NBS scale or total pH scale: What's the "right" pH?

Larry L · Mar 19, 2019

I recently learned about differences between measuring pH using typical hobby probes that use the NBS scale vs analytical measurements that use the total pH scale. Apparently (if I understand correctly) the probes we use in the hobby are affected by the high concentration of salt in the water, and measure pH as 0.2 to 0.3 units higher than what they would be on the total scale (as if there was no salt in the water).

Is that right? If so, should we be correcting for that when we make our pH measurements?

Are measurements made with pH test kits comparable to measurements made with probes?

Larry L · Mar 20, 2019

@JimWelsh any thoughts?

matthias.birkicht · Aug 2, 2020

Dear Larry,

there is a rule of thumb to convert the pH on NBS scale to total scale pH: pH(NBS) - 0.13 = pH(total).
Otherwise you need to determine temperature, Total alkalinity, Salinity, Silicate, Phosphate and calculate it with CO2sys or PyCO2sys.
It is not necessary for hobbyists...

best regards, Matthias

Randy Holmes-Farley · Aug 2, 2020

I agree with Matthias.

The scale used is determined by the buffers chosen. Since nearly everyone and everything in the hobby uses the NBS scale based on the buffers used for pH calibration, I think for any normal hobbyist, that is adequate, regardless of whether it is technically optimal or not.

If you are an exacting chemical scientist who cares about details like the activity of hydrogen ions vs concentrations of hydrogen ions, understanding the differences is certainly important.

THe introduction tot he document does a good job of explaining the technical differences and issues involved int eh several ph scales used:

Document Display | NEPIS | US EPA

nepis.epa.gov

"There are four pH scales that are commonly used in seawater pH measurement, and the lack of a consistently agreed upon scale can make interpretation between studies difficult"

Randy Holmes-Farley · Aug 2, 2020

Larry L said:
Are measurements made with pH test kits comparable to measurements made with probes?

That's a more complicated question. In short, they could be, but that doesn't mean they are. If they are made for freshwater, they may well be off, and I'm not a fan of kits anyway since it is difficult to actually check or calibrate them.

Larry L · Aug 2, 2020

Randy Holmes-Farley said:
That's a more complicated question. In short, they could be, but that doesn't mean they are. If they are made for freshwater, they may well be off, and I'm not a fan of kits anyway since it is difficult to actually check or calibrate them.

Just for some background, I posed the original question while I was alpha/beta testing a Mindstream monitor, and was seeing the pH it reported being consistently lower than my Apex pH probes. I was told it was because the Mindstream was reporting total pH while the probes were NBS, so the rule of thumb suggested by @matthias.birkicht matched pretty well to the difference I was seeing. At the time I was mostly curious if the readings from the monitor would track closer to test kits than probes.