🔬 Kjeldahl nitrogen
In short
Determination of Kjeldahl nitrogen (TKN) in water by mineralization with selenium, ammonia distillation, and titration. The test is performed according to PN-EN 25663:2001; Measurement range: Up to 10 mg N in analytical sample. The procedure comprises 7 steps (about 1 h 43 min–3 h 50 min in total); it is used for: Municipal and industrial wastewater monitoring (water permits), Biological nitrogen removal efficiency control in treatment plants, Surface water testing—eutrophication.
At a glance
- Standard: PN-EN 25663:2001
- Category: Physicochemistry
- Procedure steps: 7
- Total time of stages: 1 h 43 min–3 h 50 min
- Measurement range: Up to 10 mg N in analytical sample
- Detection limit: 0.5 mg/L N (for 50 mL sample)
- Accuracy: ±5–10%
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
-
Sample measurement
Transfer appropriate sample volume (50–250 mL depending on expected N concentration) to Kjeldahl flask.
-
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.
-
Cooling
After mineralization completion, cool flask to room temperature. Carefully add approx. 50 mL deionized water to dissolve precipitate.
-
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.
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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.
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Calculations
N [mg/L] = (V_HCl × c_HCl × 14.007 × 1000) / V_sample. Result in mg N/L. Subtract blank.
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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.
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN 25663:2001 — “Water quality — Determination of Kjeldahl nitrogen — Method after mineralization with selenium”.
How does this method work?
(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).
What is the measuring range and accuracy?
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.
How long does the test take?
The times given for the individual stages add up to approximately 1 h 43 min–3 h 50 min. The procedure comprises 7 steps.
What equipment is required?
Kjeldahl mineralization unit (heating block), Kjeldahl distillation unit, Kjeldahl flasks (250–500 mL), Burette 25 mL (class A), Analytical balance (0.1 mg), Scrubber (SO₃ vapor capture). Indicative cost of the core instrument (Kjeldahl mineralization unit (heating block)): 20,000–60,000 PLN.
Where is this test used?
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.
What safety precautions apply?
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.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN 25663.