🔬 Nitrates in Water
In short
Spectrophotometric determination of nitrates (NO₃⁻) in water by sulfosalicylic acid method. The test is performed according to ISO 7890-3:1988; Measurement range: 0.003–0.2 mg N/L (at 25 mL sample and 40 mm cuvette). The procedure comprises 7 steps (about 1 h 5 min–1 h 35 min in total); it is used for: Drinking water quality control (limit ≤50 mg NO₃⁻/L), Groundwater monitoring (agricultural nitrate contamination), Surface water monitoring (eutrophication).
At a glance
- Standard: ISO 7890-3:1988
- Category: Physicochemistry
- Procedure steps: 7
- Total time of stages: 1 h 5 min–1 h 35 min
- Measurement range: 0.003–0.2 mg N/L (at 25 mL sample and 40 mm cuvette)
- Detection limit: 0.003–0.013 mg N/L
- Wavelength: 410 nm
Overview
Nitrates (NO₃⁻) are the final oxidation product of nitrogen in water (nitrification: NH₄⁺ → NO₂⁻ → NO₃⁻). Their presence in surface and groundwater is mainly due to: agricultural runoff (nitrogen fertilizers), treated wastewater discharge (after nitrification), and natural organic matter mineralization.
Excessive nitrate concentration in drinking water poses health risks — methemoglobinemia (blue baby syndrome) in infants. Standard: ≤50 mg NO₃⁻/L (Polish Ministry of Health Regulation) = 11.3 mg N-NO₃⁻/L. Nitrates Directive (91/676/EEC) requires monitoring and limiting water nitrate contamination from agricultural sources.
ISO 7890-3 method with sulfosalicylic acid involves forming yellow sodium nitrosalicylate in alkaline medium, measured at 410 nm. It is resistant to chloride interference and suitable for raw and drinking waters.
Method principle
Nitrates react with sulfosalicylic acid (5-sulfosalicylic acid) in anhydrous environment (evaporation with H₂SO₄) forming mixture of nitrosalicylates (ortho- and para-). After sodium hydroxide addition (alkalization) stable yellow 5-nitrosalicylate sodium anion is formed.
NO₃⁻ + sulfosalicylic acid → (H₂SO₄, anhydrous) → 5-nitrosalicylic acid 5-Nitrosalicylic acid + NaOH → sodium 5-nitrosalicylate (yellow)
Yellow solution absorbance is measured spectrophotometrically at 410 nm wavelength. Color intensity is proportional to nitrate concentration.
Applications
- Drinking water quality control (limit ≤50 mg NO₃⁻/L)
- Groundwater monitoring (agricultural nitrate contamination)
- Surface water monitoring (eutrophication)
- Treated wastewater control (denitrification efficiency)
- Nitrates Directive — vulnerable zone monitoring
- Spring and mineral water testing
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0.003–0.2 mg N/L (at 25 mL sample and 40 mm cuvette) |
| Detection limit | 0.003–0.013 mg N/L |
| Wavelength | 410 nm |
| Cuvette | 10–40 mm (select for concentration) |
| Precision (CV) | 5–10% |
| Interferences | Nitrites (>0.5 mg/L), chlorides (>300 mg/L), color, turbidity |
Standard
- Standard number
- ISO 7890-3:1988
- Title (PL)
- Jakość wody — Oznaczanie azotanów — Część 3: Metoda spektrometryczna z kwasem sulfosalicylowym (brak odpowiednika PN-EN)
- Title (EN)
- Water quality — Determination of nitrate — Part 3: Spectrometric method using sulfosalicylic acid
Step-by-step procedure
-
Sample preparation
Filter sample through 0.45 μm membrane filter (if turbid). Measure 25 mL sample into porcelain evaporating dish. For higher concentration samples — dilute.
-
Evaporation
Evaporate sample to dryness in water bath at 80°C. Do not overheat — nitrates may decompose above 100°C.
-
Sulfosalicylic acid addition
To dry residue add 1 mL sulfosalicylic acid solution in H₂SO₄. Mix with glass rod. Wait 10 minutes (nitration reaction in anhydrous environment).
-
Water addition and alkalization
Carefully add 10–20 mL deionized water. Then slowly add NaOH 8 mol/L solution until pH >12 (color change to yellow). Fill to 50 mL in volumetric flask.
