Why are pH meter calibration buffers so dilute?

Rudeofus · Jan 9, 2017

After researching buffers for calibrating my pH meter, and scouring through various MSDS, I was left wondering, why commercially sold pH buffers for pH meter calibration are so dilute. Is it just product economy, or is there some advantage to a buffer with 0.5% Borate versus one with 5% ?

Photo Engineer · Jan 9, 2017

They are so dilute to offset the salt effects of concentrated solutions. As concentration goes up, pH measurements deviate from the ideal, and thus the pH of a photographic solution is only an approximation! There are correction charts.

PE

Rudeofus · Jan 10, 2017

So the reasoning appears to go as this, please correct me if this is wrong:

dilute liquids are not as well buffered, but give identical readings with properly calibrated pH meters
concentrated liquids will give different readings between properly calibrated pH meters, and are therefore not good as generic calibration standard
concentrated liquids could, however, be used to calibrate one specific pH meter, and would likely do very well there due to its high buffer strength, but would likely miscalibrate other pH meters.

Martin Rickards · Jan 10, 2017

It's also to avoid the results being affected by the possible common ion effect https://en.wikipedia.org/wiki/Common-ion_effect#Buffering_effect

sfaber17 · Jan 10, 2017

I'm not sure how much different meters would vary on high ionic strength buffers, but it is probably a factor.
I do know that many commercial buffers are made a bit higher in concentration than the National Bureau of Standards buffers to get better buffer capacity. Dilute gives more ideal behavior - low activity coefficients, check out Debye-Huckel theory, so I figure that is why NBS tends to use dilute solutions for accuracy. Dissolved CO2 is a big source of error with pH 10 or even pH 7 buffers.

Photo Engineer · Jan 10, 2017

The deviation is a constant between meters, but differs with some electrodes. It is an ionic effect, not a meter or (constant) electrode effect.

PE

Rudeofus · Jan 10, 2017

So if I don't change electrodes too often, and can calibrate both my meter and any buffer I make with a set of commercial buffer solutions, a highly concentrated buffer would work well for me, yes?

Photo Engineer · Jan 10, 2017

Just don't worry about it. It all works out. I did this for years ignoring the problem and all is ok because the difference is so small up to pH of about 11. Same thing down to about pH 3. You are making a mountain out of a molehill.

PE

Gerald C Koch · Jan 12, 2017

It may also be to prevent damage to the electrode some of which are quite delicate.

Photo Engineer · Jan 12, 2017

I've been told that electrodes recover when soaked in DW and then buffer. That is what we did and what I do.

PE

Why are pH meter calibration buffers so dilute?

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