Proper calibration of a pH probe after immersion in seawater

[RelaxingWithTheReef]

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.

 

[TokenReefer]

Probably doesn't answer your question but the paper you linked simply says "Allow sufficient time for the electrode to respond, since salty samples tend to drift as equilibrium is established at the junction." I don't see where it says how much time however (maybe I missed it). Our probes tend to 'live' in our tanks and therfore the 'drift' may not be as big of a factor since it's not being removed? Sorry, I think I need more info on what they mean (or you or I don't know) by this. Hope I didn't waste your time. Interesting schtuff

 

[gbroadbridge]

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.

I think this is a little misleading.

pH probes generally need to sit in a buffer solution for calibration until the reading stabilises which could take several minutes or longer if they have been measuring a sample like aquarium water for a while.

This is different to a probe that has been soaking in a storage solution.

I don't see that this means the calibration is any less accurate provided sufficient time is given to stabilise.

 

[Randy Holmes-Farley]

I agree that one may have to wait a reasonably long time during calibration and use, and I don’t see a good way around it.

I recommend rinsing/soaking the probe in low salinity water after removing it from the tank before calibration.

 

[Dburr1014]

I always stir it in rodi before starting calibration, dry it on a clean paper towel. I then start doing cal and rinse and dry in between solutions.
I have also noticed with my apex, the first calibration isn't very accurate. So I always do it consecutively and the second time it is much better and more stable.

It also helps to clean it gently with a soft toothbrush before starting if it was in saltwater prior.

 

[RelaxingWithTheReef]

Success! I have recalibrated my probes. Here’s the story, what I did wrong, and the procedure I used to correct my mistakes.

Quick background - The purpose of this little science experiment was to replace an aging and fouled pH probe in the sump. It’s used by Apex to control a CO2 scrubber valve on the skimmer that provides pH support during the dark period. The old probe is still usable and serves as a reference. But it’s a couple years old, dirty, and currently reads 0.14 high.

I had this bright idea to add a second pH probe “pHb”. Once pHb was working properly, I would transfer control from the original “pHa” probe to the new “pHb” probe. This would provide seamless continuity for the tank. For anyone wondering if two Apex pH probes right next to each other interact, the answer is: I found no evidence they do. They both seem well behaved.

Neptune probes were sold out at the time so I purchased BRS #210794 Double Junction.

Mistake #1 was to put the probe into the sump before calibration. The uncalibrated reading was pretty good, within 0.04. I tried to calibrate a couple days later.

Mistake #2 was using the Apex “Confidence Meter” during calibration. After multiple calibrations with Pinpoint fluid the probe calibration was off something like 0.20! Without the benefit of a second reference probe, or an Oakton pH150, you would likely never realize the calibration was so far off.

Thinking the probe was defective, I purchased an Oakton EW-59001-70 Double Junction. Did the same thing, and had the same disastrous result. That’s when I posted this thread for help, and received some good information.

It appears that in some cases hours may be needed for a probe to reach a final state of equilibrium, and provide an accurate reading. It also appears this seawater equilibrium state needs to be completely reversed for the probe to calibrate properly. This can also takes hours to accomplish. If this state is not completely reversed, it looks like the calibration error can be significant.

The calibration procedure I used is as follows:

1. Soak probe in pH probe storage solution. I used Milwaukee MA9015. The SDS states the pH as 5-7, and I measure around 6.3. When I put a “seawater saturated” probe in this solution, the pH reading is initially around 8.1. Over many hours the probe will slowly drifted back to the correct reading of 6.3. I let the probe sit overnight just to make sure this process completes.
2. Rinse the probe with DI water, and calibrate. As posters mentioned, calibration is a slow process and takes at least several minutes to complete with each fluid. I believe the Apex “confidence meter” is a little over optimistic. When it reaches 100%, I find that waiting patiently before accepting the value will usually result in additional changes as the probe slowly drifts to the final value.
3. After calibration wash the probe, and put it in the seawater. Allow a couple hours for the probe to completely settle. Mistake #3, not allowing enough time for the probe to settle in the seawater.

Is the procedure an overkill? Probably. Is there another way to accomplish an accurate calibration? Absolutely! But this procedure worked for me, and restored both of the probes. System is now running great on the pHb Oakton probe, and never skipped a beat.

It would be educational to understand exactly why pH probes respond so slowly to seawater. I have not been able to find a good explanation.

 

[Randy Holmes-Farley]

It would be educational to understand exactly why pH probes respond so slowly to seawater. I have not been able to find a good explanation.

from the link below:

High ionic strength samples, such as seawater, brines, strong acids, strong bases, foods, and beverages also present challenges. We are faced here with two challenges:

1. Change in hydrogen ion activity - ion mobility decreases in the high ionic strength samples and the activity differs from the concentration. (Note: the pH electrode responds to hydrogen ion activity, not the concentration).

2. 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.

https://assets.thermofisher.com/TFS-Assets/LSG/Application-Notes/AN-PHCONSAMP-E%201014-RevA-WEB.pdf