Description
The AquaProbe Ion Selective Electrodes (ISE) operate on the fundamental principle of potentiometry, specifically by measuring the electrical potential difference that develops across a specialised membrane when it comes to contact with a solution containing the target ion.
Nernst Equation: The relationship between the measured potential difference (E) and the activity (or concentration) of the target ion (A) in the solution is described by the Nernst equation:
Where:
- E: Measured electrode potential (mV)
- E0​: Standard electrode potential (a constant for the specific electrode)
- R: Ideal gas constant
- T: Absolute temperature (in Kelvin)
- n: Charge number of the ion (e.g., +1 for ammonium, -1 for nitrate)
- F: Faraday constant
- aA​: Activity of the target ion in the sample solution
In practical terms, the measured voltage is logarithmically proportional to the ion’s activity. A higher concentration of the target ion typically leads to a greater potential difference.
All ISE Sensors are prone to interference from ions that are similar in nature to the target ion. These are different for each electrode and the electrode may provide incorrect readings where these ions are also present. Ion Selective Electrodes are not recommended for use in brackish or salt water due to the high level of interfering ions. All ion selective electrodes work in conjunction with the pH electrode during measurement. For this reason, the main probe must have a working pH or pH/ORP electrode fitted and the conductivity (EC) of the water under test must be greater than 50μS/cm. pH ranges are given for each electrode.
Specifications
| Electrode | Range | MLD* | Interfering Ions | pH Range | Calibration Solution |
|---|---|---|---|---|---|
| Ammonium/Ammonia | 0-9,000mg/L | 1.0 ppm | Potassium, Sodium and Magnesium | 5 – 8 | AMM-CAL |
| Chloride | 0-20,000mg/L | 2.0 ppm | Bromide, Iodide, Cyanide and Sulphide | 2 – 11 | CHL-CAL |
| Fluoride | 0-1,000mg/L | 0.05 ppm | Hydroxide (OH-) | 4 – 8 | FLU-CAL |
| Nitrate | 0-30,000mg/L | 0.5 ppm | Chloride, Bromide, Fluoride, Sulphate, Chlorate and Perchlorate | 3 – 10 | NIT-CAL |
| Calcium | 0-2.000mg/L | 0.05 ppm | Magnesium, Barium, Lead, Zinc and Sodium | 4 – 9 | CAL-CAL |
* MLD (Minimum Level of Detection) is the minimum value the electrode is physically capable of measuring.
The ISE Sensors are designed to work with specific AquaProbe sensors and the correct probe should be matched to the correct probe. AP-2000, AP-5000 and AP-6000/AP-7000 probes are available on the pages linked below.
