Vernier Tech Info Library TIL #665

Question

General tips for using Ion Selective Electrodes (ISE).

Answer

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 ISEs (available at www.vernier.com/probes/ise.html if you no longer have a copy), and in our "Water Quality with�" lab manuals.

1. You must calibrate ISEs very carefully. Follow the instruction in the probe booklet or the test procedures of "Water Quality with . . ." lab manual. 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 = 1.5 V, LOW = 2.3 V, (diff of 0.8 V, negative slope)
Ammonium HIGH = 2.2 V, LOW = 1.4 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 = 1.7 V, LOW = 2.5 V, (diff of 0.8 V, negative 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.

Pre-made standards from Vernier come in 500 mL bottles.

Item Vernier Order Code
NH4 Low 1 mg/L as N NH4-LST
NH4 High 100 mg/L as N NH4-HST
NO3 Low 1 mg/L as N NO3-LST
NO3 High 100 mg/L as N NO3-HST
Ca Low 10 mg/L Ca CA-LST
Ca High 1000 mg/L Ca CA-HST
Cl Low 10 mg/L Cl CL-LST
Cl High 1000 mg/L Cl CL-HST

3. 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.

4. Keep in mind that the ISE is actually measuring voltages and then converting them to concentrations. This conversion is logarithmic, so the difference between 1000 ppm and 10 ppm is actually less than 1% variation of voltage. (this was calculated this 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 volts. 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.

5. At some point, you will need to replace the membrane of the Nitrate, Calcium 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 NO3-MOD
Ammonium NH4-MOD
Calcium CA-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 (503) 277-2299 or e-mail info@vernier.com.

Created by: dholmquist on May 04 2000
Last updated by: jhopkins on May 15 2009