For an article I am preparing I would like to catalog the ways that buffers are prepared by the chromatography community.

So, given the following mobile phase, how would you prepare it? If you think there is information not provided, make your best assumption in describing the preparation.

Mobile phase: 50/50 water-methanol 0.01M pH 4.6 acetate buffer

If after a day or two you don't see a way described that you or a coleague uses, please add it.

Thankyou for your help.

There have been some good discussions on this. Quickly: Prepare 0.01M NaOAC and HOAc solutions, mix them until you get your desired pH solution. Take the needed volume of this and add the same volume of MeOH... Report exactly what you have done.

Now I am not going to spoil a chance for the weighing people, Bill.

Hans

If you have a multichannel pumping system, you might make the buffer as described and set your system to 50%A (MeOH) and 50%B (buffer). We use NH4OAC.

George,
From your information, I would take this to be 0.01M in acetate in the final solution which would mean the aqueous portion should be 0.02M initially. This would be prepared by using ammonium acetate and adjusting the pH with either ammonium hydroxide or acetic acid to the required pH. Equal volumes of the aqueous phase and MeOH are then mixed and filtered before use.
Just my two cents worth ;->

Regards,
Mark

George,

I think many chromatographers use different methods of preparing buffers depending on what they are trying to accomplish. I often titrate buffers to adjust pH and use online mixing during method development. Then switch to more scalable methods for validation and routine use. Test methods that I'm writing now normally use a gravimetric prep. My reasoning is here.

Mark pointed out a potential flaw in the way you have written the prescription. I recommend to write it as follows: 50/50 0.01 M sodium (or ammonium or something) acetate buffer / methanol. This would state that you make it at 0.01 M in water and mix the methanol to it afterwards. In addition, you also need to say whether you use sodium or ammonium ions. You could make the owner of a MS system very unhappy, if you throw potassium acetate into it.

Of course, one can do as one pleases as long as it is described unequivocably. However, to prepare a mobile phase as in Mark´s example would bend the definition of molarity. Adding an equal amount of MeOH to a 0.02M buffer does not give a 0.01M final mobile phase. (The final volume is not doubled)
Tom, you also check the pH! Did you ever have to reject a buffer preparation on basis of weighing out contaminated (water, CO2 ...)reagents?

HW,
I have been spending all spare time shearing the sheep and almost missed this one. Anyway, Tom stole my thunder. I would prepare it his way- fast, precise, scaleable. I like Uwe's recommendation for describing buffer, concise and unambiguous.

I would not check the pH of a buffer solution prepared by weights and volumes any more than I would independently check the weight of a standard I prepared by weights and volumes.

Hans and Bill,

I can't imagine in a pharmaceutical company getting a procedure approved that required a specific pH and not checking it. I understand Bill's point but I could never make the case here. Often we even prepare check standards.

We have never had to reject a buffer due to contaminated reagents but we have had analysts select the wrong reagent a couple of times. Mostly the trihydrate instead of the monohydrate, that kind of stuff. I guess previously we would have just titrated it and never known the difference - the change in ionic strength probably would not have impacted the chromatography, at least we never noticed a case that we traced back to the wrong form of the buffer salt.

It was Bill's previous postings on the subject that got us to rethink our method of preparing buffers.

Bill, your sheep will be getting a cold! (Or are you down south?)
Anyway, thanks that you gave a new subject on which to argue: Independently checking standards. Strange that most of us (me included) don´t check a standard´s UV spec, etc. Of course, I always particularily check the peak area of a new ST. Also, one reason for this calculating the area from absorption coefficients (that other chain) is to roughly check on the weighing. (Probably, checking via UV ... is generally not done, because of the extensive linear relationship in contrast to the strongly nonlinear pH behavior)

Now dead seriously: A reexamination of Beynon´s buffer page rekindled a doubt on whether we all handle activities correctly. I am now corresponding with him, but currently assuming that his "apparent pH" are representing activities. A pH meter reads activities, weighing gives concentrations, in short one has to report even details on how the buffer itself was prepared! Any thoughts?

