Detailed description
phosphate, 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.[1][2][3]
At elevated temperatures in the solid state, phosphates can condense to form pyrophosphates.
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Chemical properties
The
phosphate ion is a polyatomic ion with the empirical
formula PO3−4 and a molar mass of 94.973 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, HPO2−4, which is the conjugate base of
H2PO−4, the dihydrogen phosphate ion, which in turn is the conjugate
base of H3PO4, phosphoric
acid. It is a hypervalent molecule (the phosphorus atom has
10 electrons in its valence shell). Phosphate is also an organophosphorus
compound with the formula OP(OR)3. 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.
Apatite is a group of phosphate minerals, usually referring to hydroxyapatite,
fluorapatite,
chlorapatite
and bromapatite, named for high
concentrations of OH−, F−, Cl−
or Br−ions, respectively, in
the crystal.
The formula of the admixture of the four most common endmembers is written as Ca10(PO4)6(OH,
F, Cl, Br)2, and the crystal unit cell formulae of the individual minerals are
written as Ca10(PO4)6(OH)2, Ca10(PO4)6(F)2,
Ca10(PO4)6(Cl)2 and Ca10(PO4)6(Br)2Apatite is one of a few minerals that are produced and used by biological micro-environmental systems. Apatite has a Mohs Scale hardness of 5. Hydroxyapatite, also known as hydroxylapatite, is the major component of tooth enamel and bone mineral. A relatively rare form of apatite in which most of the OH groups are absent and containing many carbonate and acid phosphate substitutions is a large component of bone material.
Phosphine (IUPAC name: phosphane) is the compound with the chemical formula PH3. It is a colorless, flammable, toxic gas. Pure phosphine is odourless, but technical grade samples have a highly unpleasant odor like garlic or rotting fish, due to the presence of substituted phosphine and diphosphine (P2H4). Phosphines are also a group of organophosphorus compounds with the formula R3P (R = organic derivative). Organophosphines are important in catalysts where they complex to various metal ions; complexes derived from a chiral phosphine can catalyze reactions to give chiral products.
Occurrence
Due
to its reactivity with air and many other oxygen-containing substances,
phosphorus is not found free in nature but it is widely distributed in many
different minerals.
Phosphate
rock, which is partially made of apatite (an impure tri-calcium phosphate mineral), is an
important commercial source of this element. About 50 percent of the global
phosphorus reserves are in the Arab nations.[26]
Large deposits of apatite are located in China, Russia, Morocco, Florida, Idaho, Tennessee, Utah, and elsewhere. Albright and Wilson in the United Kingdom and
their Niagara
Falls plant, for instance, were using phosphate rock in the 1890s and 1900s
from Connetable,
Tennessee and Florida; by 1950 they were using phosphate rock mainly from
Tennessee and North Africa.[13]
In the early 1990s Albright and Wilson's purified wet phosphoric acid business
was being adversely affected by phosphate rock sales by China and the entry of
their long-standing Moroccan phosphate suppliers into the purified wet
phosphoric acid business.[27]
In
2007, at the current rate of consumption, the supply of phosphorus was
estimated to run out in 345 years.[28]
However, scientists are now claiming that a "Peak Phosphorus" will
occur in 30 years and that "At current rates, reserves will be depleted in
the next 50 to 100 years."[29]
The
stability of the +5 oxidation state is illlustrated by the wide range of
phosphate materials available in the earth.
Production
White
phosphorus was first made commercially, for the match industry in the
19th century, by distilling off phosphorus vapour from precipitated phosphates,
mixed with ground coal or charcoal, which was heated in an iron pot, in retort.[30]
The precipitated phosphates were made from ground-up bones that had been
de-greased and treated with strong acids. Carbon
monoxide and other flammable gases produced during the reduction process
were burnt off in a flare stack.
This
process became obsolete when the submerged-arc furnace
for phosphorus production was introduced to reduce phosphate rock.[31][32]
Calcium phosphate (phosphate rock), mostly mined in Florida and North Africa,
can be heated to 1,200–1,500 °C with sand, which is mostly SiO2,
and coke (impure carbon) to produce vaporized tetraphosphorus, P4,
(melting point 44.2 °C), which is subsequently condensed into a white
powder under water to prevent oxidation. Even under water, white
phosphorus is slowly converted to the more stable red phosphorus allotrope
(melting point 597 °C). Both the white and red allotropes of phosphorus
are insoluble in water.
