🔬 Nitrates in Water
In short
Spectrometric determination of nitrate in raw and potable waters by the sulfosalicylic acid method. The test is performed according to ISO 7890-3:1988; Range: nitrate nitrogen up to 0.2 mg/l with a 25 ml test portion; higher concentrations in smaller test portions (1–5 µg N in the portion). The procedure comprises 9 steps; it is used for: Drinking water — nitrate 50 mg/l (NO₃) and the condition [nitrate]/50 + [nitrite]/3 ≤ 1 (Journal of Laws 2026, item 748, Annex 1 Part B), Raw waters — the precision data of the standard include river waters (Table 2), Coloured waters up to 150 mg/l Pt — with the correction for test portion absorption (6.5).
At a glance
- Standard: ISO 7890-3:1988
- Category: Physicochemistry
- Procedure steps: 9
- Range: nitrate nitrogen up to 0.2 mg/l with a 25 ml test portion; higher concentrations in smaller test portions (1–5 µg N in the portion)
- Limit of detection: 0.003–0.013 mg/l (40 mm cell, 25 ml test portion)
- Wavelength: 415 nm; 40 or 50 mm cells
Overview
STATUS (as of 28.10.2026). ISO 7890-3:1988 — first edition, published 24.11.1988; the ISO standard is current. The PKN catalogue contains no standard implementing ISO 7890-3 (searches for “ISO 7890-3” and “7890-3” — no results), so the Polish title is a working translation.
SCOPE (clause 1). A method for the determination of nitrate ion in raw and potable waters. With the maximum test portion of 25 ml, nitrate nitrogen is determined up to 0.2 mg/l; smaller test portions extend the range upwards. The limit of detection with 40 mm cells and a 25 ml test portion is 0.003–0.013 mg/l (4.65 times the within-batch standard deviation of the blank), and 0.2 mg/l gives an absorbance of about 0.68.
INTERFERENCES (1.6, Annex A). The main potential interferents are chloride, orthophosphate, magnesium and manganese(II); Annex A gives the effect of more than a dozen substances. A sample colour of up to 150 mg/l Pt does not interfere if the correction for test portion absorption is applied (6.5). Interference from nitrite is overcome by adding sodium azide (or sulfamic acid), and the EDTA added with the alkali solution prevents precipitation of calcium and magnesium salts.
SAMPLES (clause 5). Glass bottles; analysis as soon as possible after collection. Storage at 2–5 °C may preserve many types of sample, but this has to be checked for each sample type.
DRINKING WATER — LEGAL REQUIREMENTS. Regulation of the Minister of Health of 22 May 2026 (Journal of Laws 2026, item 748), Annex 1 Part B: nitrate 50 mg/l (as NO₃), provided that [nitrate]/50 + [nitrite]/3 ≤ 1 (concentrations of NO₃ and NO₂ in mg/l), and after treatment nitrite does not exceed 0.10 mg/l; measurement uncertainty for nitrate — 15 % of the parametric value (Annex 4 Part B). 50 mg/l of NO₃ equals 11.3 mg/l of nitrate nitrogen (factor 0.226 from Table 1 of the standard) — at such concentrations the test portion must be small: in Table 2 of the standard, river waters with 9–10 mg/l of nitrogen were tested in 0.5 ml test portions.
WASTE WATER. The standard does not cover waste water (scope: raw and potable waters). The waste water regulation (Journal of Laws 2019, item 1311, Annex 12, item 7) lists for nitrate nitrogen spectrophotometry without a specified standard, flow analysis PN-EN ISO 13395 and ion chromatography PN-EN ISO 10304-1.
WHAT WE DO NOT GIVE HERE. We do not reproduce the figures of the precision table (Table 2) or of the table of the effect of other substances (Annex A).
Method principle
Spectrometric measurement of the yellow compound formed by sulfosalicylic acid — produced by adding sodium salicylate and sulfuric acid to the sample — in the reaction with nitrate and after subsequent treatment with alkali. Measurement at 415 nm.
