Measuring Phosphorus Concentrations

There are several types of phosphorus and each comes with a specialized and recommended test kit. Here are some of the factors to consider when choosing and using a phosphorus test kit.

• Colour Comparators
• Detecting Low Concentrations
• Orthophosphates

Colour Comparator Kits

Students use colour comparators to measure phosphorus levels

Some phosphorus detecting equipment is more appropriate for school use. Colour comparators can be used successfully with school groups.

• Colour comparator kits are less expensive and last longer with school use.
• Colour comparator kits are most useful when testing at heavily polluted sites with high phosphorus concentrations above 0.1 mg/L.
• While they are useful, keep in mind that colour comparator kits introduce a source of error. Picking and matching colours is always subject to some personal judgment, especially at low concentrations.

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Detecting Low Concentrations

Testing for phosphorus is always challenging because it is often found in very low concentrations of less than 1.0 mg/L. Careful procedures and clean equipment are required to obtain accurate results.

• Phosphates adsorb or stick to glass, so test tubes and sample bottles should not be cleaned with phosphate detergents!
• Some test kits use plastic test tubes but these scratch more easily than glass and become hard to see through. Glass test tubes are preferable even though they can be dropped and broken.

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Orthophosphates

The procedures in all phosphorus test kits eventually finish with the measuring of orthophosphate concentration.

• This is the type of phosphorus dissolved in water and most readily available to plants.
• Orthophosphate concentrations may be the best cause-and-effect link to weed growth and algae blooms.

The orthophosphate test is the easiest phosphorus test to perform and is appropriate for clear rivers carrying low amounts of organic material. This simple test can measure low concentrations of total orthophosphates (PO4-3) in the range of 0 -1 mg/L.

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