Tom - thanks for the link there was a lot of good information in that presentation. You should post it in another thread - I think a lot of people will miss it here. Seems like common sense, but much more thought out than the procedures we use here.

HW,

In this civilized climate both the sheep and the shepherd were comfortable today, and best of all we are done with minimal wounds.

What is "Beynon's buffer page"? I don't understand your last question. Making a guess at the question there follows a response.......

In some very special cases the meter reads activity, but more generally it reads what it reads and that is defined as pH. Recall the internationally recognized definition of pH is, in words, stick the electrode in a standard (which you may define if you wish) and make the meter read the defined value. Then stick it in the sample and what ever the meter reads is the material's pH, as defined by NIST and everyone else. Unless it has been resolved in the past few months the British and US don't even use, or agree, on the same standards.

NIST definition as an equation.......

pH(sample) = pH(std) + [E(sample) - E(sample)]F/
RTln10

In very dilute solutions, mid pH range, and little junction potential the pH indicated will have some fundamental meaning, but in many cases pH is just an arbitrary number on a defined scale, from which neither H nor aH can be calculated. If schools would teach the correct definition of pH and drop the -log aH notion people would less inclined to over interpret pH measurements.

Checking buffer pH

Does any government agency require that a prepared buffer have its pH checked by a pH meter, or is this something internal company policy or tradition requires?

NIST provides directions for the preparation of primary and secondary standard pH buffers. These are accurate to 3 places. So, if a lab prepared one of these buffers by weights and volumes, say for example a phosphate buffer, would the lab then use the meter's value or the NIST value?

Bill:
Beynon´s page is Tom´s link, he calcs weights based on wanted pH and gives "apparent pH" as well.
I shouldn´t have said "reads activities", but rather "reads something related to activities, somehow, among other things". Theoretically speaking it´s impossible that the meter measurement is directly related to concentration as the H+ potential is produced by its activity. If one calibrates the meter based on weighed buffers whose pH is from a concentration calc. then it reads something related to concentration... so how is the pH ascertained of commercial liquid buffer stds?? Though electrochemistry is fascinating it is often best described by four letter words.

Your definition . . . ., does it need another std in it?

If one has a NIST standard preparation one can use it to adjust the pH meter to read that pH. I said this before: I agree with you on the gravimetric method if one has trustworthy materials to weigh. But a check is a check, even if the checking method is inferior. For instance, a lot of people check HPLC with RIA, ELISA, etc.. now that is really a sacrilege, but... but, we are all out in the cold and want to clamp on to something.
Thanks to you, I will probably prepare more stds via weighing, check it with a pH meter, if its close use the calculated value, report both. If the values are far apart I would first concentrate on the pH meter....
That still does not resolve what to do with the basic difference between concentrations (obtained via the weighing method) and activities (somehow related to pH meter). Certainly, I will not use a buffer prep. calculating software until I know what the math is (does it use Debye-Hückel corrections, exact or approximate equilibrium equations?).

HW, there is a fairly straightforeward answer to your question on how the pH of primary standards are arrived at. Yes, it is all electrochemistry, with a touch of solution thermo. Yes, I appreciate the world's view of electrochemistry.

Unfortunately you are going to be disappointed when you find out how a primary standard's pH is arrived at. Hint, the US and Britian don't agree so their standards won't read the same as ours. Everyone wants pH to have some fundamental significance, like the speed of light, but it ain't in the cards, so pH is a more limited property than one would prefer.

Please send your email and I will send on the description. (I have it as a MS Word file) mine: bjtindall@mindspring.com

Bill, hopefully, your description is easier to understand than what I have seen so far, thanks, appreciate receiving it.
Just looked at some activity coefficients of salts in my old physical chemistry book (Moore). At 0.1M many of these were around 0.6-0.7!! So if someone reports pH based on activities and another based on concentrations one shouldn´t be surprised at discrepancies. Do you, Bill, or anybody, have something on that?

Hans