The disodium salt of EDTA is added with the alkali to prevent precipitation of calcium and magnesium salts; sodium azide is added to overcome interference from nitrite.
Applications
- Drinking water — nitrate 50 mg/l (NO₃) and the condition [nitrate]/50 + [nitrite]/3 ≤ 1 (Journal of Laws 2026, item 748, Annex 1 Part B)
- Raw waters — the precision data of the standard include river waters (Table 2)
- Coloured waters up to 150 mg/l Pt — with the correction for test portion absorption (6.5)
Key parameters
| Parameter | Value |
|---|---|
| Range | nitrate nitrogen up to 0.2 mg/l with a 25 ml test portion; higher concentrations in smaller test portions (1–5 µg N in the portion) |
| Limit of detection | 0.003–0.013 mg/l (40 mm cell, 25 ml test portion) |
| Wavelength | 415 nm; 40 or 50 mm cells |
| Colour stability | absorbance constant for at least 24 h (note to 6.3.3) |
| Interferences | chloride, orthophosphate, magnesium, manganese(II); colour up to 150 mg/l Pt with correction |
| Conversion | 1 mg/l NO₃ = 0.226 mg/l N |
Standard
- Standard number
- ISO 7890-3:1988
- Title (PL)
- Jakość wody — Oznaczanie azotanów — Część 3: Metoda spektrometryczna z kwasem sulfosalicylowym (tłumaczenie robocze — normy nie wprowadzono do PN)
- Title (EN)
- Water quality — Determination of nitrate — Part 3: Spectrometric method using sulfosalicylic acid
Step-by-step procedure
-
Test portion
Samples containing suspended matter are allowed to settle, centrifuged or filtered through a washed glass fibre paper; samples with a pH above 8 are neutralized with acetic acid. Pipette into an evaporating dish a volume (at most 25 ml) containing 1–5 µg of nitrate nitrogen (6.1, 6.4).
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Removal of nitrite and evaporation
Add 0.5 ml of sodium azide solution and 0.2 ml of acetic acid, wait at least 5 min and evaporate to dryness on the boiling water bath (6.3.2).
-
Sodium salicylate
Add 1 ml of sodium salicylate solution, mix and evaporate to dryness again; remove the dish from the bath and allow it to cool to room temperature (6.3.2).
-
Sulfuric acid
Add 1 ml of sulfuric acid, dissolve the residue by gently agitating the dish and allow to stand for about 10 min (6.3.2).
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Alkalization
Add 10 ml of water, then 10 ml of alkali solution; transfer quantitatively to a 25 ml flask without making up to the mark; keep the flask in the bath at 25 ± 0.5 °C for 10 ± 2 min, then make up to the mark with water (6.3.2).
-
Measurement
Measure the absorbance at 415 nm in a 40 or 50 mm cell against distilled water; the colour is stable for at least 24 h (6.3.3).
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Blank and calibration
Blank with 5.00 ml of water instead of the test portion (6.2). Calibration: 1–5 ml of the 1 mg/l working solution (1–5 µg N) in evaporating dishes, then as for the sample; the graph of absorbance (after subtracting the blank) against the mass of nitrogen should be linear and pass through the origin — otherwise the calibration is repeated (6.3).
-
Correction for sample colour
For highly coloured samples a duplicate test portion is taken through the same procedure but without sodium salicylate, and its absorbance A_c is subtracted (6.5).
