a. phosphoric acid H3PO4.

Contents

The phosphate or orthophosphate ion [PO
4
]3−
is derived from phosphoric acid by the removal of three protons H+
. Removal of one proton gives the dihydrogen phosphate ion [H
2
PO
4
]
while removal of two protons gives the hydrogen phosphate ion [HPO
4
]2−
. These names are also used for salts of those anions, such as ammonium dihydrogen phosphate and trisodium phosphate.

In organic chemistry, phosphate or orthophosphate is an organophosphate, an ester of orthophosphoric acid of the form PO
4
RR′R″
where one or more hydrogen atoms are replaced by organic groups. An example is trimethyl phosphate, (CH
3
)
3
PO
4
. The term also refers to the trivalent functional group OP(O-)
3
in such esters. Phosphates may contain sulfur in place of one or more oxygen atoms (thiophosphates and organothiophosphates).

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. [2] 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 H(PO
4
)2−
, which in turn is the conjugate base of the dihydrogen phosphate ion H
2
(PO
4
)
, which in turn is the conjugate base of orthophosphoric acid, H
3
PO
4
.

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

Phosphoric acid speciation PiSpeciation.svg
Phosphoric acid speciation

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

EquilibriumDissociation constant Ka [4] pKa
H3PO4 ⇌ H2PO4 + H+pKa1 = 2.14
H2PO4 ⇌ HPO2−4 + H+pKa2 = 7.20
HPO2−4 ⇌ PO3−4 + H+pKa3 = 12.37

Values are at 25 °C and 0 ionic strength.

The pKa 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 pKa values) the dihydrogen phosphate ion, [H
2
PO
4
]
, is practically the only species present. Around pH 9.8 (mid-way between the second and third pKa 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
3
PO
4
, H
2
(PO
4
)
and H(PO
4
)2−
behave as separate weak acids because the successive pKa differ by more than 4.

Phosphate can form many polymeric ions such as pyrophosphate, (P
2
O
7
)4−
, and triphosphate, (P
3
O
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.

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 Pi and at physiological (homeostatic) pH primarily consists of a mixture of [HPO
4
]2−
and [H
2
PO
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
2
PO
4
]
and [HPO
4
]2−
ions are present in significant amounts in the cytosol (62% [H
2
PO
4
]
, 38% [HPO
4
]2−
). In extracellular fluid (pH = 7.4), this proportion is inverted (61% [HPO
4
]2−
, 39% [H
2
PO
4
]
).

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

[P
2
O
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.

Bones and teeth

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.

Medical and biological research uses

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]

Plant metabolism

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

Adverse health effects

Hyperphosphatemia, or a high blood level of phosphates, is associated with elevated mortality in the general population. The most common cause of hyperphosphatemia in people, dogs, and cats is kidney failure. In cases of hyperphosphatemia, limiting consumption of phosphate-rich foods, such as some meats and dairy items and foods with a high phosphate-to-protein ratio, such as soft drinks, fast food, processed foods, condiments, and other products containing phosphate-salt additives is advised. [6]

Phosphates induce vascular calcification, and a high concentration of phosphates in blood was found to be a predictor of cardiovascular events. [7]

Production

Geological occurrence

Phosphate mine near Flaming Gorge, Utah, US, 2008 Phosphate Mine Panorama.jpg
Phosphate mine near Flaming Gorge, Utah, US, 2008
Train loaded with phosphate rock, Metlaoui, Tunisia, 2012 Train loaded with phosphate rock, Metlaoui Tunisia-4298B.jpg
Train loaded with phosphate rock, Métlaoui, Tunisia, 2012

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:

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]

