🔬 Kjeldahl nitrogen
Determination of Kjeldahl nitrogen (TKN) in water by mineralization with selenium, ammonia distillation, and titration. Sum of organic and ammoniacal nitrogen.
Overview
Kjeldahl Nitrogen (TKN—Total Kjeldahl Nitrogen) is a sum parameter including organic nitrogen (in amino acids, proteins, urea, nucleic acids) and ammoniacal nitrogen (NH₄⁺/NH₃). It does not include nitrate nitrogen (NO₃⁻) or nitrite nitrogen (NO₂⁻). It is one of the key indicators of organic water and wastewater pollution.
The PN-EN 25663 (= ISO 5663) standard describes the method after mineralization with selenium—determining only nitrogen at the third negative oxidation state. The method consists of three stages: mineralization (digestion in H₂SO₄ with selenium catalyst), distillation (NH₃ release after alkalization with NaOH), and titration (quantitative NH₃ determination with acid). Range: up to 10 mg N in analytical sample.
TKN is a mandatory parameter in municipal and industrial wastewater monitoring (water permits), wastewater treatment plant efficiency assessment (nitrogen removal), and surface water testing (eutrophication).
Method principle
(1) Mineralization: sample is heated with concentrated sulfuric acid (H₂SO₄) in the presence of selenium catalyst and K₂SO₄/CuSO₄ mixture at 360–410°C. H₂SO₄ oxidatively decomposes organic matter, and organic nitrogen converts to ammonium sulfate ((NH₄)₂SO₄). (2) Distillation: after cooling, NaOH is added, which neutralizes H₂SO₄ and converts NH₄⁺ to gaseous NH₃. Ammonia distills with steam and is absorbed in boric acid (H₃BO₃) solution. (3) Titration: boric acid solution with absorbed NH₃ is titrated with hydrochloric acid (HCl) against mixed indicator (e.g., Tashiro).
Applications
- Municipal and industrial wastewater monitoring (water permits)
- Biological nitrogen removal efficiency control in treatment plants
- Surface water testing—eutrophication
- Groundwater monitoring—agricultural pollution
- Drinking water testing—indicator parameter
- Nitrogen load assessment in wastewater discharges to environment
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | Up to 10 mg N in analytical sample |
| Detection limit | 0.5 mg/L N (for 50 mL sample) |
| Accuracy | ±5–10% |
| Mineralization temperature | 360–410°C |
| Mineralization time | 1–3 hours |
| Catalyst | Selenium + K₂SO₄ + CuSO₄ |
Standard
- Standard number
- PN-EN 25663:2001
- Title (PL)
- Jakość wody — Oznaczanie azotu Kjeldahla — Metoda po mineralizacji z selenem
- Title (EN)
- Water quality — Determination of Kjeldahl nitrogen — Method after mineralization with selenium
Step-by-step procedure
1. Sample measurement
Transfer appropriate sample volume (50–250 mL depending on expected N concentration) to Kjeldahl flask.
2. Mineralization
Add 1 selenium catalyst tablet and 10–15 mL concentrated H₂SO₄. Place flask in heating block. Heat gradually to 360–410°C. Mineralize until obtaining clear, colorless or slightly greenish solution.
3. Cooling
After mineralization completion, cool flask to room temperature. Carefully add approx. 50 mL deionized water to dissolve precipitate.
4. Distillation
Place flask in distillation unit. Automatic distiller adds NaOH (alkalization) and conducts steam distillation. NH₃ is absorbed in receiving vessel containing 50 mL H₃BO₃ 4% with Tashiro indicator. Distill until collecting approx. 150–200 mL distillate.
5. Titration
Distillate (green—NH₃ present) titrate with HCl 0.01 or 0.1 mol/L until color change to violet/gray (endpoint). Record HCl volume.
6. Calculations
N [mg/L] = (V_HCl × c_HCl × 14.007 × 1000) / V_sample. Result in mg N/L. Subtract blank.
7. Quality control
Blank (deionized water + reagents), control sample with (NH₄)₂SO₄ (recovery 95–105%), duplicates. Each series verify distillation efficiency.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Kjeldahl mineralization unit (heating block) | BUCHI KjelDigester K-449, Gerhardt Kjeldatherm, VELP DKL | 20,000–60,000 PLN |
| Kjeldahl distillation unit | BUCHI KjelMaster K-375, Gerhardt Vapodest, VELP UDK 159 | 25,000–80,000 PLN |
| Kjeldahl flasks (250–500 mL) | Glass round-bottom flasks with ground joint | 30–80 PLN/piece |
| Burette 25 mL (class A) | Brand, with PTFE stopcock | 200–600 PLN |
| Analytical balance (0.1 mg) | Mettler Toledo ME204, Radwag AS 220.R2 | 5,000–15,000 PLN |
| Scrubber (SO₃ vapor capture) | BUCHI Scrubber K-415 | 5,000–15,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Concentrated sulfuric acid (H₂SO₄) (96%) | 7664-93-9 | For sample mineralization. p.a. purity, nitrogen-free. Add 10–15 mL per sample. |
| Selenium catalyst (Kjeldahl tablets) | — | Tablets: K₂SO₄ (3.5 g) + CuSO₄ (0.4 g) + Se (10 mg). Ready tablets (e.g., Merck, BUCHI). One tablet per sample. |
| Sodium hydroxide (NaOH) 32–40% | 1310-73-2 | For alkalization after mineralization—NH₃ release. Add in excess (approx. 50 mL 32% per sample). |
| Boric acid (H₃BO₃) 4% | 10043-35-3 | NH₃ absorbing solution in distillation. 40 g H₃BO₃ in 1 L water. Add mixed indicator. |
| Hydrochloric acid (HCl) 0.01–0.1 mol/L (standardized) | 7647-01-0 | For titrating absorbed NH₃. Concentration selected for expected nitrogen content. |
| Tashiro mixed indicator | — | Solution: 0.1 g methyl red + 0.05 g methylene blue in 100 mL ethanol. Color change: green (alkaline) → gray → violet (acidic). |
| (NH₄)₂SO₄ standard solution | 7783-20-2 | For recovery control. 4.716 g (NH₄)₂SO₄ (dried 105°C) in 1 L = 1000 mg N/L. |
Health and safety (OHS)
- Concentrated H₂SO₄ (96%)—strongly corrosive (H314), exothermic reaction with water! Add acid to water, not vice versa. Work under fume hood.
- NaOH 32–40%—strongly corrosive (H314). Severe skin and eye burns. Use face shield.
- Selenium—toxic (H301, H331, H373). Handle catalyst tablets with gloves.
- SO₃ and NH₃ vapors—toxic/irritating. Mineralization and distillation ONLY under fume hood or with scrubber.
- Hot mineralizate (>360°C)—burn risk. Do not tilt flasks.
- Safety glasses, acid-resistant gloves, lab coat, and face shield required.