A National Standard Methods for Determining Total Nitrogen in Water
The primary national standard methods for determining total nitrogen in water are as follows:
“Water Quality - Determination of Total Nitrogen - Potassium Persulfate Alkaline Digestion and Ultraviolet Spectrophotometric Method” (HJ 636-2012)
Scope of Application: Surface water, groundwater, industrial wastewater, and domestic sewage.
Principle: At 120-124°C, alkaline potassium persulfate solution converts nitrogen in nitrogen-containing compounds to nitrate. Absorbance is measured at wavelengths of 220 nm and 275 nm using ultraviolet spectrophotometry. The relationship between corrected absorbance and total nitrogen content is calculated.
Detection Range: 0.2–7 mg/L, detection limit 0.05 mg/L.
Interferences and Elimination: Iodide ions, bromide ions, hexavalent chromium ions, and trivalent iron ions may cause interference. These can be eliminated by dilution or addition of hydroxylamine hydrochloride solution.
Water Quality - Determination of Total Nitrogen - Gas Phase Molecular Absorption Spectroscopy (HJ 199-2023)
Scope of Application: Surface water, groundwater, domestic sewage, industrial wastewater, and seawater.
Principle: Using alkaline potassium persulfate as an oxidant, nitrogen in the sample is oxidized to nitrate nitrogen via high-temperature high-pressure digestion or online UV digestion. It is then reduced to nitric oxide by titanium trichloride and measured using a gas phase molecular absorption spectrometer.
Detection Range: Detection limit is 0.05 mg/L for both high-temperature high-pressure digestion and online UV digestion; lower limit of quantification is 0.20 mg/L.
Interferences and Mitigation: Hexavalent chromium, trivalent iron, and high concentrations of organic matter may cause interference. Mitigate by dilution or optimizing digestion conditions.
Water Quality - Determination of Total Nitrogen - Flow Injection-Hydrochloric Acid-Naphthylenediamine Spectrophotometric Method (HJ 668-2013)
Scope of Application: Surface water, groundwater, industrial wastewater, and domestic sewage.
Detection Range: 0.12–10 mg/L.
Water Quality—Determination of Total Nitrogen—Continuous Flow-Hydrochloric Acid Naphthylenediamine Spectrophotometric Method (HJ 667-2013)
Scope of Application: Surface water, groundwater, industrial wastewater, and domestic sewage.
Detection Range: 0.16–10 mg/L.
Among the above methods, HJ 636-2012 and HJ 199-2023 are currently the most commonly used laboratory determination methods. The specific selection may be determined based on sample type, detection range, and laboratory conditions.