[michellejam]

Hi guys
I work in a lab, have for 20+ years, in one capacity or another. I have always stirred my solutions while taking pH and have found over the years that others do not. When there are questions as to the outcome of a reading I always suggest that the solution be stirred, and generally the outcome is better. I am a biologist, and work with all sorts of others, including chemists. Even the chemists dont stir, and sometimes they ask me why I do. My lame answer is that I had learned to do it that way. I would like to know myself, why this makes a difference.


Can anyone help me out?
Thanks in advance,
Michelle

Thank you all so much for your input, now I dont know what to think!...such is the life!...but I will give you all snacks, because I see that I can, and I am not even sure what that means, but how can snacks be bad? But maybe it is a typeo should read smacks, like; up side the head....in which case I would never.

[Babcock_Hall]

While we await the true experts...I was taught that if I stirred the standards, I should stir the unknown.  But if the unknown could not be stirred, then the standards should not be.  The reason had to do with junction potentials, but that is as much as I remember.

[Borek]

I was taught to not stir

I remember seeing measurement result oscillating during stirring, no idea if it was some side effect of the changing magnetic field, or something else.

[Arkcon]

I was taught to stir, it was important to keep the solution over the junction in motion, to keep the solution inside the electrode in motion (that's also why the vent hole on the pH meter must be open,) so that the circuit is complete, and you get a valid reading.

Now stirring can be bad, in the electronics are loose and worn out, but at that point, you should get a new meter.  Although older meters may be more prone to wonkyness, modern ones are very robust. 

However, if Borek: isn't stirring, I may rethink the whole process.  Actually, I only use a vigorous magnetic stirrer when pH adjusting a buffer.  If I just want the pH of a sample, I just give it a quick swirl and expect the pH meter to give me an answer so promptly it doesn't really matter.

[Borek]

Don't treat what I wrote too seriously - I have not dealt with measurements for over 20 years. Also, the equipment we had was rather lousy, so could be what we were taught was partially dictated by its limitations.

[michellejam]

Just saw the way to reply..

Thank you all so much for your input, now I dont know what to think!...such is the life!...but I will give you all snacks, because I see that I can, and I am not even sure what that means, but how can snacks be bad? But maybe it is a typeo, and should read smacks, like; up side the head....in which case I would never.

[jeffmoonchop]

i think its good practise to make sure any fluid you measure is evenly distributed. Whatever is dissolved in there can sink to the bottom or top on standing. Ive always measured pH on a stirrer plate. Ive worked as an analyst and you can see how much shaking or stirring can affect data across samples. Try putting a shaken and a non-shaken sample through a HPLC and you'll see a big difference in peak area.

[Borek]

i think its good practise to make sure any fluid you measure is evenly distributed. Whatever is dissolved in there can sink to the bottom or top on standing.

It doesn't mean you have to keep stirring during measurement (unless you deal with non-homogeneous mixture, but that's a completely different situation).

That's not to say you should not, just that the conclusion that you have to doesn't follow.

[jjwinkle]

In the case of an acidic solution:

When you put the electrode into a solution, some of the solution's hydrogen ions actually leave it and go into the glass of the electrode (while metal ions move out of the glass into the solution). (A visual demonstration of this is about halfway down the screen at http://www.explainthatstuff.com/how-ph-meters-work.html.) This movement causes a change in free energy. The measurement of pH is actually a measurement of this change in free energy.  (https://en.wikipedia.org/wiki/Glass_electrode - near the bottom of the section on Galvanic cell schematic representation)

If the electrode doesn't continually access fresh solution, the hydrogen ions near the electrode become depleted. As a result, the movement of ions slows, hence change slows and becomes atypical of the solution. Stirring keeps the concentration of hydrogen ions characteristic of the solution available for ongoing movement and corresponding free energy change that is reflective of the solution.

In an alkaline solution hydrogen ions move OUT of the glass of the electrode (halfway down http://extension.psu.edu/food/entrepreneurs/equipment/what-is-ph-and-how-is-it-measured-1).

[Babcock_Hall]

An old copy of the Beckman handbook of applied electrochemistry suggests stirring both the standards and unknowns, or stirring neither.

[Borek]

An old copy of the Beckman handbook of applied electrochemistry suggests stirring both the standards and unknowns, or stirring neither.

Nice one. I guess being consistent is more important than choosing one approach over another.

[Borek]

If the electrode doesn't continually access fresh solution, the hydrogen ions near the electrode become depleted.

Have you tried to estimate changes in concentration? Or amount of H⁺ that has to be absorbed on the electrode to load it to an equilibrium potential? Measurement takes at least seconds, in this time ions will diffuse, H⁺ diffuse pretty fast. Are you sure changes won't be negligible? I have nothing against being shown I am wrong, but somehow I doubt.

[Babcock_Hall]

An old copy of the Beckman handbook of applied electrochemistry suggests stirring both the standards and unknowns, or stirring neither.

Nice one. I guess being consistent is more important than choosing one approach over another.

Their handbook discusses some of the disadvantages of stirring.  For example, when you stir, you should insulate the container from the stir plate.  They also mention that if you have an electrode with high resistance, there is the possibility of more noise and possible junction offset.  The handbook also says that faster response and a more homogeneous solution are advantages to stirring.