🔬 Ammonium Nitrogen in Water
In short
Spectrophotometric determination of ammonium nitrogen (NH₄⁺) in water by salicylate-hypochlorite method — indophenol reaction. The test is performed according to PN-EN ISO 7150-1:2002; Measurement range: 0.01–1.0 mg N/L (at 40 mL sample). The procedure comprises 7 steps (about 2 h–2 h 5 min in total); it is used for: Drinking water quality control (limit ≤0.5 mg NH₄⁺/L), Surface water monitoring — sewage contamination indicator, Treated wastewater control (nitrification efficiency).
At a glance
- Standard: PN-EN ISO 7150-1:2002
- Category: Physicochemistry
- Procedure steps: 7
- Total time of stages: 2 h–2 h 5 min
- Measurement range: 0.01–1.0 mg N/L (at 40 mL sample)
- Detection limit: 0.01 mg N/L
- Wavelength: 655 nm (or 665 nm)
Overview
Ammonium nitrogen (NH₄⁺-N / NH₃-N) is one of the most important water quality parameters. The presence of ammonia in surface water indicates fresh sewage or fertilizer contamination. In drinking water it is undesirable — standard ≤0.5 mg NH₄⁺/L (Polish Ministry of Health Regulation).
Ammonia in water exists in equilibrium NH₃ ↔ NH₄⁺ — proportions depend on pH and temperature. At pH <7 ammonium ion (NH₄⁺) dominates, at pH >9 — toxic free ammonia (NH₃). NH₃ is highly toxic to fish already at concentrations >0.02 mg/L.
ISO 7150-1 method is based on indophenol reaction (Berthelot's) — ammonium ion reacts with sodium salicylate and sodium hypochlorite in alkaline medium, in presence of sodium nitroprusside as catalyst, forming blue indophenol dye. Absorbance measured at 655 nm is proportional to NH₄⁺ concentration. Method range: up to 1 mg N/L (at 40 mL sample).
Method principle
Ammonium ion (NH₄⁺) reacts with sodium hypochlorite (NaOCl) forming monochloramine (NH₂Cl), which then reacts with sodium salicylate in alkaline environment (pH ~12.6) in presence of catalyst (sodium nitroprusside), forming blue indophenol complex (5-aminosalicylate).
NH₄⁺ + OCl⁻ → NH₂Cl + H₂O NH₂Cl + salicylate → 5-aminosalicylate (blue dye)
Blue dye absorbance is measured spectrophotometrically at 655 nm (or 665 nm) wavelength. Color intensity is proportional to ammonium nitrogen concentration. Color development time: 60 minutes at room temperature.
Applications
- Drinking water quality control (limit ≤0.5 mg NH₄⁺/L)
- Surface water monitoring — sewage contamination indicator
- Treated wastewater control (nitrification efficiency)
- Groundwater testing (ammonium nitrogen as contamination indicator)
- Water treatment process monitoring
- Aquaculture water quality control (NH₃ toxicity to fish)
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0.01–1.0 mg N/L (at 40 mL sample) |
| Detection limit | 0.01 mg N/L |
| Wavelength | 655 nm (or 665 nm) |
| Color development time | 60 minutes (room temperature) |
| Reaction pH | ~12.6 (strongly alkaline environment) |
| Precision (CV) | 3–10% (depends on concentration) |
Standard
- Standard number
- PN-EN ISO 7150-1:2002
- Title (PL)
- Jakość wody — Oznaczanie azotu amonowego — Część 1: Manualna metoda spektrometryczna
- Title (EN)
- Water quality — Determination of ammonium — Part 1: Manual spectrometric method
Step-by-step procedure
-
Solution preparation
Prepare fresh reagents: salicylate solution (valid 1 week), hypochlorite solution (valid 1 week), nitroprusside solution (valid 1 month in darkness). Use ammonia-free water.
-
Calibration curve
Prepare standard series: 0 (blank), 0.05, 0.1, 0.2, 0.5, 1.0 mg N/L from NH₄Cl standard solution. Sample/standard volume: 40 mL in 50 mL flasks.
-
Reagent addition
To each flask add: 4 mL salicylate solution, 4 mL NaOH solution, 2 mL hypochlorite solution, 0.5 mL nitroprusside solution. Fill to 50 mL with water. Mix thoroughly.
-
Color development (incubation)
Leave flasks in darkness at room temperature (20–25°C) for 60 minutes. Color changes from colorless to blue-green proportionally to NH₄⁺ concentration.
