Common in natural systems, such as lakes, oceans, and soil, phosphorus is an essential element for plant and animal growth. However, when present in large concentrations, phosphorus can cause excessive microorganism and algae growth. For this reason many wastewater treatment plants monitor the total phosphorus that is discharged. It is one of the primary nutrients that is removed chemically or biologically.

For hobbyists with saltwater or freshwater aquaria, a high amount of phosphorus can be problematic to fish, plants, and coral.

The amount of phosphorus can be expressed in two different ways:

  • Phosphate (PO43-) also known as orthophosphate, or as
  • Phosphorus (P) also known as orthophosphate as phosphorus (PO4-P).

orthophosphate is a molecule with a phosphorus atom surrounded by four oxygen atoms, and a negative three charge. Phosphorus or orthophosphate, is a measure of the amount of orthophosphate (PO43-) molecules in a sample. While phosphorus or orthophosphate as phosphorus (PO4-P), is a measure of the amount of phosphorus (P) ions in a sample. This means that when measuring phosphorus, the four oxygen atoms bound to the phosphorus atom are included in the measurement. However, it is possible to easily convert between the two measurements with simple multiplication or division. To convert a result from phosphorus (P) to phosphorus (PO43-) you simply need to multiply by 3.06. Conversely, to convert from phosphorus to phosphorus you just need to divide by 3.06. The reason this works is because the orthophosphate molecule (PO43-) is approximately 3.06 times heavier than the phosphorus atom.

Hanna offers a variety of chemical based methods to measure phosphorus. These include reactive, acid hydrolyzable, and total phosphorus.

Reactive Phosphorus

Reactive phosphorus is the simplest form, known as phosphate or orthophosphate. It is considered reactive since it easily reacts or bonds with cations. Orthophosphate is commonly added to water to inhibit corrosion of pipes in the distribution of water.

Acid Hydrolyzable Phosphorus

Acid Hydrolyzable Phosphorus also known, as condensed phosphate is a complex form of orthophosphate that are bound together. These forms include polyphosphate, pyrophosphate, and metaphosphate, which are used in boiler and cooling tower water treatment for corrosion inhibition of pipes.

Total Phosphorus

Total phosphorus (Total P) is the sum of all phosphorus including inorganic (orthophosphate and acid hydrolyzable) and organic matter such as the phosphorus found in living matter (i.e.ATP/ADP). In order to measure the organic phosphorus, the sample needs to be digested with an acid and heat in order to breakdown the organically bound phosphorus to the simplest form, orthophosphate.

Available technologies to measuring phosphorus include spectrophotometers, benchtop and portable photometers, Checker HC and chemical test kits.



Spectrophotometers are available that have phosphorus methods in several ranges pre-programmed into the meter. The spectrophotometer offers the highest precision due to the quality of the optical system that has a wavelength accuracy of +/- 1.5 nm. The spectrophotometer allows for custom methods to be used as well.



Benchtop Photometers

Benchtop photometers include multiparameter versions for water and wastewater treatment. These photometers can measure all three forms of phosphorus and are able to display results in multiple chemical forms. All benchtops have a digital pH electrode input allowing it to be used as a traditional pH meter.



Portable Photometers

A portable photometer is available to measure phosphorus and display results in multiple chemical forms. The portable photometer has the CAL Check feature that allows for performance verification and, if needed, recalibration using the CAL Check standards. The meter is available as a meter only or as a kit. The kit version includes a rugged carrying case and CAL Check standards.



Checker HC

Checker HC, like a chemical test kit, are reagent based designs with a color change based on concentration. The handheld colorimeters, like portable and benchtop instrumentation, use the Beer-Lambert principle to determine the color change. The change in color is not subjective as with using the naked eye. A marine version is available with chemistry that is compatible for use with saltwater aquariums.



Chemical Test Kits

Multiparameter Chemical Test Kits (CTK) are available to measure phosphorus in soil. These kits include other key parameters including nitrogen and potassium.

For educational use there is the Hanna Backpack Lab for Soil Quality. The Backpack Lab includes a phosphorus test kit along with a teacher’s guide that discusses the importance of the parameter.




Solutions for phosphorus measurement include the CAL Check standards for verifying and, if needed, the calibration of our portable photometers. Each CAL Check standard is supplied with a Certificate of Analysis stating the accuracy and traceability of the standard.




Accessories include the replacement cuvettes and caps for the photometers and Checker HC. Also included is the COD reactor that is used to digest the samples for acid hydrolysable and total phosphorus measurements.


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