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 |
|---|---|---|
| Iodide (I⁻) | variable | Strong interference — avoid |
| Bromide (Br⁻) | 0.02 | — |
| Chloride (Cl⁻) | 0.0002 | — |
| Silver (Ag⁺) | variable | Precipitates AgSCN on membrane |
| Sulphide (S²⁻) | variable | Strong interference |
| Thiosulphate (S₂O₃²⁻) | variable | — |
Apparatus Required
- Ion-Selective Electrode: ELIT 8229
- 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 SCN solutions plus ISAB. Use standard addition method for complex matrices.
Sample Preparation & Measurement
Add 10 ml ISAB to 100 ml sample. Remove silver, sulphide, and iodide before measurement if present.
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 range (2–12) makes this electrode suitable for many sample types.
- Chloride can be tolerated up to 500× the thiocyanate concentration.
- Used in clinical analysis for saliva thiocyanate (smoking biomarker).