Ammonium
NH₄⁺The ELIT ammonium electrode measures NH₄⁺ ions directly in aqueous solution without the need for alkaline pre-treatment. Suitable for water quality monitoring, environmental analysis, aquaculture, and agricultural applications.
The Selectivity Coefficient (SC) represents the approximate apparent increase in the measured concentration caused by 1 unit of the interferent. Effect (% error) = (interferent concentration × SC / analyte concentration) × 100.
| Interfering Ion | Selectivity Coefficient | Notes |
|---|---|---|
| Potassium (K⁺) | 0.1 | Primary interferent — special correction required if K/NH₄ ratio > 0.5 |
| Sodium (Na⁺) | 0.002 | — |
| Magnesium (Mg²⁺) | 0.0002 | Note: 0.1M MgSO₄ ISAB can give apparent NH₄ signal up to 2 ppm — use CuSO₄ instead for samples < 50 ppm NH₄ |
| Calcium (Ca²⁺) | 0.00006 | — |
| Lithium (Li⁺) | 0.00003 | — |
Apparatus Required
- Ion-Selective Electrode: ELIT 8051
- Reference electrode: Double junction lithium acetate (ELIT 003n)
- Dual electrode head (ELIT 201)
- ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter
- 150 ml polypropylene beakers, 100 ml volumetric flask, 1, 2, 5, 10 ml pipettes
Calibration
Calibrate with serial dilutions of 1000 ppm standard: 1000, 100, 10, 1, 0.1 ppm NH₄. For high ionic strength samples (>0.01 M), add 10 ml ISAB to each 100 ml standard. Note: ISAB addition raises slope, raises lower detection limit, and lowers pH.
Sample Preparation & Measurement
Low ionic strength: no preparation required — immerse electrodes directly in 50–100 ml sample. High ionic strength: add 10 ml ISAB to 100 ml sample and stir well.
Results
Results are displayed as ppm and mol/l. If buffer solution has been added equally to standards and samples, figures will not need adjusting as all are affected by the same dilution factor. Allow 2–3 minutes stabilisation after electrode immersion. Wash and dry electrodes between samples to avoid cross-contamination.
Samples with High Potassium Content (K/NH₄ > 0.5)
Rather than relying on the general selectivity coefficient, measure the interference directly: (1) measure NH₄ and K in a representative sample; (2) add additional K to double the concentration and remeasure the apparent NH₄; (3) calculate the selectivity coefficient from the increase. Alternatively, spike all standards with the same K concentration as the samples.
pH and Temperature Relationship
In aqueous solution there is a pH-dependent equilibrium between NH₄⁺ and NH₃. Below pH 6.5 there is essentially 100% NH₄⁺; above pH 11.5 essentially 100% NH₃. For Total Ammonia Nitrogen (TAN) measurements, keep pH below 7. For dissolved NH₃ toxicity, measure pH and temperature of each sample and calculate the NH₄/NH₃ ratio.
- The ELIT ammonium electrode measures NH₄⁺ directly in solution — no pre-treatment with caustic chemicals or liberation of noxious gases is required.
- Stir solutions gently during measurement; allow 2–3 minutes stabilisation after electrode immersion.
- For highest precision, recalibrate frequently.
- 1000 ppm standard is stable for extended periods; dilute working standards should be freshly prepared.