How can measuring mV in a pH buffer be used to troubleshoot an Ammonia Ion Selective Electrode (ISE)?

Document ID

Document ID TE9268

Published Date

Published Date 10/04/2019
Question
How can measuring mV in a pH buffer be used to troubleshoot an Ammonia Ion Selective Electrode (ISE)?
Summary
A troubleshooting step to verify if an issue is caused by the probe itself or if additional troubleshooting is necessary.
Answer
The Ammonia ISEs work by volatile Ammonia from the sample passing through the gas permeable membrane to enter the filling solution inside. Once inside the filling solution this ammonia causes the pH to change. This change in pH is measured potentiometrically using the internal sensor, which is then converted to a mg/L result using the calibration curve.

Because this internal sensor is just a pH sensor, the sensor can be checked by collecting mV measurements on pH buffers just like any other pH probe. Below is a simple procedure for doing this.
  1.  Remove the membrane from the electrode.
  2.  Rinse the internal sensor with clean DI water.
  3.  Fully submerge the internal sensor in a pH buffer (note that the entire glass section that would normally be encased by the membrane would need to be submerged).
  4.  Take a reading, ignoring the mg/L value, and record the mV result.
  5.  Repeat steps 2-4 for 4, 7, and 10 pH buffers.
  6.  Compare results to what would be expected. See also: How can mV results for pH be used for troubleshooting?

Assuming the pH buffers are in good condition, if the probe does not provide what would be expected in terms of mV difference between buffers or if the buffers are more than +/-30 mV from the expected values, then it is confirmed that the probe is faulty. If within warranty (See also: What is the warranty period for the IntelliCAL probes?) please contact Hach Tech Support. If outside of warranty, then the probe would need to be replaced.

 If the probe provides what would be expected in terms of mV difference between buffers and buffers being within +/-30 mV of the expected values, then any issues the probe may be having are not caused by the probe itself, and if within warranty a claim for the probe would be declined. Additional troubleshooting steps would be required to help Identify what the cause of the issue could be, and this additional troubleshooting can be done with Hach Tech Support. Often the first step would be to replace the filling solution and membrane followed by additional conditioning. This would be in addition to verifying sample and standard preparations as well as calibration and measurement procedures from the User Manuals.

 Additional articles useful to Ammonia ISE troubleshooting:
  What could be causing erratic mV readings using the IntelliCAL Ammonia ISENH3181 probe?
  How often should I replace an Ammonia ISE's membrane module?
  What volume of filling solution should be dispensed into the ISENH3181 membrane module when replacing the membrane?
  How is Ammonia ISE filling solution reset and why is it important?