-
Cooling
Wait until solution reaches room temperature (alkalization is exothermic). Fill to mark in volumetric flask.
-
Absorbance measurement
Measure absorbance at 410 nm against blank (blank sample prepared identically with deionized water). Read concentration from calibration curve.
-
Quality control
Control standard — deviation max ±10%. Blank — absorbance <0.02. Duplicates — RPD <15%. Calibration curve — R² >0.999.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| UV-VIS spectrophotometer | Hach DR6000, Shimadzu UV-1900, WTW photoLab 7600 | 15 000–45 000 PLN |
| Optical cuvettes 10–40 mm | Glass or quartz cuvettes | 50–300 PLN/pc |
| Hot plate or water bath | For sample evaporation at 80°C | 1 000–5 000 PLN |
| Automatic pipettes | Eppendorf Research Plus 1–10 mL | 800–2 000 PLN |
| Beakers / porcelain evaporating dishes | Evaporating dishes 50–100 mL for bath evaporation | 20–60 PLN/pc |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Sulfosalicylic acid (5-sulfosalicylic) | 5965-83-3 | Coloring reagent: 10 g sulfosalicylic acid in 100 mL concentrated H₂SO₄. Prepare fresh. |
| Sulfuric acid H₂SO₄ concentrated | 7664-93-9 | Anhydrous reaction medium for nitration. Highly corrosive and hygroscopic. |
| Sodium hydroxide NaOH | 1310-73-2 | Solution 8 mol/L (320 g/L) for alkalization. Highly corrosive, exothermic reaction with water. |
| Sodium potassium tartrate (Rochelle salt) | 304-59-6 | Additive for masking metal interferences (Fe, Mn). 50 g/L. |
| NO₃⁻ standard solution 1000 mg N/L | 7631-99-4 | Certified CRM (KNO₃ or NaNO₃ in water). For calibration curve. |
Health and safety (OHS)
- Concentrated sulfuric acid — highly corrosive, causes severe burns. Exothermic reaction with water. Work under fume hood.
- Sodium hydroxide 8 mol/L — highly corrosive. Use goggles, gloves, and coat.
- Sulfosalicylic acid — irritating, avoid skin and eye contact.
- Safety goggles, nitrile gloves, and coat mandatory.
- Sample alkalization with H₂SO₄ — strongly exothermic! Add NaOH slowly and carefully.
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?
Nitrates react with sulfosalicylic acid (5-sulfosalicylic acid) in anhydrous environment (evaporation with H₂SO₄) forming mixture of nitrosalicylates (ortho- and para-). After sodium hydroxide addition (alkalization) stable yellow 5-nitrosalicylate sodium anion is formed.
What is the measuring range and accuracy?
Measurement range: 0.003–0.2 mg N/L (at 25 mL sample and 40 mm cuvette); Detection limit: 0.003–0.013 mg N/L; Wavelength: 410 nm; Cuvette: 10–40 mm (select for concentration).
How long does the test take?
The times given for the individual stages add up to approximately 1 h 5 min–1 h 35 min. The procedure comprises 7 steps.
What equipment is required?
UV-VIS spectrophotometer, Optical cuvettes 10–40 mm, Hot plate or water bath, Automatic pipettes, Beakers / porcelain evaporating dishes. Indicative cost of the core instrument (UV-VIS spectrophotometer): 15 000–45 000 PLN.
Where is this test used?
Drinking water quality control (limit ≤50 mg NO₃⁻/L); Groundwater monitoring (agricultural nitrate contamination); Surface water monitoring (eutrophication); Treated wastewater control (denitrification efficiency); Nitrates Directive — vulnerable zone monitoring; Spring and mineral water testing.
What safety precautions apply?
Concentrated sulfuric acid — highly corrosive, causes severe burns. Exothermic reaction with water. Work under fume hood.; Sodium hydroxide 8 mol/L — highly corrosive. Use goggles, gloves, and coat.; Sulfosalicylic acid — irritating, avoid skin and eye contact.; Safety goggles, nitrile gloves, and coat mandatory.; Sample alkalization with H₂SO₄ — strongly exothermic! Add NaOH slowly and carefully.
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.