Overview
ISO 11905-1:1997 - Water quality - Determination of nitrogen - Part 1: Method using oxidative digestion with peroxodisulfate - specifies a laboratory reference method to determine nitrogen in water samples by converting ammonia, ammonium, nitrite and many organic nitrogen compounds to nitrate via peroxodisulfate oxidative digestion, followed by nitrate measurement. The method is intended for natural fresh water, seawater, drinking water, surface water and many sewage effluents. Results are reported as milligrams of nitrogen per litre (mg/L).
Key topics and technical requirements
- Principle: Oxidative digestion with potassium peroxodisulfate in buffered alkaline solution under pressure/heat converts nitrogen species to nitrate. The digest is reduced (copperized cadmium) to nitrite and measured colorimetrically (pink azo dye) at ~540 nm in a continuous-flow system.
- Range & sensitivity: Using a 50 mL test portion the method measures up to 5 mg N/L; typical lower limit ≈ 0.02 mg N/L (dependent on the nitrate measurement technique).
- Sample constraints: Test portions must keep TOC ≤ 2 mg C or COD ≤ 6 mg O2 (per test portion). If original sample COD > 120 mg O2/L or TOC > 40 mg/L, dilution is required to ensure excess oxidant.
- Exclusions & limitations: Dissolved nitrogen gas is not measured. Some organic N compounds (e.g., with C=NH groups or multiple N–N bonds) may have incomplete recovery; typical recoveries are generally good (not less than ~87% for some amino compounds).
- Reagents & apparatus: Specifies reagent grades, preparation of peroxodisulfate oxidizing solution, buffer and colour reagents, copperized cadmium reduction column, PTFE digestion vessels, autoclave (≤200 kPa, 120 °C) and continuous-flow analysis equipment.
- Quality & performance: Includes blank tests, calibration, verification of organic-N recovery and precision data in annexes. References ISO 3696 (lab water), ISO 5725‑2 (precision assessment) and ISO 2854 (comparison procedures).
- Safety: Notes corrosive acids/alkali and toxic cadmium-use appropriate PPE and waste controls.
Applications and users
Who uses ISO 11905-1:1997:
- Environmental and regulatory laboratories monitoring water quality and total nitrogen.
- Wastewater treatment operators verifying effluent nitrogen loads.
- Drinking water laboratories and research groups assessing nitrogen species and organic-N contributions.
- Consultants and authorities performing compliance testing, method validation and inter-laboratory comparisons.
Practical value:
- Provides a standardized, validated oxidative digestion protocol suitable for routine monitoring and comparative studies.
- Useful where instrumental high‑temperature oxidation is not available or when a recognized reference method is required.
Related standards
- ISO 11905-2 (other parts of the same series - instrumental high-temperature oxidation/chemiluminescence)
- ISO 3696 - Water for analytical laboratory use
- ISO 5725-2 - Precision (trueness and precision) evaluation
- ISO 2854 - Comparison procedures between methods
Keywords: ISO 11905-1:1997, water quality, nitrogen determination, peroxodisulfate oxidation, oxidative digestion, total nitrogen, nitrate measurement, continuous-flow analysis, copperized cadmium, TOC, COD.
Open preview (PDF)Water quality — Determination of nitrogen — Part 1: Method using oxidative digestion with peroxodisulfate