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A few comments about this that may be tangentially helpful for those that aren't scientists/engineers (those that are scientists will likely roll their eyes and say "yeah, dude, we know that!"):
While the definition of pH solely depends on solution chemistry and the concentration of the hydronium ion, actually assaying for pH has a lot more complexity than that. In particular, typical pH meters depend on the electrical potential setup between a reference electrode solution (often a reference electrode gel in the case of reef tank pH probes) and the test sample that is highly dependent on ion flow across a crushed-glass junction. Chemical pH assays (such as Salifert's) depend on the color change of a reference dye, which, while obviously related to pH, is a very different detection method than an electronic pH probe.
There are multiple conclusions from the above mechanistic description. The first is that pH probes are electrical devices that are subject to all of such device's potential pitfalls. The electrical potential I mentioned is quite small - in the millivolt range. That means that a LOT of details can have an effect on their accuracy. Some of those details are whether or not the crushed glass junction is plugged (which inhibits the ion flow), whether the probe is in a reasonable flow environment (in laboratories, one is often directed to stir the sample as the pH is determined), whether the reference solution/gel has been contaminated, and one of the bigger issues - whether the electrical connection between the probe's BNC connection and the actual meter is pristine, tight, and has little to no surface corrosion.
To obtain a reliable pH measurement by use of an electronic meter/pH probe, considerations of the condition of the pH probe, junction, electrical connection and pH measurement environment are important in addition to the pH calibration procedure itself. In particular, I'd advise a reefer using a continuous pH measurement device to examine the BNC junction connection on both the probe and the controller once every 3 months and ensure there's no visible corrosion. I'd also advise someone setting one of these devices up to pick a spot in their sump/tank with reasonably high flow rates past the probe - relatively stagnant locations may result in quite large pH swings.
With respect to the pH calibration procedure, there are several important factors. The first is one already discussed in this thread, which is temperature compensation. If one were to read the table of pH value versus temperature for a typical pH 7 and pH 10 standard solution, you will notice that the pH 7 standard's pH value changes very little with a few dozen degrees temperature difference, while the pH 10 standard's pH value changes quite a lot over that same temp range. This is one reason that equilibrating the pH standards to the temperature of the environment that you'll be calibrating the meter in, and turning the temperature compensation "on" in the device is important. Another factor associated with pH calibration and pH standard solutions is CO2 dissolution - this is likely why the Neptune folks advised not to allow the probe to set in the pH calibration solution for lengthy periods while waiting for the millivolt values to settle. Particularly when using small-volume, pouched pH standards, the pH value of the standard could conceivably change quite a bit from adsorption of CO2 from the air.
While the definition of pH solely depends on solution chemistry and the concentration of the hydronium ion, actually assaying for pH has a lot more complexity than that. In particular, typical pH meters depend on the electrical potential setup between a reference electrode solution (often a reference electrode gel in the case of reef tank pH probes) and the test sample that is highly dependent on ion flow across a crushed-glass junction. Chemical pH assays (such as Salifert's) depend on the color change of a reference dye, which, while obviously related to pH, is a very different detection method than an electronic pH probe.
There are multiple conclusions from the above mechanistic description. The first is that pH probes are electrical devices that are subject to all of such device's potential pitfalls. The electrical potential I mentioned is quite small - in the millivolt range. That means that a LOT of details can have an effect on their accuracy. Some of those details are whether or not the crushed glass junction is plugged (which inhibits the ion flow), whether the probe is in a reasonable flow environment (in laboratories, one is often directed to stir the sample as the pH is determined), whether the reference solution/gel has been contaminated, and one of the bigger issues - whether the electrical connection between the probe's BNC connection and the actual meter is pristine, tight, and has little to no surface corrosion.
To obtain a reliable pH measurement by use of an electronic meter/pH probe, considerations of the condition of the pH probe, junction, electrical connection and pH measurement environment are important in addition to the pH calibration procedure itself. In particular, I'd advise a reefer using a continuous pH measurement device to examine the BNC junction connection on both the probe and the controller once every 3 months and ensure there's no visible corrosion. I'd also advise someone setting one of these devices up to pick a spot in their sump/tank with reasonably high flow rates past the probe - relatively stagnant locations may result in quite large pH swings.
With respect to the pH calibration procedure, there are several important factors. The first is one already discussed in this thread, which is temperature compensation. If one were to read the table of pH value versus temperature for a typical pH 7 and pH 10 standard solution, you will notice that the pH 7 standard's pH value changes very little with a few dozen degrees temperature difference, while the pH 10 standard's pH value changes quite a lot over that same temp range. This is one reason that equilibrating the pH standards to the temperature of the environment that you'll be calibrating the meter in, and turning the temperature compensation "on" in the device is important. Another factor associated with pH calibration and pH standard solutions is CO2 dissolution - this is likely why the Neptune folks advised not to allow the probe to set in the pH calibration solution for lengthy periods while waiting for the millivolt values to settle. Particularly when using small-volume, pouched pH standards, the pH value of the standard could conceivably change quite a bit from adsorption of CO2 from the air.