Cyanide
CN⁻The ELIT cyanide electrode requires strict pH control (11–13) due to the toxic HCN gas liberated at lower pH. Essential for monitoring industrial effluents, mining waste, and electroplating rinse waters.
- KCN is highly toxic — all work must be performed in a fume cupboard with appropriate PPE.
- NEVER add acid to cyanide-containing samples — risk of generating lethal HCN gas.
- 10M NaOH ISAB is highly caustic — handle with care and eye protection.
- This electrode has a narrow, high pH range (11–13) and a limited concentration range (0.03–260 ppm).
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. |
| Iodide (I⁻) | 1 | Equally sensitive to I⁻ and CN⁻ — causes ~10% error if I⁻ is > 1/10th the cyanide concentration. |
Apparatus Required
- Ion-Selective Electrode: ELIT 8291
- Reference electrode: Double junction (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 200, 20, 2, 0.2 ppm CN solutions. Add 2 ml 10M NaOH ISAB to each 100 ml standard — this ensures correct pH and eliminates risk of toxic HCN fumes.
Sample Preparation & Measurement
MANDATORY: add 2 ml NaOH ISAB to 100 ml of every sample before measurement. This keeps pH > 11 and prevents HCN gas liberation.
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 Containing Heavy Metal Complexes
Cyanide bound in complex formation with heavy metals is liberated by EDTA displacement. Prepare EDTA solution: dissolve 7.44g EDTA in 1 litre deionised water. Add 5 ml EDTA to 50 ml of solution and heat to 50°C in a fume cupboard for 5 minutes. Immediately add 50 ml 0.2M NaOH and cool to room temperature. Standards must receive identical treatment.
- Note narrow and high pH range (11 to 13) — ISAB addition is mandatory, not optional.
- Limited concentration range compared to other ISEs (maximum 260 ppm).
- Always perform cyanide work in a well-ventilated area or fume cupboard.