How is Ammonia ISE filling solution reset and why is it important?

Document ID

Document ID TE8366

Version

Version 2.0

Status

Status Published

Published Date

Published Date 07/26/2018
Question
How is Ammonia ISE filling solution reset and why is it important?
Summary
Resetting filling solution for Ammonia ISE.
Answer
These probes function when volatile Ammonia passes through the gas permeable membrane and into the filling solution, which changes the pH of the filling solution. The change in pH is measured by the electrode and then converted to Ammonia concentration through calculation using the calibration curve.

When going from one sample/standard to the next some residual Ammonia from the previous sample(s) will be left in the filling solution which leads to positive bias in later tests. This positive bias is negligible when going from lower concentrations to higher concentrations, which is why it's so important that the standards are read in order of lowest concentration to higher concentration during calibration (in addition to going from low to high is best practice for all applications and test to limit the impact of cross contamination).

When going from higher concentration samples to lower concentration samples it would be recommended to reset the filling solution to prevent bias from the higher concentration samples. It's also recommended to do so after calibration before sampling to prevent bias from the highest standard of the calibration, and always reset before running a check standard.

Not doing so is most commonly recognized by check standards reading high after using the probe to test samples.

Resetting the filling solution is done by simply letting the probe set in 1000 mg/L NH3-N standard without ISA added for about 30 seconds to 1 minute. Doing this removes/limits the amount of residual Ammonia in the filling solution.

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