🔬 Total Nitrogen in Water
In short
Determination of nitrogen in water after oxidative digestion with peroxodisulfate in a closed vessel, in which ammonia, nitrite and organic nitrogen compounds are converted to nitrate. The test is performed according to PN-EN ISO 11905-1:2001; Range: up to 5 mg/l N with a 50 ml test portion; higher concentrations with a smaller portion (clause 3). The procedure comprises 7 steps; it is used for: Natural fresh water, sea water, drinking water, surface water and treated sewage effluent (clause 1), Sewage and trade wastes, if organic matter in the test portion does not exceed 40 mg/l C (TOC) or 120 mg/l O₂ (COD) (clause 1), Municipal wastewater — total nitrogen as an annual mean with a limit value of 10–30 mg/l or a reduction of 70–80 % (Journal of Laws 2019, item 1311, Annex 2).
At a glance
- Standard: PN-EN ISO 11905-1:2001
- Category: Physicochemistry
- Procedure steps: 7
- Range: up to 5 mg/l N with a 50 ml test portion; higher concentrations with a smaller portion (clause 3)
- Lower limit of detection: typically 0.02 mg/l N — depends on the method used to determine nitrate (clause 3)
- Wavelength: 540 nm; analyser spectrometer measuring in the range 520–550 nm (clause 5, 7.4)
Overview
STATUS (as of 29.10.2026). PN-EN ISO 11905-1:2001 — Polish version, published on 27.12.2001, adopts EN ISO 11905-1:1998 [IDT] and ISO 11905-1:1997 [IDT]. The standard is current — no withdrawal note. ISO 11905-1:1997 (first edition) has the status of a current standard in the catalogue; part 2 of this series covers the determination of bound nitrogen after combustion to nitrogen dioxide with chemiluminescent detection.
SCOPE (clause 1). Nitrogen present in water as free ammonia, ammonium, nitrite, nitrate and organic nitrogen compounds that are converted to nitrate under the oxidative conditions described. Dissolved nitrogen gas is not determined by this method. It applies to natural fresh water, sea water, drinking water, surface water and treated sewage effluent, and also to sewage and trade wastes in which the amount of organic matter in the test portion can be kept below 40 mg/l as carbon (TOC) or 120 mg/l as oxygen (COD).
RANGE (clauses 3–4). With the maximum test portion (50 ml), nitrogen is determined up to 5 mg/l N; higher concentrations with a smaller test portion. The lower limit of detection is typically 0.02 mg/l N, and it as well as the sensitivity depend on the method used to measure the nitrate formed by the oxidation.
INTERFERENCES (clause 4). Dissolved and suspended organic matter consumes the oxidizing reagent — if the COD of the sample exceeds 120 mg/l O₂ or the TOC 40 mg/l C, the sample is diluted. Not all organic nitrogen compounds are converted quantitatively to nitrate: compounds with double- and triple-bonded nitrogen and with a >C=NH group give poor recoveries, and compounds with free amino groups give less than quantitative recovery, but not less than 87 %. On real samples with large amounts of organic matter, the results do not differ significantly from those of instrumental methods with high-temperature oxidation or reduction.
REFERENCE METHOD (introduction of the standard, PKN catalogue card). The reference method is continuous flow analysis with initial reduction of nitrate to nitrite on copperized cadmium. Annex C gives examples of other techniques for determining nitrate in the digest; they may be used if their precision (determined according to ISO 5725-2 and compared according to ISO 2854) is at least as good as the precision data in Annex A. All concentrations are expressed in milligrams of nitrogen per litre (mg/l N).
