Total Phosphate (TP)

Total Phosphate (TP)

02 Sep 2022

What is Total Phosphate?

Total phosphorus (TP) is the measurement of all phosphorus within a sample and whether Phosphorus is dissolved or particulate. Composed of orthophosphate, polyphosphate and organic Phosphorous, the test measures both dissolved and suspended orthophosphate as the sample is not filtered.

Water Total Phosphorus is examined after all forms of phosphates converted to orthophosphate and is commonly expressed as milligrams of phosphorus per litre of water (mg/L of P).

The EPA (Environmental Protection Administration) established a recommended limit of 0.05 mg/L for Total Phosphates in streams that enter lakes and 0.1 mg/L for total phosphorus in flowing waters.


Harm of Phosphorus in Water

As phosphates are chemical compounds containing Phosphorous, a key nutrient for both plants and animal growth. Whilst phosphate is essential for plants and animals, too much of it can cause a form of water pollution known as Eutrophication.

Eutrophication also referred to as “nutrient-induced increase in phytoplankton productivity” can lead to depletion of dissolved oxygen in water and decreased biodiversity.

In the nitrogen cycle with sufficient oxygen, ammonia nitrogen can be converted into nitrite and nitrate, which may cause cancers in the human body. Dissolved ammonia can also be toxic to fish, animals and humans.


Principle of Molybdovanadate Method

Reactive Phosphorus combines with Molybdate in an acid medium to form a phosphomolybdate complex. Vanadium, contained in Molybdovanadate Reagent, reacts with the complex to form Vanadomolybdophosphoric acid. Intensity of the resulting yellow colour is proportional to the concentration of reactive phosphorus.


Steps of Molybdovanadate Method

Using Rocker CR 25 COD Reactor, Rocker WD 100 Reader and HACH Phosphorus (Total) TNT Reagent Set, Molybdovanadate (HR) as example.

  1. Turn on CR 25 reactor then start TP program to preheat to 150°C.
  2. Select the appropriate concentration of reagent according to the phosphate concentration of sample.
  3. Add 5 ml sample to one reagent vial as the test sample.
  4. Add one pack of Potassium Persulfate powder pillow to each vial.
  5. Close the vials and shake until all powders are finely dissolved.
  6. Put the vials into CR 25 reactor then start to countdown from 30 mins at 150°C.
  7. Remove the vials after digestion and cool vials down to room temperature.
  8. Add 2 ml of 1.54N Sodium Hydroxide Standard Solution to each vial. Close the vials and invert to mix.
  9. Add 0.5 ml Molybdovanadate reagent to each vial. Close the vials and invert to mix.
  10. Wait 7 minutes for reaction.
  11. Turn on WD 100 colorimeter, then select test item and concentration range.
  12. Insert the blank sample vial into WD 100 colorimeter to start a zero calibration.
  13. Insert the sample vial into WD 100 colorimeter and start reading.




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