-
Absorbance measurement
Measure absorbance at 655 nm against blank (blank sample prepared identically with deionized water). Cuvette 10–50 mm (select for concentration). Read concentration from calibration curve.
-
Calculations
NH₄⁺-N concentration [mg/L] = calibration curve reading × dilution factor. Conversion: NH₄⁺ [mg/L] = NH₄⁺-N × 1.288.
-
Quality control
Control standard (e.g., 0.5 mg N/L) — deviation max ±10%. Blank <0.01 mg N/L. Duplicates — RPD <15%.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| UV-VIS spectrophotometer | Hach DR3900, DR6000, Shimadzu UV-1900, WTW photoLab 7600 | 15 000–45 000 PLN |
| Optical cuvettes 10–50 mm | Glass or quartz cuvettes 10, 20, 40, 50 mm | 50–300 PLN/pc |
| Automatic pipettes | Eppendorf Research Plus 1–10 mL and 100–1000 μL | 800–2 000 PLN |
| Volumetric flasks 50 mL | Class A, borosilicate glass | 30–80 PLN/pc |
| Water bath (optional) | To accelerate color development at 25°C | 3 000–8 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Sodium salicylate C₇H₅NaO₃ | 54-21-7 | Coloring reagent: 130 g sodium salicylate + 130 g trisodium citrate per liter. Store in darkness, shelf life 1 week. |
| Sodium hypochlorite NaOCl | 7681-52-9 | Oxidizing solution: sodium dichloroisocyanurate 2 g/L or hypochlorite solution ~0.2 g Cl₂/L. |
| Sodium nitroprusside Na₂[Fe(CN)₅NO] | 13755-38-9 | Indophenol reaction catalyst: 0.5 g/L. Store in darkness (photosensitive!). |
| Sodium hydroxide NaOH | 1310-73-2 | For alkalizing reaction environment to pH ~12.6. Solution 320 g/L. |
| NH₄⁺ standard solution 1000 mg N/L | 12125-02-9 | Certified CRM (NH₄Cl in water). For calibration curve and quality control. |
Health and safety (OHS)
- Sodium hypochlorite — corrosive, strong oxidizer. Causes skin and eye burns. Do not mix with acids (Cl₂ release)!
- Sodium hydroxide — highly corrosive, causes severe burns. Use goggles and gloves.
- Sodium nitroprusside — toxic, releases cyanide ions. Store in darkness.
- Sodium salicylate — irritating, avoid skin contact
- Safety goggles, nitrile gloves, and laboratory coat mandatory
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 7150-1:2002 — “Water quality — Determination of ammonium — Part 1: Manual spectrometric method”.
How does this method work?
Ammonium ion (NH₄⁺) reacts with sodium hypochlorite (NaOCl) forming monochloramine (NH₂Cl), which then reacts with sodium salicylate in alkaline environment (pH ~12.6) in presence of catalyst (sodium nitroprusside), forming blue indophenol complex (5-aminosalicylate).
What is the measuring range and accuracy?
Measurement range: 0.01–1.0 mg N/L (at 40 mL sample); Detection limit: 0.01 mg N/L; Wavelength: 655 nm (or 665 nm); Color development time: 60 minutes (room temperature).
How long does the test take?
The times given for the individual stages add up to approximately 2 h–2 h 5 min. The procedure comprises 7 steps.
What equipment is required?
UV-VIS spectrophotometer, Optical cuvettes 10–50 mm, Automatic pipettes, Volumetric flasks 50 mL, Water bath (optional). Indicative cost of the core instrument (UV-VIS spectrophotometer): 15 000–45 000 PLN.
Where is this test used?
Drinking water quality control (limit ≤0.5 mg NH₄⁺/L); Surface water monitoring — sewage contamination indicator; Treated wastewater control (nitrification efficiency); Groundwater testing (ammonium nitrogen as contamination indicator); Water treatment process monitoring; Aquaculture water quality control (NH₃ toxicity to fish).
What safety precautions apply?
Sodium hypochlorite — corrosive, strong oxidizer. Causes skin and eye burns. Do not mix with acids (Cl₂ release)!; Sodium hydroxide — highly corrosive, causes severe burns. Use goggles and gloves.; Sodium nitroprusside — toxic, releases cyanide ions. Store in darkness.; Sodium salicylate — irritating, avoid skin contact; Safety goggles, nitrile gloves, and laboratory coat mandatory.
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 7150-1.