REGULATIONS. Municipal wastewater: total nitrogen is the sum of Kjeldahl, nitrite and nitrate nitrogen, assessed as an annual mean: 10 mg/l (PE from 100 000) and 15 mg/l (PE 15 000–99 999) or a minimum reduction of 70–80 %; 15 mg/l (PE 2000–14 999) and 30 mg/l (PE below 2000) apply only to discharges into lakes and their tributaries and directly into artificial reservoirs on flowing waters, and for PE 10 000–14 999 discharging into other waters — a reduction of 70–80 % (Journal of Laws 2019, item 1311, Annex 2 item 4 with explanatory notes 4–5); industrial wastewater — 30 mg/l (Annex 4 item 11). Among the reference methodologies of this regulation (Annex 12), total nitrogen has no separate entry — Kjeldahl, nitrate and nitrite nitrogen are listed. In the monitoring of surface water and groundwater, PN-EN ISO 11905-1 is the flow-analysis methodology for total nitrogen (indicator 3.5.5) and Kjeldahl nitrogen (indicator 3.5.2) (Journal of Laws 2021, item 1576, Annex 8). For water intended for human consumption, the regulation gives no value for total nitrogen, only for nitrate (50 mg/l) and nitrite (0.50 mg/l) (Journal of Laws 2026, item 748, Annex 1 Part B).
WHAT WE DO NOT GIVE HERE. The publicly available text of the standard ends at 9.1 (test portion). The blank test (9.2), cleaning of vessels (9.3), the digestion procedure — volume of oxidizing solution and time in the autoclave (9.4), verification of the recovery of organic nitrogen (9.5), starting and setting up the analyser (9.6–9.7), calibration and determination (9.8–9.9), calculation and precision data (clause 10, Annex A), recovery data for nitrogen compounds (Annex B) and alternative techniques (Annex C) are beyond it — we know only their titles from the table of contents, so we do not give volumes, times or the number of standards.
Method principle
Ammonia, nitrite and many organic nitrogen compounds in the sample are oxidized to nitrate with peroxodisulfate in a buffered alkaline system by boiling at elevated pressure in a closed container. The digest is passed through a mixing coil containing copperized cadmium, where nitrate is reduced to nitrite. The nitrite reacts with 4-aminobenzene sulfonamide and N-(1-naphthyl)-1,2-diaminoethane dihydrochloride to give a pink colour, which is measured photometrically at 540 nm (clause 5).
Applications
- Natural fresh water, sea water, drinking water, surface water and treated sewage effluent (clause 1)
- Sewage and trade wastes, if organic matter in the test portion does not exceed 40 mg/l C (TOC) or 120 mg/l O₂ (COD) (clause 1)
- Municipal wastewater — total nitrogen as an annual mean with a limit value of 10–30 mg/l or a reduction of 70–80 % (Journal of Laws 2019, item 1311, Annex 2)
- Monitoring of surface water and groundwater — flow-analysis methodology for total nitrogen (Journal of Laws 2021, item 1576, Annex 8, indicator 3.5.5)
Key parameters
| Parameter | Value |
|---|---|
| Range | up to 5 mg/l N with a 50 ml test portion; higher concentrations with a smaller portion (clause 3) |
| Lower limit of detection | typically 0.02 mg/l N — depends on the method used to determine nitrate (clause 3) |
| Wavelength | 540 nm; analyser spectrometer measuring in the range 520–550 nm (clause 5, 7.4) |
| Organic matter limit per test portion | not more than 2 mg C (TOC) or 6 mg O₂ (COD); samples with COD > 120 mg/l or TOC > 40 mg/l — dilute (clause 4, 9.1) |
| Recovery | compounds with free amino groups — not less than 87 %; poor with multiple-bonded nitrogen and the >C=NH group (clause 4) |
| Sample storage | 2–5 °C for up to 48 h; after acidification with sulfuric acid to pH 2 — up to 8 days (clause 8) |
Standard
- Standard number
- PN-EN ISO 11905-1:2001
- Title (PL)
- Jakość wody — Oznaczanie azotu — Część 1: Metoda mineralizacji nadtlenodwusiarczanem
- Title (EN)
- Water quality — Determination of nitrogen — Part 1: Method using oxidative digestion with peroxodisulfate
Step-by-step procedure
-
Sampling and storage
Plastics or glass bottles. Analyse as quickly as possible or store at 2–5 °C for up to 48 h; acidification with sulfuric acid to pH 2 (preventing absorption of atmospheric ammonia) allows storage for up to 8 days (clause 8).
-
Choice of test portion
The maximum test portion — 50 ml — is used for concentrations up to 5 mg/l N; for higher concentrations a smaller portion. The test portion must not contain more than 2 mg of organic carbon (TOC) or have a COD above 6 mg O₂ (9.1).