Mining

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

Production and global reserves of natural phosphate by country in 2019
(USGS, 2021) [21]
CountryProduction
(millions kg)
Share of
global
production (%)
Reserves
(millions kg)
Algeria 1,3000.542,200,000
Australia 2,7001.171,100,000
Brazil 4,7003.001,600,000
China 95,00044.833,200,000
Egypt 5,0002.472,800,000
Finland 995-1,000,000
India 1,4800.4946,000
Iraq 2000.09430,000
Israel 2,8101.4857,000
Jordan 9,2203.36800,000
Kazakhstan 1,5000.72260,000
Mexico 5580.7630,000
Morocco and Western Sahara 35,50013.4550,000,000
Peru 4,0001.79210,000
Russia 13,1005.60600,000
Saudi Arabia 6,5001.481,400,000
Senegal 3,4200.4550,000
South Africa 2,1000.991,400,000
Syria 2,0000.341,800,000
Togo 8000.4530,000
Tunisia 4,1101.79100,000
Uzbekistan 900-100,000
United States 23,30012.371,000,000
Vietnam 4,6501.2130,000
Other countries1,1401.17840,000
Total227,00010071,000,000

Ecology

Sea surface phosphate from the World Ocean Atlas WOA09 sea-surf PO4 AYool.png
Sea surface phosphate from the World Ocean Atlas
Relationship of phosphate to nitrate uptake for photosynthesis in various regions of the ocean. Note that nitrate is more often limiting than phosphate. See the Redfield ratio. PhosphatetoNitrate.png
Relationship of phosphate to nitrate uptake for photosynthesis in various regions of the ocean. Note that nitrate is more often limiting than phosphate. See the Redfield ratio.

In ecological terms, because of its important role in biological systems, phosphate is a highly sought after resource. Once used, it is often a limiting nutrient in environments, and its availability may govern the rate of growth of organisms. This is generally true of freshwater environments, whereas nitrogen is more often the limiting nutrient in marine (seawater) environments. Addition of high levels of phosphate to environments and to micro-environments in which it is typically rare can have significant ecological consequences. For example, blooms in the populations of some organisms at the expense of others, and the collapse of populations deprived of resources such as oxygen (see eutrophication) can occur. In the context of pollution, phosphates are one component of total dissolved solids, a major indicator of water quality, but not all phosphorus is in a molecular form that algae can break down and consume. [22]

Calcium hydroxyapatite and calcite precipitates can be found around bacteria in alluvial topsoil. [23] As clay minerals promote biomineralization, the presence of bacteria and clay minerals resulted in calcium hydroxyapatite and calcite precipitates. [23]

Phosphate deposits can contain significant amounts of naturally occurring heavy metals. Mining operations processing phosphate rock can leave tailings piles containing elevated levels of cadmium, lead, nickel, copper, chromium, and uranium. Unless carefully managed, these waste products can leach heavy metals into groundwater or nearby estuaries. Uptake of these substances by plants and marine life can lead to concentration of toxic heavy metals in food products. [24]

See also

Related Research Articles

<span class="mw-page-title-main">Phosphorus</span> Chemical element, symbol P and atomic number 15

Phosphorus is a chemical element; it has symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Earth. It has a concentration in the Earth's crust of about one gram per kilogram. In minerals, phosphorus generally occurs as phosphate.

<span class="mw-page-title-main">Pyrophosphate</span> Class of chemical compounds

In chemistry, pyrophosphates are phosphorus oxyanions that contain two phosphorus atoms in a P−O−P linkage. A number of pyrophosphate salts exist, such as disodium pyrophosphate and tetrasodium pyrophosphate, among others. Often pyrophosphates are called diphosphates. The parent pyrophosphates are derived from partial or complete neutralization of pyrophosphoric acid. The pyrophosphate bond is also sometimes referred to as a phosphoanhydride bond, a naming convention which emphasizes the loss of water that occurs when two phosphates form a new P−O−P bond, and which mirrors the nomenclature for anhydrides of carboxylic acids. Pyrophosphates are found in ATP and other nucleotide triphosphates, which are important in biochemistry. The term pyrophosphate is also the name of esters formed by the condensation of a phosphorylated biological compound with inorganic phosphate, as for dimethylallyl pyrophosphate. This bond is also referred to as a high-energy phosphate bond.

<span class="mw-page-title-main">Phosphoric acid</span> Chemical compound (PO(OH)3)

Phosphoric acid is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula H3PO4. It is commonly encountered as an 85% aqueous solution, which is a colourless, odourless, and non-volatile syrupy liquid. It is a major industrial chemical, being a component of many fertilizers.

