A phosphate as an inorganic chemical is a salt of phosphoric acid.
In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid.
Organic phosphates are important in biochemistry and biogeochemistry or ecology.
Inorganic phosphates are mined to obtain phosphorus for use in agriculture and industry.
At elevated temperatures in the solid state, phosphates can condense to form pyrophosphates.
Chemical properties
The phosphate ion is a polyatomic ion with the empirical formula PO43− and a molar
mass of 94.97 g/mol.
It consists of one central phosphorus atom surrounded by four oxygen atoms in a tetrahedral
arrangement.
The phosphate ion carries a negative three formal charge and is the conjugate base of
the hydrogen phosphate ion, HPO42−, which is the conjugate base of H2PO4−, the dihydrogen
phosphate ion, which in turn is the conjugate base of H
3PO 4, 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 not 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.
The pyrophosphates are mostly water soluble.
Aqueous phosphate exists in four forms.
In strongly basic conditions, the phosphate ion predominates, whereas in weakly basic
conditions, the hydrogen phosphate ion is prevalent.
In weakly acid conditions, the dihydrogen phosphate ion is most common.
In strongly acidic conditions, trihydrogen phosphate (H
3PO 4) is the main form.
More precisely, considering the following three equilibrium reactions:
H 3PO
4 H+ + H2PO4− H2PO4− H+ + HPO42−
HPO42− H+ + PO43− the corresponding constants at 25°C are:
The speciation diagram obtained using these pK values shows three distinct regions.
In effect H 3PO
4, H 2PO−
4 and HPO2− 4 behave as separate weak acids.
This is because the successive pK values differ by more than 4.
For each acid the pH at half-neutralization is equal to the pK value of the acid.
The region in which the acid is in equilibrium with its conjugate base is defined by pH ≈ pK
± 2.
Thus the three pH regions are approximately 0–4, 5–9 and 10–14.
This is idealized as it assumes constant ionic strength, which will not hold in reality at
very low and very high pH values.
For a neutral pH as in the cytosol, pH=7.0
so that only H 2PO−
4 and HPO2− 4 ions are present in significant amounts
(62% H 2PO−
4, 38% HPO2− 4 Note that in the extracellular fluid, this
proportion is inverted (61% HPO2− 4, 39% H
2PO− 4).
Phosphate can form many polymeric ions such as diphosphate, P
2O4− 7, and triphosphate, P
3O5− 10.
The various metaphosphate ions have an empirical formula of PO−
3 and are found in many compounds.
Biochemistry of phosphates 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 at physiological pH primarily consists
of a mixture of HPO2− 4 and H
2PO− 4 ions.
Inorganic phosphate can be created by the hydrolysis of pyrophosphate, which is denoted
PPi: P
2O4− 7 + H2O 2 HPO2−
4 However, phosphates are most commonly found
in the form of adenosine phosphates, 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.
Compounds such as substituted phosphines have uses in organic chemistry but do not seem
to have any natural counterparts.
The addition and removal of phosphate from proteins in all cells is a pivotal strategy
in the regulation of metabolic processes.
Phosphate is useful in animal cells as a buffering agent.
Phosphate salts that are commonly used for preparing buffer solutions at cell pHs include
Na2HPO4, NaH2PO4, and the corresponding potassium salts.
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 consists of fluoroapatite, an hydroxy calcium phosphate
where some of the hydroxyl groups have been replaced by fluoride ions.
Plants uptake phosphorus through several pathways: the arbuscular mycorrhizal pathway and the
direct uptake pathway.
Occurrence and mining
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.
The largest phosphorite or rock phosphate deposits in North America lie in the Bone
Valley region of central Florida, the Soda Springs region of Idaho, and the coast of
North Carolina.
Smaller deposits are located in Montana, Tennessee, Georgia and South Carolina near Charleston
along Ashley Phosphate road.
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, Morocco, 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: Algeria, Egypt, Morocco, mainly near Khouribga and Youssoufia; Senegal, Togo, Tunisia
and Western Sahara, at the town of Bu Craa.
Middle East: Israel, Saudi Arabia, Jordan, Syria and Iraq, at the town of Akashat, near
the Jordanian border.
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.
However, some scientists believed that a "peak phosphorus" will occur in 30 years and Dana
Cordell from Institute for Sustainable Futures said in Times that at "current rates, reserves
will be depleted in the next 50 to 100 years."
Reserves refer to the amount assumed recoverable at current market prices, and, in 2012, the
USGS estimated 71 billion tons of world reserves, while 0.19 billion tons were mined globally
in 2011.
Phosphorus comprises 0.1% by mass of the average rock, and consequently there are quadrillions
of tons of phosphorus in Earth's 3 * 1019 ton crust, albeit at predominantly lower concentration
than the deposits counted as reserves from being inventoried and cheaper to extract.
Some phosphate rock deposits are notable for their inclusion of significant quantities
of radioactive uranium isotopes.
This syndrome is noteworthy because radioactivity can be released into surface waters in the
process of application of the resultant phosphate fertilizer.
In December 2012 Cominco Resources announced an updated JORC compliant resource of their
Hinda project in Congo-Brazzaville of 531Mt making it the largest measured and indicated
phosphate deposit in the world.
Ecology
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 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 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 which
algae can break down and consume.
Calcium hydroxyapatite and calcite precipitates can be found around bacteria in alluvial topsoil.
As clay minerals promote biomineralization, the presence of bacteria and clay minerals
resulted in calcium hydroxyapatite and calcite precipitates.
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.
In Germany, the use of uranium-contaminated standard phosphate fertilizers in farming
has been linked to significantly raised uranium levels in drinking water.
In some areas, it has led to recommendations to use bottled water, instead of tap water,
to prepare food for babies and small children.
See also
References
External links US Minerals Databrowser provides data graphics
covering consumption, production, imports, exports and price for phosphate and 86 other
minerals Taking Stock of Phosphorus and Biofuels, global
phosphate mining, use and shortages.
Phosphate at Lab Tests Online Phosphate: analyte monograph - The Association
for Clinical Biochemistry and Laboratory Medicine