To stir or not to stir when taking a pH

[r6180]

Hello,

My lab runs a very sensitive ion exchange assay. The mobile phase needs to 6.50. I have found that if you ph the solution while it is stirring (which is the way our analysts currently pH) the reading is consistently .03 pH units lower than if the solution is sitting still. The reason our analysts stir the solution is that the pH is adjusted with 1N NaOH and it is stirred to mix that into the reagent to adjust the pH.

Any input would be appreciated. Thanks.

[tom jupille]

Obvious first question is "what are you using as a buffer?". First suggestion is to simply define the buffer by weight (you can weigh more precisely than you can measure pH).

[HW Mueller]

Also, it has been pointed out many times that one will likely provoke problems when placing an electrode into the buffer that is to be used in LC. One should take out an aliquot into a test tube and determine the pH there. If you search the forum you may find extensive discussions on buffer problems.

[Markus Laeubli, Metrohm]

Dear r6180

First of all: congratulations to you pH measurements. A difference of only 0.03 pH units is very good. The reason for such changes are the diffusion potentials which change in stirred or unstirred solutions.
Wich is the correct value? I do not know.
But just go the easy way. You do need pH = 6.50 (unstirred) just adjust the eluent to pH = 6.53 (stirred).

[Rafael Chust]

I fully agree with Tom and Hans.

Anyway, in HPLC eluents, is much more important the ionic strenght than the actual pH itself... and, of course, different pH meters would generate different measurements...

[Paulhurleyuk]

For HPLC frankly, I would expect to have more trouble from mobile phase contamination as a result of putting your pH probe in your mobile phase than I would from a 0.03 pH unit difference.

Buffers and cleaning solutions from the pH probe will contaminate the mobile phase, and especially with gradient elution can cause big problems.

Best way, work out exactly how much 1N NaOH you have to add to a batch of mobile phase to get the right pH, either by experimentation or by an online mobile phase calculator, and then always make up your mobile phase by Mass / volume. Then you don't have to worry about reading the pH of a stirred solution (which is problematic, as it takes quite a while to fully mix a large volume of solution)

Paul.

(just my $0.02)

[HW Mueller]

Rafael, your first statement is great, thanks, your second one less so. Ionic strength is more important if pH doesn´t play a role. For acids or bases the pH can be of enormous influence especially near the pKa....

[Rafael Chust]

I eventually did not express myself correctly when I refer ionic strenght (problems of dealing with foreign languages...).

What I wanted to state is that pH means a content of ionic species in equilibrium (read buffers) so, what is important is the actual "number" of molecules around more than the pH value itself.

It is easy to set a given pH by adding a given amount of salt in a given volume of water...

Did I make myself clear now?

[HW Mueller]

If you mean with this that pH is not so important as long as the H+ activity is right than you are on the right track (Got this from Henry Ford, freely: You can have any color of car as long as it´s black).

[Rafael Chust]

Yeah, you certainly get my drift! Precise and concise!