-
Preparing the test portion
Mix the laboratory sample thoroughly; homogenize an inhomogeneous sample with the homogenizer. With strongly acid samples (pH < 2), after adding the oxidizing solution carefully adjust the pH to 9.7 with sodium hydroxide solution (9.1).
-
Oxidative digestion
The test portion with the oxidizing solution (potassium peroxodisulfate, boric acid, sodium hydroxide) is boiled at elevated pressure in a closed container — in an autoclave operating at 120 °C, in vessels withstanding up to 200 kPa (clause 5, 7.2–7.3). Volumes and digestion time are given in 9.4 — beyond the available text of the standard.
-
Reduction and colour reaction
In the flow analyser the digest passes through the column of copperized cadmium (nitrate → nitrite) and is then mixed with the colour reagent — a pink azo dye forms (clause 5, 7.4–7.5).
-
Measurement
The absorbance is measured in the flow cell at 540 nm (clause 5). Calibration, determination and calculation of the result — 9.8–10.1, beyond the available text of the standard.
-
Blank and recovery check
At least one blank with water instead of the test portion (6.8); the recovery of organic nitrogen is checked with a glycine solution of a concentration matched to the expected range (6.7, 9.5).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Homogenizer | for homogenizing inhomogeneous samples before taking the test portion (7.1, 9.1) | — |
| Digestion vessels | bottles of PTFE or another suitable material, with screw caps, nominal capacity 100–125 ml, withstanding pressures of up to 200 kPa (7.2) | — |
| Autoclave | for pressures up to 200 kPa and operation at 120 °C (7.3) | — |
| Continuous flow analyser | sampler, multichannel peristaltic pump, analytical cartridge with pump tubes, mixing coils, reduction column and dialyser, spectrometer with a flow cell for measurements in the range 520–550 nm, recorder (7.4) | — |
| Reduction column | glass or plastics tubing with the same internal diameter as the mixing coils; the volume of copperized cadmium granules (ml) as close as possible to the total liquid flow rate through the column (ml/min) (7.5) | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Water | — | purity grade 3 according to ISO 3696; reagents of recognized analytical grade (clause 6) |
| Sulfuric acid, c(H₂SO₄) ≈ 4 mol/l | 7664-93-9 | carefully mix (110 ± 0.5) ml of concentrated acid (ρ = 1.84 g/ml) into about 350 ml of water, cool, dilute to (500 ± 10) ml; stable indefinitely (6.1) |
| Sodium hydroxide solution, c(NaOH) ≈ 0.375 mol/l | 1310-73-2 | (15.0 ± 0.5) g NaOH in about 900 ml of water, after cooling make up to (1000 ± 10) ml; in a polyethylene container, stable for at least 6 months (6.2) |
| Oxidizing solution | 7727-21-1 | (5.0 ± 0.1) g of potassium peroxodisulfate (K₂S₂O₈, containing not more than 0.001 % (m/m) nitrogen) and (3.00 ± 0.05) g of boric acid (H₃BO₃) in (100 ± 5) ml of NaOH solution (6.2); in polyethylene, in the dark at room temperature, stable for up to one week (6.3) |
| Hydrochloric acid, c(HCl) ≈ 5 mol/l and ≈ 0.1 mol/l | 7647-01-0 | add (450 ± 10) ml or (10.0 ± 0.5) ml of concentrated acid (ρ = 1.18 g/ml) to water and make up to (1000 ± 10) ml (6.4–6.5) |
| Glycine solutions, 200 mg/l and 2 mg/l N | 56-40-6 | (1.072 ± 0.001) g of glycine to 1 l (in a refrigerator at 0–5 °C stable for at least a month); 10 ml of it to 1 l, fresh for each batch — for verifying the recovery of organic nitrogen (6.6–6.7) |
| Copperized cadmium granules | 7440-43-9 | wash (10 ± 0.1) g of cadmium granules (250–425 µm) with 50 ml of hydrochloric acid 5 mol/l, rinse with water, treat with copper sulfate solution (20 g CuSO₄·5H₂O in 1 l) until a brown colloidal precipitate forms, wash with water at least 10 times; store under water without contact with air (6.9–6.10) |
