🔬 Total Nitrogen in Water
Determination of total nitrogen in water after oxidative digestion with peroxodisulfates. Sum of organic nitrogen, ammonium, nitrite and nitrate nitrogen.
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
| Parameter | Value |
|---|---|
| Measuring range | 1–100 mg N/L (without dilution) |
| Detection limit | 0.1 mg N/L |
| Accuracy | ±5–10% of measured value |
| Wavelength | 540 nm (after reduction to nitrites) |
| Mineralization temperature | 120°C (autoclave, 30 min) |
| Analysis time | 1–2 hours (with mineralization) |
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.
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.
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.
4. Autoclave — oxidation
Carry out mineralization in autoclave at 120°C under pressure 1.1 atm for 30 minutes. Cool to room temperature.
5. Acidification
After mineralization, acidify the sample with hydrochloric acid to pH 2–3. Make up to volume with deionized water to 25 mL.
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.
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.
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).
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.
10. Quality control (QC)
Analyze blank (deionized water), duplicate every 10 samples, CRM control sample. Recovery 90–110%.
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
| Equipment | Example | Indicative price |
|---|---|---|
| UV-VIS Spectrophotometer | Hach DR6000, Shimadzu UV-1900i, Thermo Scientific GENESYS 150 | 15,000–45,000 PLN |
| Laboratory autoclave | Tuttnauer ELV 2540, Systec VX-65 | 20,000–45,000 PLN |
| Block digester | Gerhardt Kjeldatherm, Büchi SpeedDigester K-439 | 25,000–55,000 PLN |
| Flow analysis system CFA/FIA | SEAL Analytical AA500, Skalar San++ | 80,000–200,000 PLN |
| Cadmium reduction column | Custom packed Cd/Cu column | 500–1,500 PLN |
| Analytical balance | Mettler Toledo ME204, Sartorius Quintix 224 | 5,000–12,000 PLN |
| Automatic pipettes | Eppendorf Research Plus, Gilson Pipetman L | 800–2,500 PLN/pc |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Potassium peroxodisulfate (K₂S₂O₈) | 7727-21-1 | ACS grade, oxidizer for mineralization, working concentration 0.045 mol/L |
| Sodium hydroxide (NaOH) | 1310-73-2 | ACS grade, for buffering alkaline mineralization environment |
| Boric acid (H₃BO₃) | 10043-35-3 | ACS grade, alkaline buffer preventing loss of ammonium nitrogen |
| Sulfanilamide | 63-74-1 | ACS grade, reagent for diazotization reaction (Griess reagent I) |
| N-(1-naphthyl)ethylenediamine (NED) | 1465-25-4 | Dihydrochloride, Griess reagent II, for forming colored azo complex |
| Potassium nitrate (KNO₃) | 7757-79-1 | Nitrate standard CRM for calibration curve, 1000 mg N/L |
| Hydrochloric acid (HCl) | 7647-01-0 | 3 mol/L, for acidifying sample after mineralization |
| Metallic cadmium (Cd) | 7440-43-9 | Granules 0.3–1.0 mm for reduction column (reduction NO₃⁻ → NO₂⁻) |
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