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 |
|---|---|---|
| Silver (Ag⁺) / Sulphide (S²⁻) | very high | All poly-crystalline membranes — unreliable in presence of Ag or S ions. |
| Cyanide (CN⁻) | 1 | Equally sensitive to CN⁻ and I⁻ — causes significant positive error if CN > 1/100th the iodide concentration. |
| Bromide (Br⁻) | ~0.0004 | — |
| Chloride (Cl⁻) | 0.000001 | — |
Apparatus Required
- Ion-Selective Electrode: ELIT 8281
- Reference electrode: Double junction lithium acetate (ELIT 003)
- 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, 0.1 ppm I solutions. For high ionic strength samples (> 0.01 M), add 2 ml ISAB to each 100 ml standard.
Sample Preparation & Measurement
Low ionic strength: immerse in 50–100 ml sample. High ionic strength: add 2 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.
- Wide pH operating range (2–12).
- The selectivity sequence for this electrode is CN > I ≫ Br ≫ Cl.
- Note that the iodide electrode has a strong interference on the chloride electrode — iodide samples must not contaminate chloride measurements.