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General Tips for Using Ion-Selective Electrodes

Vernier Ion-Selective Electrodes

Vernier offers five Ion-Selective Electrodes (ISEs): Ammonium (NH4+), Potassium (K+), Calcium (Ca2+), Nitrate (NO3), and Chloride (Cl). ISEs require careful use; follow these guidelines for best results:

  • Calibrate ISEs very carefully and often. Two calibration standards ship with each ISE. It is important to soak the electrode in the High Standard solution for at least 30 minutes prior to calibration. If, at any time, the reported reading is a constant 1.0 mg/L (and the electrode is not in a 1.0 mg/L solution), you need to recalibrate.

  • Be certain that your standards are uncontaminated. Because the High Standard is 100 times more concentrated than the Low Standard, it is very easy to cross-contaminate the solutions. To keep your standards from being contaminated, thoroughly rinse and blot dry the sensor tip prior to placing it in a standard. You can purchase fresh standards from Vernier or make your own using the instructions found in the sensor booklet.

  • If you know the approximate concentrations of your samples, you will save time if you analyze them from lowest concentration to highest. If there is great variation in concentration from one sample to the next, it can take several minutes for the reading to stabilize.

  • Choose a specific time at which to calibrate and take your reading for each sample. For example, if you calibrate when the ISE has been in a solution for 60 seconds, you should also collect data for your samples after the sensor has been in the sample for 60 seconds.

  • The ISE actually measures voltages and then converts the voltage values to concentration values. This conversion is logarithmic, so keep in mind that a small voltage change can cause a large variation in the concentration reading.

  • Replace the membranes of the Nitrate, Calcium, Potassium, and Ammonium ISEs as needed. Each of these ISEs has a PVC membrane with a limited life expectancy; depending upon the amount of use and how well they were taken care of, the membranes should give good readings for 1–2 years. If you notice distinctly different voltages or slowed response during calibration, it is probably time to replace the membrane module. The replacement modules have a limited shelf-life, so they should be purchased immediately prior to use.

If you have additional questions, email us at chemistry@vernier.com


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