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Phosphorus Types, Analyses, and Terminology
Phosphorus as an element makes up about 0.1% of the elements in the earth’s crust so
in terms of abundance, it is minor compared to oxygen, sodium, or aluminum, but in
terms of importance to life, it is a critical element. Phosphorus is an important
component of ATP (energy), DNA (genes), RNA (protein synthesis), NADP (energy
transfer), phospholipids (cell membrane), and many more. Phosphorus is critical for
growth and survival of all life forms and there is no exception.
In the analyses of phosphorus, there are many terms in print and different regulations
are in place to monitor levels of phosphate in water since excess phosphate can lead to
excessive algal blooms. Generally, phosphorus does not exist by itself (elemental
phosphorus) and instead is usually bound to oxygen or other elements. The most
common form is as phosphate (PO4) and many times phosphorus and phosphate are
used interchangeably, but they are not the same chemical. Phosphate types are also
subdivided into organic phosphate and inorganic phosphate. Organic phosphate is the
type of phosphate attached to and part of molecular essential life. These would include
ATP, DNA, RNA, phospholipids, and others. Inorganic phosphate, on the other hand, is
defined as a salt of phosphoric acid. Phosphoric acid has the molecular formula of
H3PO4, but the hydrogen atoms can be replaced by other cations such as sodium,
potassium, calcium, and others and form molecules such as sodium phosphate,
potassium phosphate, or calcium phosphate.
Other terms for phosphate include orthophosphate and polyphosphate.
Orthophosphate is the PO4-3 molecule composed of four (4) oxygen atoms and one
phosphorus atom. A polyphosphate is when more than three (3) orthophosphate
molecules are linked together. In other words, the orthophosphate is the building block
of the polyphosphate molecule.
Other terms utilized include soluble phosphate and insoluble phosphate. Soluble
phosphate is the form of phosphorus that is soluble in water, while insoluble phosphates
are not soluble. From a chemical standpoint, a soluble phosphate is a molecule that
can pass through a 0.45 µ filter.
Many of the terms can be used interchangeably because they refer to the same
chemical molecule. For example, inorganic phosphate, inorganic phosphorus, soluble
phosphate, and orthophosphate all refer to the same chemical, namely PO 4-3 or
phosphate. It is this form of phosphate that can be taken up by the plants and other
living organisms. The cells use this form of phosphate (orthophosphate) to build ATP,
DNA, NADP, phospholipids, and other biological chemicals.
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For all intensive purposes, orthophosphorus and soluble phosphorus are the same
chemical. In the same manner, organic phosphate, insoluble phosphate, and
polyphosphates are also terms used interchangeably, since they are made up of larger
molecules. The basic analytical process is to analyze orthophosphate. Thus, when a
test is requested for soluble phosphate, the laboratory prepares samples just to
determine the level of orthophosphate; however, different procedures may term it as
inorganic phosphorus, soluble phosphate, or orthophosphate. They are all the same.
When total phosphate is required, the sample is treated with acid and heat to break the
organic, insoluble and polyphosphates down to orthophosphate and then the total
orthophosphate determined. Having knowledge of the total phosphate and
orthophosphorus, it is possible to calculate the organic phosphate.
For example:
Total P – Ortho P = Organic P
Neither the equation nor the methods can provide which category (ATP, DNA, etc.) for
the organic phosphate. Additional separation of the fats or nucleic acids would be
necessary to determine the chemical form the phosphate is in.
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