The
electric furnace method allowed production to increase to the point where
phosphorus could be used in weapons of war.[7][13]
In World
War I it was used in incendiaries, smoke
screens and tracer bullets.[13]
A special incendiary bullet was developed to shoot at hydrogen-filled
Zeppelins
over Britain (hydrogen being highly inflammable
if it can be ignited).[13]
During World War II, Molotov
cocktails of benzene
and phosphorus were distributed in Britain to specially selected civilians
within the British resistance operation, for defence; and phosphorus incendiary
bombs were used in war on a large scale. Burning phosphorus is difficult to
extinguish and if it splashes onto human skin it has horrific effects (see precautions
below).[12]
Today
phosphorus production is larger than ever. It is used as a precursor for
various chemicals,[33]
in particular the herbicide glyphosate sold under the brand name Roundup. Production
of white phosphorus takes place at large facilities and it is transported
heated in liquid form. Some major accidents have occurred during
transportation, train derailments at Brownston,
Nebraska and Miamisburg, Ohio led to large fires. The
worst accident in recent times was an environmental one in 1968 when phosphorus
spilled into the sea from a plant at Placentia Bay, Newfoundland.[34]
Applications
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Widely used compounds
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Use
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Ca(H2PO4)2·H2O
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Baking
powder and fertilizers
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CaHPO4·2H2O
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Animal food
additive, toothpowder
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H3PO4
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Manufacture
of phosphate fertilizers
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PCl3
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Manufacture
of POCl3 and pesticides
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POCl3
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Manufacturing
plasticizer
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P4S10
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Manufacturing
of additives and pesticides
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Na5P3O10
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Detergents
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Phosphorus,
being an essential plant nutrient, finds its major use as a constituent of fertilizers
for agriculture
and farm production in the form of concentrated phosphoric acids, which can
consist of 70% to 75% P2O5. Global demand for fertilizers
led to large increase in phosphate (PO43–) production in the
second half of the 20th century. Due to the essential nature of phosphorus to
living organisms, the low solubility of natural phosphorus-containing
compounds, and the slow natural cycle of phosphorus, the agricultural industry
is heavily reliant on fertilizers that contain phosphate, mostly in the form of
superphosphate
of lime. Superphosphate of lime is a mixture of two phosphate salts,
calcium dihydrogen phosphate Ca(H2PO4)2 and
calcium sulfate dihydrate CaSO4·2H2O produced by the
reaction of sulfuric acid and water with calcium phosphate.
- Phosphorus
is widely used to make organophosphorus compounds, through
the intermediates phosphorus chlorides and two phosphorus
sulfides: phosphorus pentasulfide, and phosphorus sesquisulfide.[13]
Organophosphorus compounds have many applications, including in plasticizers,
flame retardants, pesticides,
extraction
agents, and water
treatment.[12]
- Phosphorus
is also an important component in steel
production, in the making of phosphor bronze, and in many other
related products. Phosphorus is added to metallic copper during its
smelting process to react with oxygen present as an impurity in copper and
to produce oxygen-free copper or phosphorus-containing
copper (CuOFP)
alloys with a higher thermal and electrical conductivity than normal
copper.
- Phosphates
are utilized in the making of special glasses that are
used for sodium lamps.[35]
- Bone-ash,
calcium phosphate, is used in the production
of fine china.[35]
- Sodium tripolyphosphate made from
phosphoric acid is used in laundry detergents in some countries, but
banned for this use in others.[35]
- Phosphoric
acid made from elemental phosphorus is used in food applications such as
some soda beverages. The acid is also a starting point to make food grade
phosphates.[13]
These include mono-calcium phosphate that is employed in baking
powder and sodium tripolyphosphate and other
sodium phosphates[13].
Among other uses these are used to improve the characteristics of
processed meat and cheese. Others are used in toothpaste.[13]
Trisodium phosphate is used in cleaning
agents to soften water and for preventing pipe/boiler
tube corrosion.
- White
phosphorus, called "WP" (slang term "Willie Peter") is
used in military
applications as incendiary bombs, for smoke-screening
as smoke pots and smoke bombs, and in tracer ammunition. It is also a
part of an obsolete M34 White Phosphorus US hand grenade. This
multipurpose grenade was mostly used for signalling, smoke screens and
inflammation; it could also cause severe burns and had a psychological
impact on the enemy.[36][37]
- Red
phosphorus is essential for manufacturing matchbook strikers, flares,[13]
safety matches, pharmaceutical grade and street methamphetamine,
and is used in cap gun caps.
- Phosphorus
sesquisulfide is used in heads of strike-anywhere matches.[13]
- In
trace amounts, phosphorus is used as a dopant for n-type semiconductors.
- 32P and 33P are used as radioactive tracers
in biochemical laboratories (see Isotopes).
- Phosphate
is a strong complexing agent for the hexavalent
uranyl (UO22+)
species and this is the reason why apatite and
other natural phosphates can often be very rich in uranium.
- Tributylphosphate is an organophosphate soluble in kerosene and used to extract uranium in the Purex process applied in the reprocessing of spent nuclear fuel.
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