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Result
A_r = A_s − A_b (or A_s − A_b − A_c); from the graph the mass of nitrogen m(N) in µg; ρ_N = m(N) / V in mg/l; result as ρ_N, ρ_NO₃ or c(NO₃) — Table 1 (clause 7).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Spectrometer | for operation at 415 nm, with cells of optical path length 40 or 50 mm (4.1) | — |
| Evaporating dishes | about 50 ml; new or rarely used dishes are rinsed with water and taken through the first two paragraphs of 6.3.2 to clean them (4.2) | — |
| Boiling water bath | capable of accepting at least six dishes (4.3) | — |
| Thermostatic water bath | 25 ± 0.5 °C (4.4) | — |
| 25 ml one-mark volumetric flasks | for the solutions prepared for measurement (6.3.2) | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Concentrated sulfuric acid | 7664-93-9 | c(H₂SO₄) = 18 mol/l, ρ = 1.84 g/ml (3.1) |
| Glacial acetic acid | 64-19-7 | c = 17 mol/l, ρ = 1.05 g/ml; for neutralizing samples with a pH above 8 and during the determination (3.2) |
| Alkali solution | — | 200 ± 2 g of sodium hydroxide in about 800 ml of water, then 50 ± 0.5 g of EDTA disodium salt (dihydrate); after cooling make up to 1 l; in a polyethylene bottle, stable indefinitely (3.3) |
| Sodium azide solution, 0.5 g/l | 26628-22-8 | 0.05 g of sodium azide in about 90 ml of water, dilute to 100 ml; stable indefinitely. A sulfamic acid solution of 0.75 g/l may be used instead (3.4) |
| Sodium salicylate solution, 10 g/l | 54-21-7 | 1 ± 0.1 g in 100 ± 1 ml of water; prepared fresh on each day of operation (3.5) |
| Potassium nitrate — standard solutions | 7757-79-1 | stock 1000 mg/l of nitrogen: 7.215 ± 0.001 g of KNO₃ dried at 105 °C for at least 2 h, made up to 1 l — stored for at most 2 months; 100 mg/l — at most 1 month; working 1 mg/l — fresh on each occasion of use (3.6–3.8) |
Health and safety (OHS)
- Eye protection and protective clothing are essential when using concentrated sulfuric acid, acetic acid and the alkali solution; reagents are never pipetted by mouth (clauses 3 and 6)
- Sodium azide is very toxic if swallowed, and contact of the solid reagent with acids liberates a very toxic gas (warning to 3.4)
Frequently asked questions
Which standard describes this test?
The test is performed according to ISO 7890-3:1988 — “Water quality — Determination of nitrate — Part 3: Spectrometric method using sulfosalicylic acid”.
How does this method work?
Spectrometric measurement of the yellow compound formed by sulfosalicylic acid — produced by adding sodium salicylate and sulfuric acid to the sample — in the reaction with nitrate and after subsequent treatment with alkali. Measurement at 415 nm.
What is the measuring range and accuracy?
Range: nitrate nitrogen up to 0.2 mg/l with a 25 ml test portion; higher concentrations in smaller test portions (1–5 µg N in the portion); Limit of detection: 0.003–0.013 mg/l (40 mm cell, 25 ml test portion); Wavelength: 415 nm; 40 or 50 mm cells; Colour stability: absorbance constant for at least 24 h (note to 6.3.3).
How long does the test take?
The procedure comprises 9 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Spectrometer, Evaporating dishes, Boiling water bath, Thermostatic water bath, 25 ml one-mark volumetric flasks.
Where is this test used?
Drinking water — nitrate 50 mg/l (NO₃) and the condition [nitrate]/50 + [nitrite]/3 ≤ 1 (Journal of Laws 2026, item 748, Annex 1 Part B); Raw waters — the precision data of the standard include river waters (Table 2); Coloured waters up to 150 mg/l Pt — with the correction for test portion absorption (6.5).
What safety precautions apply?
Eye protection and protective clothing are essential when using concentrated sulfuric acid, acetic acid and the alkali solution; reagents are never pipetted by mouth (clauses 3 and 6); Sodium azide is very toxic if swallowed, and contact of the solid reagent with acids liberates a very toxic gas (warning to 3.4).
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 ISO 7890-3.