🔬 Total Nitrogen in Water

Physicochemistry PN-EN ISO 11905-1

Overview

Total nitrogen (TN — Total Nitrogen) is one of the key parameters for assessing water and wastewater quality. It includes the sum of all forms of nitrogen present in the sample: organic nitrogen (proteins, amino acids, urea), ammonium nitrogen (NH₄⁺), nitrite nitrogen (NO₂⁻) and nitrate nitrogen (NO₃⁻). Excess nitrogen content in surface waters leads to eutrophication — massive development of algae and cyanobacteria, which degrades aquatic ecosystems.

The method according to PN-EN ISO 11905-1 consists of oxidative mineralization of the water sample with peroxodisulfates (persulfates) in an alkaline environment. As a result of this process, all forms of nitrogen present in the sample are oxidized to nitrates (NO₃⁻). The concentration of nitrates is then determined spectrophotometrically after reduction to nitrites — directly or using flow analysis (CFA/FIA).

Determination of total nitrogen is required by Polish regulations concerning monitoring of surface waters, municipal and industrial wastewater, and groundwater. This parameter is mandatory in water permits and WIOŚ (Inspectorate for Environmental Protection) reports. The permissible value of total nitrogen in wastewater discharged to waters is 10–30 mg N/L depending on the size of the treatment plant.

The peroxodisulfate method is preferred due to lower toxicity compared to the classic Kjeldahl method — it does not require the use of sulfuric acid, selenium catalysts or toxic reagents. It provides accurate results for natural waters, drinking water, seawater and wastewater with organic matter content below 40 mg C/L.

Method principle

The water sample is subjected to oxidative mineralization with potassium peroxodisulfate (K₂S₂O₈) in an alkaline environment buffered with sodium hydroxide. At a temperature of 120°C (autoclave) or 100°C (boiling), peroxodisulfates decompose, generating highly reactive sulfate radicals (SO₄⁻•), which oxidize all forms of organic and inorganic nitrogen to nitrates (NO₃⁻). Then nitrates are reduced to nitrites (NO₂⁻) on a cadmium column or enzymatically (nitrate reductase), and nitrites are determined spectrophotometrically in a diazotization reaction with Griess reagent (sulfanilamide + N-(1-naphthyl)ethylenediamine) at wavelength 540 nm.

Applications

• Monitoring of surface water quality (rivers, lakes, reservoirs)
• Control of municipal and industrial wastewater (water permits)
• Assessment of eutrophication risk in water reservoirs
• Drinking water testing (MoH Regulation — permissible limit NO₃: 50 mg/L)
• Monitoring of groundwater and water intakes
• Control of biological nitrogen removal processes in treatment plants
• Environmental studies — assessment of agriculture impact on waters

Key parameters

Standard

Standard number

PN-EN ISO 11905-1:1998

Title (PL)

Jakość wody — Oznaczanie azotu — Część 1: Metoda z zastosowaniem utleniania nadsiarczynami

Title (EN)

Water quality — Determination of nitrogen — Part 1: Method using oxidative digestion with peroxodisulfate

Step-by-step procedure

1. Sample preparation

   Filter the water sample through a 0.45 µm filter (for dissolved nitrogen) or leave unfiltered (total nitrogen). Store sample at 4°C, analysis within 28 days.

   ⏱ Time: 10 min

2. Preparation of mineralizing solution

   Dissolve 10 g K₂S₂O₈ and 6 g NaOH in 200 mL deionized water. Add 30 g H₃BO₃. Prepare fresh on the day of analysis.

   ⏱ Time: 15 min

3. Sample mineralization

   Measure 10 mL of sample into a test tube and add 5 mL of mineralizing solution. Seal tightly and place in autoclave.

   ⏱ Time: 5 min

4. Autoclave — oxidation

   Carry out mineralization in autoclave at 120°C under pressure 1.1 atm for 30 minutes. Cool to room temperature.

   ⏱ Time: 60 min • 🌡 Temperature: 120°C

5. Acidification

   After mineralization, acidify the sample with hydrochloric acid to pH 2–3. Make up to volume with deionized water to 25 mL.

   ⏱ Time: 5 min

6. Reduction of nitrates to nitrites

   Pass the sample through an activated Cd/Cu reduction column or add nitrate reductase enzyme solution with NADH. Wait 15–30 min.

   ⏱ Time: 15–30 min

7. Color reaction — Griess reagent

   Add 0.5 mL sulfanilamide solution (1% in 3 mol/L HCl). Mix. After 2 min add 0.5 mL NED (0.02%). Mix and wait 10 min for color development.

   ⏱ Time: 15 min

8. Spectrophotometric measurement

   Measure absorbance at 540 nm in 1 cm or 5 cm cuvette. Read concentration from calibration curve (6–8 points, range 0–10 mg N/L).

   ⏱ Time: 5 min

9. Calibration curve preparation

   Prepare a series of KNO₃ standard solutions: 0, 0.5, 1, 2, 5, 10 mg N/L. Subject to identical mineralization and reduction procedure.

   ⏱ Time: 60 min

10. Quality control (QC)

    Analyze blank (deionized water), duplicate every 10 samples, CRM control sample. Recovery 90–110%.

    ⏱ Time: 15 min

11. Calculations and reporting

    Calculate total nitrogen concentration from calibration curve. Report result in mg N/L to 0.1 mg/L. Account for dilutions.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Potassium peroxodisulfate — strong oxidizer, may cause skin and eye irritation. Use gloves and safety goggles
• Sodium hydroxide — highly corrosive (H314). Use gloves, goggles, lab coat. In case of skin contact, rinse immediately with water
• Cadmium — toxic and carcinogenic (cat. 1B). Use dust extraction, nitrile gloves, work under fume hood
• Hydrochloric acid — corrosive, irritating to respiratory tract. Work under fume hood, use personal protective equipment
• Autoclave — risk of burns from hot steam. Use heat-resistant gloves, do not open before pressure drops

Frequently asked questions