Chloride
Cl⁻The ELIT chloride electrode uses a poly-crystalline AgCl/AgS membrane for direct chloride determination across a very wide pH range. Widely used in food, water, and industrial applications.
- Do NOT add sodium bromate buffer to samples containing significant cyanide — the acid solution may liberate lethal HCN gas.
- Irreversible membrane damage will result from exposure to high iodide concentrations.
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⁻) | >1 | CRITICAL: the Cl membrane is far more sensitive to I than to Cl. High iodide concentrations will irreversibly damage the membrane. |
| Bromide (Br⁻) | high | Strong interference — must be absent or at insignificant concentrations. |
| Cyanide (CN⁻) | high | Strong interference. Note: do NOT add acid to samples containing significant cyanide — risk of lethal HCN gas. |
| Sulphide (S²⁻) | high | All poly-crystalline (AgS-based) membranes are unreliable in presence of Ag or S ions. |
| Silver (Ag⁺) | high | — |
Apparatus Required
- Ion-Selective Electrode: ELIT 8261
- 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 1000, 100, 10, 1 ppm Cl solutions. For high ionic strength samples (> 0.01 M), add 2 ml ISAB to each 100 ml standard. A sodium bromate buffer can reduce interference from Br, I, CN, and S if samples contain these ions.
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.
Butter
Heat ~10g butter with 100 ml dilute HNO₃ until fat melts. Stir to extract chloride without emulsifying. Cool until fat solidifies. Pipette 25 ml of the solution through the fat layer and dilute 1:1 with deionised water. Standards must be mixed 1:1 with dilute HNO₃ ISAB (60 ml conc. HNO₃ in 1 litre).
Fruit Juice
Use Sample Addition method due to high Cl content, complex matrix and high ionic strength. Calibrate with two standards spanning the expected range (10-fold difference in concentration). Enter slope, standard concentration and expected sample concentration into the software.
Mayonnaise
Disperse ~1g mayonnaise in 500 ml 1M nitric acid in a 1000 ml flask. Dilute to 1 litre, filter 50–100 ml into sample beaker. Mix standards 1:1 with 1M HNO₃.
Milk
Mix 1 volume milk with 2 volumes dilute HNO₃ (60 ml conc. HNO₃ per litre) and shake for 1 minute. Nitric acid acts as ISAB. Measure by Direct Potentiometry. Add HNO₃ ISAB to all standards in 2:1 ratio.
Meat
Liquidise ~1g meat with 100 ml water. Filter, mix 25 ml with 25 ml of 0.5M HNO₃ ISAB. Result: ppm × 100 ÷ sample weight = µg/g.
Soil
Shake ~4g air-dried soil with 50 ml deionised water for 1 hour. Filter, wash, dilute to 100 ml. Add 2 ml NaNO₃ ISAB. Result: ppm × 100 ÷ sample weight = mg/kg.
- The chloride electrode will only give reliable results if iodide, bromide, cyanide, sulphide, and silver are absent or at insignificant levels.
- A sodium bromate buffer (15.1g NaBrO₃ in 800 ml water + 75 ml conc. HNO₃, diluted to 1 L) can remove up to 1000 ppm Br or I and 500 ppm sulphide. Prepare and use in a well-ventilated area — may liberate bromine gas.