It appears there are some challenges when using a pH probe to measure the pH of seawater. Seawater can be classified as having High Ionic Strength, and Thermo Fisher even published an app note on the subject.
https://assets.thermofisher.com/TFS-Assets/LSG/Application-Notes/AN-PHCONSAMP-E 1014-RevA-WEB.pdf
The article makes the statement, “High ionic strength solutions change the liquid junction potential. This may lead to bias and considerable time may be required to establish a stable reading.”
The difficulty I am having is when a pH probe is first put in seawater, it takes some time for it to stop drifting and settle to the final value as Thermo Fisher points out. But after it settles, and you put the probe into calibration solutions, the probe appears to be out of calibration. If you put the probe back into a storage solution for a period of time, it appears to very slowly drift back into calibration. It would appear that the seawater induces a medium term voltage drift or bias into the probe. If you pull the probe out of seawater and immediately recalibrate, it appears the calibration is way off due to this induced bias.
@Randy Holmes-Farley or other guru’s of pH probe wisdom Is this observation accurate? Is there a suggested procedure to condition the probe to remove the bias prior to recalibration? At some point does the bias become permanent if the probe as been in seawater for a long time? Is recalibration of an older probe something that should not even be attempted because it could result in a more inaccurate reading?
I ask that we keep the thread focused on the proper use and procedures related to pH probes, and not drift into a discussion of why calibration or a given accuracy of the pH measurement is important in reefkeeping.
When I read previous discussions, I think this subject is causing issues and confusion with many reefers. Lets all try to understand our pH probes a little better.
https://assets.thermofisher.com/TFS-Assets/LSG/Application-Notes/AN-PHCONSAMP-E 1014-RevA-WEB.pdf
The article makes the statement, “High ionic strength solutions change the liquid junction potential. This may lead to bias and considerable time may be required to establish a stable reading.”
The difficulty I am having is when a pH probe is first put in seawater, it takes some time for it to stop drifting and settle to the final value as Thermo Fisher points out. But after it settles, and you put the probe into calibration solutions, the probe appears to be out of calibration. If you put the probe back into a storage solution for a period of time, it appears to very slowly drift back into calibration. It would appear that the seawater induces a medium term voltage drift or bias into the probe. If you pull the probe out of seawater and immediately recalibrate, it appears the calibration is way off due to this induced bias.
@Randy Holmes-Farley or other guru’s of pH probe wisdom Is this observation accurate? Is there a suggested procedure to condition the probe to remove the bias prior to recalibration? At some point does the bias become permanent if the probe as been in seawater for a long time? Is recalibration of an older probe something that should not even be attempted because it could result in a more inaccurate reading?
I ask that we keep the thread focused on the proper use and procedures related to pH probes, and not drift into a discussion of why calibration or a given accuracy of the pH measurement is important in reefkeeping.
When I read previous discussions, I think this subject is causing issues and confusion with many reefers. Lets all try to understand our pH probes a little better.