pH probe calibration questions (temperature and biofilm effects)

[drolmaeye]

Regarding ph probe calibration:

Q1. Does temperature really make that much of a difference for calibration?

Most of the guides/instructions I read for probe calibration stress the importance of bringing the calibration solution up to the appropriate temperature (usually 25 C, which corresponds nicely to our approximate reef temperatures), but I recently took a peak at correction charts and it looks like the a variation of +/- 5 C (or ~10 F) only changes the pH (for a pH of 7.00@25C) by ~0.01/0.02. Are there additional temperature considerations? If not, it does not seem like it is critical to have the calibration solution precisely at 25 C.

Q2. Does a biofilm over the probe tip have a significant affect on the measured pH?

I have read (anecdotally) that it is good practice to calibrate (or recalibrate) the probe after it has been submerged in tank water for a week or so. The reasoning is that a biofilm forms over the probe tip and it is desirable to calibrate under these normal-use conditions (as opposed to when the probe is new).

Any insight on either question is much appreciated.

 

[Randy Holmes-Farley]

The other factor is the actual pH of the buffer itself, which will change with temperature. The degree of change depends on the chemistry, and the best products show the temp effects right on the bottle.

I discuss such issues here:

Aquarium Chemistry: Measuring pH with a Meter

This article describes how pH meters work, how to select one to use, and how to best use it to measure the pH of a reef aquarium.

reefs.com

From it:

Buffers will often have such pH values as a function of temperature printed on the bottle. For example, a standard phosphate buffer has a pH of 7.000 at 25 ºC, but 7.04 at 15 ºC (a small difference). At the same time, a carbonate buffer with a pH of 10.01 at 25 ºC has a pH of 10.12 at 15 ºC (a larger difference).

 

[Randy Holmes-Farley]

Regarding ph probe calibration:

Q1. Does temperature really make that much of a difference for calibration?

Most of the guides/instructions I read for probe calibration stress the importance of bringing the calibration solution up to the appropriate temperature (usually 25 C, which corresponds nicely to our approximate reef temperatures), but I recently took a peak at correction charts and it looks like the a variation of +/- 5 C (or ~10 F) only changes the pH (for a pH of 7.00@25C) by ~0.01/0.02. Are there additional temperature considerations? If not, it does not seem like it is critical to have the calibration solution precisely at 25 C.

Q2. Does a biofilm over the probe tip have a significant affect on the measured pH?

I have read (anecdotally) that it is good practice to calibrate (or recalibrate) the probe after it has been submerged in tank water for a week or so. The reasoning is that a biofilm forms over the probe tip and it is desirable to calibrate under these normal-use conditions (as opposed to when the probe is new).

Any insight on either question is much appreciated.

A biofilm has the potential to have substantial effects. Algae that is photosynthesizing, for example, will locally raise the pH.

Likewise, bacteria that are metabolizing organics have the potential to lower it.