For a homeowner I would still recommend pH 7 and 4 for calibration. The swimming pool is slightly alakline being ideally between pH 7.2 ot pH 7.6. That is pretty close to the isopotential point of pH 7.
pH 10 is not a very stable buffer. The carbon dioxide in the atmosphere diffuses into the solution and will cause the formation of carbonic acid which will dissociate into carbonate and hydrogen ions resulting in a decrease in pH.
I actually had a water treatment plant that was reporting a whole pH unit discrepency from the field versus the lab. The people in the filed were storing the probe in groundwater which caused an offset error (>60 mV in pH 7) and slope was very low (85%). The thing was, the slope of the pH electrode was okay but the pH 10 buffer was actually pH 9.5 due to the 10 buffer being bad.
Once they cleaned the probe and calibrated in fresh buffers, the offset and slope were back to values as a new probe. Being a municipality they have to calibrate to pH 7 and 10 since their readings are greater than pH 7 (EPA standard). They now calibrate to three points and monitor the differences between the two slopes. There should be very minor difference between the two slope values so when they see a difference then they know to change the pH 10 buffer.
I cannot speak to other manufacturers but I know where I have worked we use an average slope based on calibration. If calibrating in pH 7 and 4 the line will extend to 10. It wasn't until the high end of the meters that the user has an option to use linear or segmented slope. With a segmented slope the calibration of pH 7 and 4 only impact that section. The slope between 7 and 10 is independent. If it has not been calibrated then segement will use adefault value of 100% or 59.16 mV/pH at 25 oC.
Lastly sodium error can be an issue but it happens at very high pH such as pH 12 or above. The high temperature glass formulation has lower sodium error than general purpose formulation used in standard pH electrodes. Attached is a chart that shows alkaline error of solutions that have 0.1M and 1 M Sodium.
