What's the difference between Phosphate tests for Orthophosphate, Total Phosphorous, Acid Hydrolyzable Phosphates and Organic Phosphates?

Phosphate tests are often stated as reporting Orthophosphate or Total Phosphorous, but can also give other parameters like Acid Hydrolyzable Phosphates. We've taken a look at the definitions behind these terms, so you can tell which Phosphorous or Phosphate test to use.

What are Phosphates and Phosphorous?

The element Phosphorous, P, exists in water courses in a wide range of forms - as dissolved ions or in larger organic and inorganic molecules. Different Phosphate and Phosphorous tests will show up different forms, hence the variety seen in Phosphate test methods.  

Forms of Phosphorous / Phosphate

The most common forms of phosphates, their structure, origin and how to test for them are listed below;

1. Orthophosphate (aka Reactive Phosphate) PO₄^3-

The simplest form is Orthophosphate, which is also known as reactive phosphorous. This form will react directly with colorimetric phosphate reagents, and is the only form of Phosphorous to do so. This is used by bacteria, plants and algae as a vital nutrient in surface waters such as lakes. It is also found in fertilizers, so can get into water courses by runoff from agricultural land. Recommended electronic test methods;

• Lovibond MD100 Pocket Colorimeter for 0.05 to 4.0 mg/L with tablet reagents
• Hach Pocket Colorimeter for LCK Phosphate - for 0.02 to 3 mg/ with LCK vial reagents
• Hach Chemkey 25 pieces Orthophosphate  for 2.0-3.0 mg/L

Recommended manual test methods;

• Hach Test Strips for Ortho and Reactive Phosphate for 0 to 50 mg/L
• Lovibond Phosphate Checkit test kit for 0 to 4 mg/L

2. Condensed Phosphates (aka Meta, Pyro or Poly -phosphates or Acid Hydrolyzable Phosphates)

These are two or more orthophosphate units linked together. They are used in treatment systems for boiler water, and are found in many detergents. The test for this type of phosphates is known as "acid hydrolyzable phosphorous" - this requires heating the sample with strong acid to break down the linked units into single orthophosphate units. The test will then detect all present, so take a result before and after the acid hydrolysis step if you need to know how much is due to condensed and how much is due to orthophosphate units.

(condensed phosphates) = (acid hydrolyzable phosphates) - (orthophosphates)

Recommended test methods;

• Hach Test Kit PO-24 for Ortho and Meta phosphates - for Meta or Condensed Phosphates an additional acid digestion step is needed, see the user manual here

3. Organic Phosphates (aka Total Phosphorus)

These species have one or more orthophosphate groups attached to an organic molecule. They are usually formed by biological processes and are found in organic matter such as plant or animal tissues, sewage from animal or human waste, or food or pesticide residues.

Tests for Total Phosphorous will allow organic phosphates to be measured. These involve heating a sample with a strong acid and a strong oxidizer (like persulphate) to convert the organic phosphates into orthophosphates. Again this means that the results will show all phosphorous forms, so test a sample from a total phosphorous test and from a condensed phosphates test if you need to assess the different forms, then calculate using the equation;

(organic phosphates) = (total phosphates) - (acid hydrolyzable phosphates)

Since this test requires heating with acid and oxidizer, there are no portable methods.

Recommended test methods;

• Total Phosphorous vials by Hach - versions available for 0.06 to 3.5 mg/L or high range 1 to 100 mg/L for reading on a DR900 or DR1900 spectrophotometer (NB a micropipette and digestor reactor such as the DRB200 are also required) - see also own brand equivalents to these vials.
• Total Phosphorus test vials by Palintest - for 0 to 12 mg/L, the Palintest tubetest heater and a Palintest Photometer are also required

 

Which Phosphorus test should I select?

If you are testing Phosphorus to report to a regulation agency, such as the Environment Agency, DEFRA or EPA, you should check with them as to which Phosphate or Phosphorus test they require you to perform. Due to the variety of forms and reasons for testing, this is the only way to be certain you have selected the correct test.   For more information on Phosphate test methods, follow the links above or contact us if you have any questions;