Ion-Selective Electrodes require careful use. The following are some general tips to keep in mind that will supplement the information found in the probe booklet that comes with the ISE and in our “Water Quality with…” lab manuals.
Updated booklets for each ISE can be found on the respective webpages for your ISE (see https://www.vernier.com/product-category/?category=ion-selective-electrodes&page_num=1).

1. You must calibrate ISEs very carefully. See How do I calibrate my sensor?

If you have a LabQuest ISE probe with a cable and white, BTA end: Below are some typical voltages one might see when the ISE is in the High and Low standards. These values are approximate, and will vary from probe to probe.
Nitrate HIGH = 1.6 V, LOW = 2.4 V, (diff of 0.8 V, negative slope)
Ammonium HIGH = 2.1 V, LOW = 1.3 V (diff of 0.8 V, positive slope)
Calcium HIGH = 1.9 V, LOW = 1.5 V, (diff of 0.4 V, positive slope)
Chloride HIGH = 2.0 V, LOW = 2.8 V, (diff of 0.8 V, negative slope)
Potassium HIGH = 2.7 V, LOW = 1.9 V, (diff of 0.8 V, positive slope)

If you have a Go Direct ISE: Below are some typical voltages one might see when the ISE is in the High and Low standards. These values are approximate, and will vary from probe to probe. See How do I change the displayed measurement channel on my Go Direct Electrode Amplifier or ISE Amplifier?
Nitrate HIGH = 44 mV, LOW = 160 mV, (diff of 116 mV, negative slope)
Ammonium HIGH = 116 mV, LOW = 0 mV (diff of 116 mV, positive slope)
Calcium HIGH = 80 mV, LOW = 30 mV, (diff of 50 mV, positive slope)
Chloride HIGH = 114 mV, LOW = 230 mV, (diff of 116 mV, negative slope)
Potassium HIGH = 166 mV, LOW = 50 mV, (diff of 116 mV, positive slope)

2. Be certain that your standards are uncontaminated. With the High Standard 100 times more concentrated than the Low Standard, contamination is very easy to do. To keep your standards from being contaminated, be very thorough in your rinsing and gently blot dry prior to placing in any standard. You can purchase fresh standards from Vernier (see below) or make your own using the instructions found in the sensor booklet.

Ready-made standards from Vernier come in 500 mL bottles, and are available as follows:

NH₄ 1 mg/L Standard NH₄ ISE Solution (NH4-LST)
NH₄ 100 mg/L Standard NH₄ ISE Solution (NH4-HST)

NO₃ 1 mg/L Standard NO₃ ISE Solution (NO3-LST)
NO₃ 100 mg/L Standard NO₃ ISE Solution (NO3-HST)

Ca 10 mg/L Standard Ca ISE Solution (CA-LST)
Ca 1000 mg/L Standard Ca ISE Solution (CA-HST)

Cl 10 mg/L Standard Cl ISE Solution (CL-LST)
Cl 1000 mg/L Standard Cl ISE Solution (CL-HST)

K 10 mg/L Standard Potassium Solution (K-LST)
K 1000 mg/L Standard Potassium Solution (K-HST)

3. Make sure the reference junctions (two white dots on either side of the electrode) are submerged during calibration and while taking readings.

4. Another issue with ISEs is probe response time. Any sensor such as these will have certain amount of time it takes to totally stabilize. With ISEs, this can vary, but is generally reproducible on a particular probe. So if you chose a specific time at which to take your reading – say 45 seconds – you should always be hitting the response at the same point. The values might drift a little more, but because you calibrated at the 45 second mark and took your sample reading at the 45 second mark, your answer should be correct.

5. Keep in mind that the ISE is actually measuring potential and then converting it to concentration. This conversion is logarithmic, so the difference between 1000 ppm and 10 ppm is actually less than 1% variation of voltage. (This was calculated using the average of 1.900 V at 1000 ppm and 1.500 V at 10 ppm.) Any sensor will be noisy at the millivolt level. Unfortunately for ISEs, a few millivolts can translate into 100 ppm at the higher levels. Some other companies just have you read ISE values in potential. This makes them look nice, but you still have to perform the logarithmic conversion on them to get your concentration. We opted for what we think is the more user-friendly route of calculating them for you, even though it could give the appearance of a noisier, or “driftier”, sensor.

Another example that helps put this idea in perspective is what is really happening when you measure pH. The difference between pH 3.9 and pH 4.0 doesn’t seem too big. If your pH sensor was fluctuating between these, you probably wouldn’t think too much about it. But what is really being measured, the activity of the H+ ions is actually a 25% fluctuation. It’s just hidden because here the logarithmic relationship works in our favor. In ISEs, it works against us.

The Go Direct ISEs or an ISE-BNC connected to an EA-BTA can give you ion concentration in potential. These values are slightly more stable since there is no conversion here.

6. At some point, you will need to replace the membrane of the Nitrate, Calcium, Potassium, and Ammonium ISEs. Each of these has a PVC membrane with a limited life expectancy. When distinctly different voltages or voltage ranges are noticed during calibration, it is probably time to replace the membrane module/sensor. Depending upon use/care, these membranes will give accurate readings for up to 2 years. The Chloride ISE has a solid-state membrane with a longer membrane life (up to 5 years). This sensor does not have a replacement module. Replacement modules have order codes of:
Nitrate: Nitrate Replacement Module (NO3-MOD)
Ammonium Ammonium Replacement Module (NH4-MOD)
Calcium Calcium Replacement Module (CA-MOD)
Potasium Potassium Replacement Module (K-MOD)
Important: Replacement membranes degrade over time even they are not being used, so should not be ordered far in advance of the time they will be used.

If, after implementing all of these tips, you are still unable get your ISE to function properly, contact Vernier Tech Support at (888) 837-6437 or e-mail info@vernier.com.

Related ISE TILs:
How can I have my ISE read mV output instead of mg/L?
Can I use my third-party ISE with my Vernier equipment?
Do you sell ISE Standards?
Will I get interference when I put two sensors in the same solution?
Nitrate Ion-Selective Electrode Troubleshooting and FAQs
Chloride Ion-Selective Electrode Troubleshooting and FAQs
Calcium Ion-Selective Electrode Troubleshooting and FAQs
Ammonium Ion-Selective Electrode Troubleshooting and FAQs
ISEs connected to a Go Wireless Link are identified as a different sensor.
Does Vernier offer a sensor to test for lead in water?
Go Direct Ion Selective Electrode Amplifier Troubleshooting and FAQs
Go Direct Ammonium Ion-Selective Electrode Troubleshooting and FAQs
Go Direct Potassium Ion-Selective Electrode Troubleshooting and FAQs
Go Direct Nitrate Ion-Selective Electrode Troubleshooting and FAQs
Go Direct Calcium Ion-Selective Electrode Troubleshooting and FAQs
Go Direct Chloride Ion-Selective Electrode Troubleshooting and FAQs