🔬 Kjeldahl nitrogen

Physicochemistry PN-EN 25663

Overview

STATUS (as of 29.10.2026). PN-EN 25663:2001 — Polish version, published 23.07.2001, implements EN 25663:1993 and ISO 5663:1984 [IDT]. The standard is current — no withdrawal note. EN 25663:1993 and ISO 5663:1984 are current in the CEN and ISO catalogues, with no newer edition.

DEFINITION AND SCOPE (clauses 1, 3). Kjeldahl nitrogen — the content of organic and ammoniacal nitrogen in a sample determined after mineralization; it does not include nitrate and nitrite nitrogen and does not necessarily include all organically bound nitrogen. Only trivalent negative nitrogen is determined: organic nitrogen in the form of azide, azine, azo, hydrazone, nitrite, nitro, nitroso, oxime or semicarbazone is not determined quantitatively, and nitrogen in heterocyclic compounds may be incompletely recovered. The method applies to raw, potable and waste waters; up to 10 mg of nitrogen can be determined in the test portion — with a 10 ml test portion this corresponds to concentrations up to 1000 mg/l. The limit of detection (100 ml test portion) is 1 mg/l; with a 100 ml test portion, 1.0 ml of 0.02 mol/l hydrochloric acid corresponds to 2.8 mg/l of nitrogen.

INTERFERENCES (clause 10). Nitrate and nitrite may cause errors in both directions: under the test conditions they may be reduced to ammonium (high results) or form ammonium salts that decompose at the mineralization temperature with loss of nitrogen in gaseous form (low results). If this is likely to cause unacceptable bias, a separate reduction to ammonium precedes the mineralization. Over-prolonged mineralization also gives low results — the procedure in clause 8 must be followed carefully.

REPEATABILITY (9.2, Table 2, data from France). Standard deviations: urea solution 2 mg/l (500 ml test portion) — 0.027 mg/l; 50 mg/l (100 ml) — 0.31 mg/l; 150 mg/l (100 ml) — 2.69 mg/l; sewage 79 mg/l (50 ml) — 0.68 mg/l; chemical waste 16 mg/l (200 ml) — 0.19 mg/l and 62 mg/l (100 ml) — 0.48 mg/l.

LEGAL REQUIREMENTS. PN-EN 25663 is the reference method for Kjeldahl nitrogen (Norg + N-NH₄) in waste water (regulation of 12 July 2019, Journal of Laws 2019, item 1311, Annex 12 item 9) and the titrimetric and spectrophotometric method for Kjeldahl nitrogen in the monitoring of surface and ground waters (Journal of Laws 2021, item 1576, Annex 8, indicator 3.5.2). The regulation on water intended for human consumption (Journal of Laws 2026, item 748) has no Kjeldahl nitrogen parameter.

Method principle

The sample is mineralized with sulfuric acid containing a high concentration of potassium sulfate, which raises the boiling point of the mixture, in the presence of selenium as catalyst; the nitrogen compounds covered by the test are converted to ammonium sulfate. After addition of alkali, ammonia is distilled into a boric acid/indicator solution and the ammonium in the distillate is titrated with a standard acid. Alternatively, ammonium is determined directly in the mineralizate by spectrometry at 655 nm (clause 11). Selenium was chosen as catalyst instead of mercury because of the toxicity of mercury, but the toxicity of selenium must not be overlooked (clause 4).

Applications

• Raw, potable and waste waters (1.2)
• Waste water — reference method for Kjeldahl nitrogen (Journal of Laws 2019, item 1311, Annex 12)
• Monitoring of surface and ground waters (Journal of Laws 2021, item 1576)
• Does not include nitrate and nitrite nitrogen (clause 3)

Key parameters

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. Samples

   Polyethylene or glass bottles; analyse as quickly as possible or store at 2–5 °C until analysed. Acidification with sulfuric acid to pH < 2 may aid preservation, provided absorption of atmospheric ammonia is avoided (clause 7).

2. Preparing the apparatus

   If the distillation apparatus has been out of use for more than a few days: about 350 ml of water and a few granules in the distillation flask, distil at least 100 ml; discard the distillate and the residue (clause 6).

3. Test portion

   If the approximate nitrogen concentration is known, the volume of the test portion is selected from Table 1 (titration with 0.02 mol/l acid): up to 10 mg/l — 250 ml, 10–20 mg/l — 100 ml, 20–50 mg/l — 50 ml, 50–100 mg/l — 25 ml (8.1).

4. Mineralization

   Boil the test portion in a Kjeldahl flask with 10 ml of sulfuric acid, (5.0 ± 0.5) g of catalyst mixture and a few granules rapidly under fume extraction; the volume decreases as water boils away, then white fumes appear. After fume evolution has ended, observe the mineralizate and, when it has become clear and colourless or light brown, continue heating for a further 60 min. After the water has boiled away, the heating should reflux the acid half-way up the neck of the flask — this ensures a sufficiently high mineralization temperature (8.3, note 1).

5. Distillation

   Cool the flask to room temperature. Measure (50 ± 5) ml of boric acid/indicator solution into the receiving flask; the condenser tip must be below the surface of the solution. Carefully add (250 ± 50) ml of ammonium-free water and a few granules to the flask, then 50 ml of sodium hydroxide solution, and immediately attach the flask to the apparatus (the contents of a flask not compatible with the apparatus are transferred quantitatively to a distillation flask, conveniently at the water addition stage). Heat so that the distillate collects at about 10 ml/min; stop when about 200 ml have been collected (8.3, note 2).

6. Titration

   Titrate the distillate with 0.02 mol/l hydrochloric acid to a purple end point; for distillates from samples with high nitrogen concentrations 0.10 mol/l acid may be used (8.3, note 3).

7. Blank test

   As in steps 4–6, with about 250 ml of ammonium-free water instead of the test portion (8.2).

8. Calculation

   ρN = (V₁ − V₂) / V₀ · c · 14.01 · 1000 [mg/l], where V₀ is the volume of the test portion (ml), V₁ and V₂ the volumes of acid used for the sample and the blank (ml), c the exact acid concentration (mol/l), 14.01 the relative atomic mass of nitrogen; amount-of-substance concentration cN [µmol/l] = ρN · 71.4 (9.1).

9. Spectrometric variant

   After the mineralizate has cooled: (50 ± 10) ml of water, 2 drops of 4-nitrophenol solution (1 g/l), then very slowly sodium hydroxide solution, mixing and cooling, until a persistent pale yellow colour develops, and a few drops of sulfuric acid until it is discharged; transfer quantitatively to a 200 ml flask and make up to the mark. Ammonium is determined spectrometrically (ISO 7150-1), with calibration and blank solutions prepared in the same way; the result is multiplied by 200/V₀ (11.1).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Sulfuric acid and the 500 g/l sodium hydroxide solution cause severe burns (warnings at 5.3 and 5.4)
• The catalyst mixture is toxic — avoid inhaling dust during its preparation and use; all residues containing selenium are collected for selenium recovery or controlled disposal (5.8)
• Mineralization may evolve toxic sulfur dioxide, and hydrogen sulfide or hydrogen cyanide may be liberated from polluted samples — mineralization is carried out under an effective fume extraction system (8.3)
• Procedure suggested by the standard for selenium residues: collect them in a glass bottle labelled “Toxic”, acidify to pH 2 and add solid tin(II) chloride — red selenium precipitates, which is filtered off and disposed of in a controlled manner (11.2)

Frequently asked questions