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⁺) | ~0.6 | Significant — Na/K ratio must be > 20 to avoid significant error. |
| Ammonium (NH₄⁺) | ~0.2 | Na/NH₄ ratio must be > 10. |
| Magnesium (Mg²⁺) | ~0.03 | Na/Mg ratio must be > 1. |
| Calcium (Ca²⁺) | ~0.02 | Na/Ca ratio must be > 1. |
Apparatus Required
- Ion-Selective Electrode: ELIT 8230
- Reference electrode: Single junction silver chloride (ELIT 001n)
- 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 1000, 100, 10, 1 ppm Na solutions. For high ionic strength samples, use the Standard Addition method — no satisfactory ISAB exists for this electrode.
Sample Preparation & Measurement
Best used for pure Na solutions or where Na is far more concentrated than other ions. For complex matrices with significant K, NH₄, Ca or Mg, use the Na071 glass combination electrode instead.
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.
- No satisfactory ISAB exists for the sodium PVC electrode — use Standard Addition for high ionic strength samples.
- The Na071 glass combination electrode is far less sensitive to other cations (K⁺ SC = 0.0006 vs ~0.6 for PVC) and is recommended for critical measurements.
- pH must be maintained in the 3–10 range.