Phosphate
In
inorganic chemistry, a phosphate is a
salt of
phosphoric acid. Phosphates are also important in
biochemistry.
Encyclopedia
In
inorganic chemistry, a
phosphate is a
salt of
phosphoric acid. Phosphates are also important in
biochemistry.
Chemical properties
The
phosphate ion is a polyatomic ion with the empirical formula
PO43- and a molecular mass of 94.97 daltons; it consists of one central phosphorus
atom surrounded by four identical oxygen atoms in a
tetrahedral arrangement. The phosphate ion carries a negative three
formal charge and is the conjugate base of the hydrogenphosphate ion, H
PO
42-, which is the conjugate base of H
2PO
4-, the dihydrogen phosphate ion, which in turn is the conjugate base of H
3PO
4, phosphoric acid. It is a hypervalent molecule .
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 insoluble in
water at standard temperature and pressure, except for the alkali metal salts.
In dilute
aqueous solution, phosphate exists in four forms. In strongly basic conditions, the phosphate ion predominates, while in weakly basic conditions, the
hydrogen phosphate ion is prevalent. In weakly acid conditions, the
dihydrogen phosphate ion is most common. In strongly acid conditions, aqueous phosphoric acid 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 :
For a strongly basic pH , we find
showing that only PO43- and HPO42− are in significant amounts.
For a neutral pH , we find
so that only H2PO4- and HPO42- ions are in significant amounts . Note that in the extracellular fluid , this proportion is inverted .
For a strongly acid pH , we find
showing that H3PO4 is dominant with respect to H2PO4−. HPO42− and PO43− are practically absent.
Phosphate can form many polymeric ions, diphosphate , P2O74-, triphosphate, P3O105-, et cetera. The various metaphosphate ions have an empirical formula of PO3- and are found in many compounds.
Phosphate deposits can contain significant amounts of naturally occurring uranium. Subsequent uptake of such soil amendments can lead to crops containing uranium concentrations.
Occurrence
Phosphates are the naturally occurring form of the element phosphorus, found in many phosphate minerals. Elemental phosphorus and phosphides are not found . In mineralogy and geology, phosphate refers to a rock or ore containing phosphate ions.
The largest rock phosphate deposits in North America lie in the Bone Valley region of central Florida, United States, 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, which used to have massive phosphate deposits of the best quality, has been mined excessively. Rock phosphate can also be found on Navassa Island. Morocco, Tunisia, Israel, Togo, and Jordan have large phosphate mining industries as well.
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 can be created by the hydrolysis of pyrophosphate, which is denoted PPi:
- P2O74- + H2O ? 2HPO42-
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.
In ecological terms, because of its important role in biological systems, phosphate is a highly sought after resource. Consequently, it is often a limiting reagent in environments, and its availability may govern the rate of growth of organisms. Addition of high levels of phosphate to environments in which it is typically rare can have significant ecological consequences. For example, booms in the populations of some organisms at the expense of others, and the collapse of populations deprived of resources such as oxygen . In the context of pollution, phosphates are a principal component of total dissolved solids, a major indicator of water quality.
Uses
Phosphates are often used in laundry detergent as a water softener, but, because of boom-bust cycles tied to emission of phosphates into watersheds, phosphate detergent sale or usage is restricted in some areas.
In agriculture phosphate refers to one of the three primary plant nutrients, and it is a component of fertilizers. Rock phosphate is quarried from phosphate beds in sedimentary rocks. In former times it was simply crushed and used as is, but the crude form is now used only in organic farming. Normally it is chemically treated to make superphosphate, triple superphosphate, or ammonium phosphates, which have higher concentration of phosphate and are also more soluble, therefore more quickly usable by plants.
Fertilizer grades normally have three numbers; the first is the available nitrogen, the second is the available phosphate , and the third is the available potash . Thus a 10-10-10 fertilizer would contain ten percent of each, with the remainder being filler.
Surface runoff of phosphates from excessively fertilized farmland can be a cause of phosphate pollution leading to eutrophication , algal bloom and consequent oxygen deficit. This can lead to anoxia for fish and other aquatic life in the same manner as phosphate-based detergents.
Notes
References