A polyphosphate is a salt or ester of polymeric oxyanions formed from tetrahedral PO4 (phosphate) structural units linked together by sharing oxygen atoms. Polyphosphates can adopt linear or a cyclic (also called, ring) structures. In biology, the polyphosphate esters ADP and ATP are involved in energy storage. A variety of polyphosphates find application in mineral sequestration in municipal waters, generally being present at 1 to 5 ppm. GTP, CTP, and UTP are also nucleotides important in the protein synthesis, lipid synthesis, and carbohydrate metabolism, respectively. Polyphosphates are also used as food additives, marked E452.

An oxyanion, or oxoanion, is an ion with the generic formula A
x
Oz
y
. Oxyanions are formed by a large majority of the chemical elements. The formulae of simple oxyanions are determined by the octet rule. The corresponding oxyacid of an oxyanion is the compound H
z
A
x
O
y
. The structures of condensed oxyanions can be rationalized in terms of AOn polyhedral units with sharing of corners or edges between polyhedra. The oxyanions adenosine monophosphate (AMP), adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are important in biology.

<span class="mw-page-title-main">Calcium phosphate</span> Chemical compound

The term calcium phosphate refers to a family of materials and minerals containing calcium ions (Ca2+) together with inorganic phosphate anions. Some so-called calcium phosphates contain oxide and hydroxide as well. Calcium phosphates are white solids of nutritional value and are found in many living organisms, e.g., bone mineral and tooth enamel. In milk, it exists in a colloidal form in micelles bound to casein protein with magnesium, zinc, and citrate–collectively referred to as colloidal calcium phosphate (CCP). Various calcium phosphate minerals are used in the production of phosphoric acid and fertilizers. Overuse of certain forms of calcium phosphate can lead to nutrient-containing surface runoff and subsequent adverse effects upon receiving waters such as algal blooms and eutrophication (over-enrichment with nutrients and minerals).

<span class="mw-page-title-main">Pyrophosphoric acid</span> Chemical compound

Pyrophosphoric acid, also known as diphosphoric acid, is the inorganic compound with the formula H4P2O7 or, more descriptively, [(HO)2P(O)]2O. Colorless and odorless, it is soluble in water, diethyl ether, and ethyl alcohol. The anhydrous acid crystallizes in two polymorphs, which melt at 54.3 and 71.5 °C. The compound is a component of polyphosphoric acid, an important source of phosphoric acid. Anions, salts, and esters of pyrophosphoric acid are called pyrophosphates.

<span class="mw-page-title-main">Phosphorous acid</span> Chemical compound (H3PO4)

Phosphorous acid is the compound described by the formula H3PO3. This acid is diprotic, not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in the preparation of other phosphorus compounds. Organic derivatives of phosphorous acid, compounds with the formula RPO3H2, are called phosphonic acids.

<span class="mw-page-title-main">Phosphoric acids and phosphates</span> Class of chemical species; phosphorus oxoacids and their deprotonated derivatives

In chemistry, a phosphoric acid, in the general sense, is a phosphorus oxoacid in which each phosphorus (P) atom is in the oxidation state +5, and is bonded to four oxygen (O) atoms, one of them through a double bond, arranged as the corners of a tetrahedron. Two or more of these PO4 tetrahedra may be connected by shared single-bonded oxygens, forming linear or branched chains, cycles, or more complex structures. The single-bonded oxygen atoms that are not shared are completed with acidic hydrogen atoms. The general formula of a phosphoric acid is Hn+2−2xPnO3n+1−x, where n is the number of phosphorus atoms and x is the number of fundamental cycles in the molecule's structure, between 0 and n + 2/2.

In chemistry, phosphorus oxoacid is a generic name for any acid whose molecule consists of atoms of phosphorus, oxygen, and hydrogen. There is a potentially infinite number of such compounds. Some of them are unstable and have not been isolated, but the derived anions and organic groups are present in stable salts and esters. The most important ones—in biology, geology, industry, and chemical research—are the phosphoric acids, whose esters and salts are the phosphates.

<span class="mw-page-title-main">Tricalcium phosphate</span> Chemical compound

Tricalcium phosphate (sometimes abbreviated TCP), more commonly known as Calcium phosphate, is a calcium salt of phosphoric acid with the chemical formula Ca3(PO4)2. It is also known as tribasic calcium phosphate and bone phosphate of lime (BPL). It is a white solid of low solubility. Most commercial samples of "tricalcium phosphate" are in fact hydroxyapatite.

