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 |
|---|---|---|
| Rubidium (Rb⁺) | 2 | Concentration should be < 1/100th of potassium to avoid significant error. |
| Caesium (Cs⁺) | 0.4 | Concentration should be < 1/10th of potassium. |
| Ammonium (NH₄⁺) | 0.01 | Should be less than potassium concentration to avoid significant error. |
| Sodium (Na⁺) | 0.0004 | NaCl ISAB causes ~0.5 ppm apparent K signal — acceptable for most applications. |
| Calcium (Ca²⁺) | 0.0003 | — |
| Magnesium (Mg²⁺) | 0.0003 | — |
| Lithium (Li⁺) | 0.0001 | — |
Apparatus Required
- Ion-Selective Electrode: ELIT 8031
- 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 1000, 100, 10, 1 ppm K solutions. Add 2% v/v ISAB to all standards and samples. Best results in still (unstirred) solutions.
Sample Preparation & Measurement
Add 2 ml 2.5M NaCl ISAB to each 100 ml sample. For K < 5 ppm in high ionic strength samples, use 5M TEAC instead. Filter turbid samples.
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.
- Best results when readings taken in still (un-stirred) solutions.
- Although sodium interferes, NaCl can be used as ISAB as it only produces ~0.5 ppm apparent K signal — negligible for most applications.
- For K < 5 ppm in high ionic strength samples, switch ISAB to 5M TEAC (Tetraethylammoniumchloride).