| Buffer solution | 12125-02-9 | (85.0 ± 0.5) g of ammonium chloride and (1.0 ± 0.1) g of wetting agent (nonionic surfactant) in 1000 ml of water; stable for at least a month (6.11, 6.13) |
| Colour reagent | 63-74-1 | (40 ± 0.5) g of 4-aminobenzene sulfonamide in a mixture of 100 ml of orthophosphoric acid (ρ = 1.71 g/ml) and about 500 ml of water, with (2.00 ± 0.2) g of N-(1-naphthyl)-1,2-diaminoethane dihydrochloride (CAS 1465-25-4), to 1000 ml; in an amber glass bottle at 2–5 °C stable for a month (6.12) |
| Nitrate solutions, 1000 mg/l and 100 mg/l N | 7757-79-1 | (7.215 ± 0.001) g of potassium nitrate (dried at 105 °C for at least 2 h) to 1 l — stable for two months; dilution of 50 ml to 500 ml — stable for a month (6.14–6.15) |
| Nitrite solutions, 100 mg/l and 4 mg/l N | 7632-00-0 | (0.492 ± 0.002) g of sodium nitrite (dried at 105 °C for at least 2 h) to 1000 ml, in an amber glass bottle at 2–5 °C stable for at least a month; 2 ml to 50 ml — prepare as required (6.16–6.17) |
Health and safety (OHS)
- Sulfuric acid and hydrochloric acid can cause severe burns — wear gloves and eye protection when preparing and using them (6.1, 6.4–6.5)
- The sodium hydroxide solution and the oxidizing solution are corrosive — gloves and eye protection (6.2–6.3)
- Cadmium is toxic by inhalation, by contact with skin and by ingestion (6.10)
- N-(1-naphthyl)-1,2-diaminoethane dihydrochloride is toxic by inhalation, by contact with skin and by ingestion (6.12)
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 11905-1:2001 — “Water quality — Determination of nitrogen — Part 1: Method using oxidative digestion with peroxodisulfate”.
How does this method work?
Ammonia, nitrite and many organic nitrogen compounds in the sample are oxidized to nitrate with peroxodisulfate in a buffered alkaline system by boiling at elevated pressure in a closed container. The digest is passed through a mixing coil containing copperized cadmium, where nitrate is reduced to nitrite.
What is the measuring range and accuracy?
Range: up to 5 mg/l N with a 50 ml test portion; higher concentrations with a smaller portion (clause 3); Lower limit of detection: typically 0.02 mg/l N — depends on the method used to determine nitrate (clause 3); Wavelength: 540 nm; analyser spectrometer measuring in the range 520–550 nm (clause 5, 7.4); Organic matter limit per test portion: not more than 2 mg C (TOC) or 6 mg O₂ (COD); samples with COD > 120 mg/l or TOC > 40 mg/l — dilute (clause 4, 9.1).
How long does the test take?
The procedure comprises 7 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Homogenizer, Digestion vessels, Autoclave, Continuous flow analyser, Reduction column.
Where is this test used?
Natural fresh water, sea water, drinking water, surface water and treated sewage effluent (clause 1); Sewage and trade wastes, if organic matter in the test portion does not exceed 40 mg/l C (TOC) or 120 mg/l O₂ (COD) (clause 1); Municipal wastewater — total nitrogen as an annual mean with a limit value of 10–30 mg/l or a reduction of 70–80 % (Journal of Laws 2019, item 1311, Annex 2); Monitoring of surface water and groundwater — flow-analysis methodology for total nitrogen (Journal of Laws 2021, item 1576, Annex 8, indicator 3.5.5).
What safety precautions apply?
Sulfuric acid and hydrochloric acid can cause severe burns — wear gloves and eye protection when preparing and using them (6.1, 6.4–6.5); The sodium hydroxide solution and the oxidizing solution are corrosive — gloves and eye protection (6.2–6.3); Cadmium is toxic by inhalation, by contact with skin and by ingestion (6.10); N-(1-naphthyl)-1,2-diaminoethane dihydrochloride is toxic by inhalation, by contact with skin and by ingestion (6.12).
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 ISO 11905-1.