FAQ on Ion Selective Electrodes
Calibration
Recalibrate at least once a week, or daily if in doubt of results being accurate or reproducible.
Shelf-Life
The stock standard will last at least 12 months before discarding, whereas diluted standards treated with ISA/pH buffer should be prepared weekly.
Storage
It is best to store them dry when they are not to be used in the more than 2 weeks or so. Empty out the filling solution in the gas-sensing electrode. For shorter periods, store in dilute standard approximating the sample concentration, and ISA/pH buffer added when required.
Temperature Compensation
It is possible, but somewhat difficult or unnecessary if used in ambient temperature. First, you have to know the isopotential point for a given electrode system. Second, the concentration of the sample has to be similar in concentration to the isopotential point or else the temperature correction will be very inaccurate. Third, the temperature of the sample can not exceed the operational temperature range of the ISE. Fourth, very little isopotential point data for ISE's is available at this time. It is best to standardize and measure samples at the same temperature without using temperature compensation.
In-line Continuous ISE Measurement
Yes, direct measurement is possible in many cases without need for ISA/buffer addition to the sample stream. However, Fluoride, Sulfide, Ammonia, and Sodium Electrodes do require pH adjustment and must have ISA/ buffer added to the sample stream if certain pH range is being met.
Standards Of ISE
The most obvious choice will be determined by what concentration units are desired (e.g. ppm or Molar). Also, if an electrode is being used to measure another ion (e.g. sulfate with a lead electrode), use a stock standard of the ion to be measured (e.g. sulfate).
Combination vs Half-cell ISE
There is more than one advantage: no external reference electrode needed, more economical than the price of both, and one less electrode to fit in the process. Unless half-cell ISEs is used for measuring certain organic samples.
Cleaning & Maintenance
Polymer membrane ISE - Treat with distilled water, followed by soaking in standard.
Polymer membrane ISE include Ammonia, Ammonium, Calcium, Carbon dioxide, Fluoroborate, Nitrate, Nitrogen oxide, Perchlorate, Potassium, Sodium, Surfatant, and Water Hardnes.
Solid State ISE - Polish membrane with polishing strips, followed by soaking in dilute ISA solution
Solid State ISE include Bromide, Cadmium, Chloride, Copper, Cyanide, Fluoride, Iodide, Lead, and Silver/Sulfide
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