<span class="mw-page-title-main">Sodium monofluorophosphate</span> Chemical compound

Sodium monofluorophosphate, commonly abbreviated SMFP, is an inorganic compound with the chemical formula Na2PO3F. Typical for a salt, MFP is odourless, colourless, and water-soluble. This salt is an ingredient in some toothpastes.

Phosphinates or hypophosphites are a class of phosphorus compounds conceptually based on the structure of hypophosphorous acid. IUPAC prefers the term phosphinate in all cases, however in practice hypophosphite is usually used to describe inorganic species, while phosphinate typically refers to organophosphorus species.

<span class="mw-page-title-main">Dicalcium phosphate</span> Chemical compound

Dicalcium phosphate is the calcium phosphate with the formula CaHPO4 and its dihydrate. The "di" prefix in the common name arises because the formation of the HPO42– anion involves the removal of two protons from phosphoric acid, H3PO4. It is also known as dibasic calcium phosphate or calcium monohydrogen phosphate. Dicalcium phosphate is used as a food additive, it is found in some toothpastes as a polishing agent and is a biomaterial.

<span class="mw-page-title-main">Dihydrogen phosphate</span> Inorganic ion

Dihydrogen phosphate is an inorganic ion with the formula [H2PO4]. Phosphates occur widely in natural systems.

<span class="mw-page-title-main">Fluorapatite</span> Phosphate mineral

Fluorapatite, often with the alternate spelling of fluoroapatite, is a phosphate mineral with the formula Ca5(PO4)3F (calcium fluorophosphate). Fluorapatite is a hard crystalline solid. Although samples can have various color (green, brown, blue, yellow, violet, or colorless), the pure mineral is colorless, as expected for a material lacking transition metals. Along with hydroxylapatite, it can be a component of tooth enamel, but for industrial use both minerals are mined in the form of phosphate rock, whose usual mineral composition is primarily fluorapatite but often with significant amounts of the other.

<span class="mw-page-title-main">Monosodium phosphate</span> Chemical compound

Monosodium phosphate (MSP), also known as monobasic sodium phosphate and sodium dihydrogen phosphate, is an inorganic compound with the chemical formula NaH2PO4. It is a sodium salt of phosphoric acid. It consists of sodium cations (Na+) and dihydrogen phosphate anions (H2PO−4). One of many sodium phosphates, it is a common industrial chemical. The salt exists in an anhydrous form, as well as monohydrate and dihydrate (NaH2PO4·H2O and NaH2PO4·2H2O respectively).

<span class="mw-page-title-main">Monohydrogen phosphate</span> Chemical compound

Hydrogen phosphate or monohydrogen phosphate(systematic name) is the inorganic ion with the formula [HPO4]2-. Its formula can also be written as [PO3(OH)]2-. Together with dihydrogen phosphate, hydrogenphosphate occurs widely in natural systems. Their salts are used in fertilizers and in cooking. Most hydrogenphosphate salts are colorless, water soluble, and nontoxic.

<span class="mw-page-title-main">Ammonium dihydrogen phosphate</span> Chemical compound

Ammonium dihydrogen phosphate (ADP), also known as monoammonium phosphate (MAP) is a chemical compound with the chemical formula (NH4)(H2PO4). ADP is a major ingredient of agricultural fertilizers and dry chemical fire extinguishers. It also has significant uses in optics and electronics.

<span class="mw-page-title-main">Hypophosphoric acid</span> Chemical compound

Hypophosphoric acid is a mineral acid with the formula H4P2O6, with phosphorus in a formal oxidation state of +4. In the solid state it is present as the dihydrate, H4P2O6·2H2O. In hypophosphoric acid the phosphorus atoms are identical and joined directly with a P−P bond. Isohypophosphoric acid is a structural isomer of hypophosphoric acid in which one phosphorus has a hydrogen directedly bonded to it and that phosphorus atom is linked to the other one by an oxygen bridge to give a phosphorous acid/phosphoric acid mixed anhydride. The two phosphorus atoms are in the +3 and +5 oxidation states, respectively.

References

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