what is "apparent Ph value"

[ym3142]

pH measurement has been discussed here extensively. My understanding is that we are not supposed to measure pH of a solution with organic. Then I found in the USP
"For these reasons, the values so obtained with solutions that are only partially aquesous in charater(meaning to contain so organic) can be regarded only as apparent pH values."

So what is the meaning of "apparent pH value"? Is the measurement of organic aqueous still a useful practice?
Thanks for your input.

[Mark Tracy]

This topic has been discussed at length before. Look in the history.

To make a long story short, the familiar 0-14 scale gets rather bent when you mix in organic solvents. The pKa values all shift around. That means that the same buffer diluted in 10% MeOH has a different apparent pH than in 25% MeCN. As long as the water/organic composition is defined, you can use these apparent pH values. There are a few tricks to getting better measurements, but nothing really critical.

[Uwe Neue]

Let me get on my horse again...

The absolutely best way to measure the pH of the mobile phase is to measure the pH of the aqueous component of your mobile phase.
Reasons:
1. You know the pH scale.
2. You know the pKa's of your buffers.
3. Knowing both, you can adjust the pH of your buffer to create buffers with good buffering capability (good buffer capacity).

The quality of the buffer, i.e. it's buffer capacity, does not change in the presence of an organic solvent (at equal buffer concentrations). This is what one wants.

Once I throw in an organic solvent, the pH scale changes, and I have no clue where the pKa of my buffer is. This makes the selection of high-quality buffers with good buffering capability difficult.

Occasionally, it is not possible to measure the pH in 100% water due to solubility issues with the base used. You can still make good buffers, by weighing out the buffer components.

Measuring the pH in the presence of an organic solvent is good for the theoreticians (like me), but not for practical chromatography.

OK, getting off my horse again...

[Mark Tracy]

The practical problem is that we define the buffer in the aqueous system because it is familiar and well documented, then we use it in a semi-aqueous system because that is where the chromatography happens. Fortunately, buffers are usually forgiving, that is why we like them.

[Uwe Neue]

Rereading my last post, I realized that I may not have answered the original question properly.

There is nothing like an "apparent" pH value!

Let me restate first, what I said above: Unless there is a good reason to do otherwise, pH values should be measured in water. This is simple, and everybody understands this.

If for whatever reason you need to measure a pH value in the presence of an organic solvent, this can be done, provided you follow correct protocols. There is nothing "apparent" in such a pH value. If properly measured, with properly calibrated equipment, the pH values measured in the presence of an organic solvent are as real as the pH values measured in water.

What this value means with respect to the question if you have created a good buffer or not, this question can be answered only if you know the pKa of your buffer at this concentration of the organic modifier that you are using. If you know the pKa of your buffer at this organic composition, and you measure your pH with a properly calibrated pH meter within say about 1 pH unit away from this pKa, you have a good buffer.

[ym3142]

Uwe and Mark,
Thanks for taking time to explaining the old question again and again. Nice weekend,

[james little]

Bill Tindall wrote a bunch about this topic, see links in LC/GC at:

http://www.lcresources.com/wiki/index.p ... Q:PHAdjust

[HW Mueller]

For transferring purposes (publishing, etc..) it would be extremely helpful if everybody reported the pH of the purely aqueous part, that is before the mobile phase is completed by addition of organic modifyers. After that it is left to anybody whether he wants to report the phase of the moon during pH determination etc....
Now the pH is related to the chemical potential of H+. That reminds me of trying to understand Debye-Hückel, which if memory doesn´t fail, doesn´t even dare to go into nonaqu. solvents. In short, I am still extremely weary about explanations on what some measurements of pH mean.

Again: Can´t we agree to report the pH in the aqu. part, giving the exact concentrations of other salts (some buffer prep methods don´t allow one to know that)? Any other measurements are fine if it keeps the measuring person happy (this is not talking about studies of pH itself).

[tom jupille]

Hans, you are quite right. Consistency is more important than correctness. The catch is that it's easier to be consistent if you do things correctly.

A good analogy is a quote attributed to the American author Mark Twain: "Always tell the truth; it makes it much easier to keep your story straight."

[HW Mueller]

Sorry Tom, garbled it once more, thought I was rooting for consistency + correctness.

[tom jupille]

Hans, nothing to apologize for for. You are right, but I just wanted to emphasize the point.