Removing Nitrogen Interference in TP Measurement

Total phosphorus (TP) determination by the molybdate blue method suffers from significant interference when nitrite exceeds 1 mg/L. Nitrite reacts with molybdate under acidic conditions to form a blue complex identical to phosphomolybdenum blue, causing overestimation. Ammonia and nitrate do not directly interfere, but ammonia may oxidize to nitrite during digestion, while nitrate at very high levels can suppress color development.

Two practical elimination strategies are widely used.

Oxidative digestion enhances the standard persulfate digestion step. Increasing the persulfate dosage or extending heating time generates more atomic oxygen, converting nitrite to non‑interfering nitrate. This method is simple and requires no extra reagents, yet it may fail for extremely high nitrite loads.

Sulfamic acid treatment is more selective. After digestion, adding sulfamic acid to the sample rapidly converts nitrite into nitrogen gas and water under acidic conditions. This reaction is fast and does not affect the subsequent phosphomolybdate color formation. A short waiting period allows gas bubbles to escape before adding the color reagent.

For samples with nitrite far above the threshold, dilution is a straightforward backup, provided that phosphorus remains within the calibration range. Proper sample pretreatment—filtration through 0.45 μm membranes and prompt analysis—also minimizes interference from particulates and nitrite transformation during storage.

In practice, a combined approach works best: strengthen the oxidation during digestion, then apply sulfamic acid before color development. This dual strategy ensures complete nitrite removal and yields accurate TP results across most wastewater and surface water matrices.