Buffer solution

[yeevoon]

1. I need to prepare ammonium formate buffer solution at pH 3.5. May I know what is the correct way to prepare it?

2. What is the difference if I only use formic acid as the buffer solution (pH 3.5)?

[Markus Laeubli, Metrohm]

Dear Yeevoon

You may use ammonium formate and adjust the pH with either formic acid or ammonium hydroxide solution to pH = 3.5.

Or you may use formic acid and adjust the pH with ammonium hydroxide solution to pH = 3.5.
The result is the same.

[Tomasz]

Formic acid at pH 3.5 is not a buffer solution.

[danko]

Tomasz,

Why not?

Best Regards

[Bryan Evans]

When it comes to buffer preparation, there are 2 ways: The Right Way and The Wrong Way:

(Example) Wrong Way:
25mM HCOONH4 Buffer, pH 3.5
(implies to weigh salt, and titrate with acid to desired pH)

(Example) Right Way:
25mM HCOOH / 25mM HCOONH4 = 60 / 40

The Right Way is reproducible. The Wrong Way is not.

[HW Mueller]

A third method, namely weighing conjugate base and acid, then diluting to the mark in a volumetric flask is the most accurate (if possible). See

http://www.liv.ac.uk/buffers/buffercalc.html

Also, the material given by Tom removes some of the myths which are presented here again.

[Bryan Evans]

Hans,

I agree that you can make (equal) concentrated solutions of A (acid) and B (conjugate base).
You can make (equal) serial dilutions of each and then mix. This could be done, for example, if one needs to make dilute buffer but
needs to weigh more acid / salt in order to be within calibration range of their balance.

I believe this link you posted over complicates the process for a lot of end users.
The method is designed for accuracy, not precision.

The methodology that Imtakt suggests will result in consistant pH AND ionic strength.
This is what really matters for HPLC.

[yeevoon]

Thank you all for the feedback!

But I'm just wondering if there is any difference in using different salt as buffer, i.e. what is the difference when I use ammonium formate and ammonium acetate to prepare the buffer solution?

what are the factors/criteria to be taken into consideration when it comes to the choice of buffer to be used?

[HW Mueller]

Bryan, I am talking about preparing buffers by weighing. The calcs that go into Beynons calculator are tedious, but the user doesn´t see this. It has been mentioned before: This is how standard buffers are prepared. Since when are standard buffer not precise?? (Or, why is Beynons calculator associated with high accuracy, but not with precision?)

yeevoon, you need to take Tom´s seriously and/or look at some analytical chemistry texts, chemical equilibrium texts, Schaum´s practicing series.

[lmh]

Hang on a moment, I'm not sure I'm understanding.

(1) I do understand that Bryan's "wrong way" is wrong, because it isn't a 25mM buffer. You started with 25mM formate salt, and added more formic acid, so it's now a more concentrated solution of formic-stuff, and you will need to spend some time with a calculator to work out exactly what you've made.

(2) I don't understand why Bryan's "right way" is more reproducible. If my ammonium formate is a bit damp today (being very hygroscopic), my 25mM solution won't be quite 25mM, and the pH will not be what I am expecting. This is also a weakness of the first approach suggested by the excellent web buffer-calculator.

(3) I don't understand the differences in accuracy and precision between the methods (A) of preparing 25mM each of acid and base, and mixing to pH, versus (B) preparing slightly less than the correct volume for 25mM formic acid, setting pH with ammonium hydroxide, and topping up to the correct volume. Both should make the same thing.

Given volatile and hygroscopic acids and bases, it seems to me that the most reproducible approach is to use (B) starting with the correct amount of the most reliably measured chemical, and setting pH with the less reliably measured chemical (because then the pH depends on the calibration of the pH meter, and the ionic strength depends on the chemical you can weigh/pipette accurately).

Personally, I've always used (A) for phosphate buffers where both forms are available and accurately measurable, and (B) for things like Hepes/NaOH or Tris/HCl, where I usually only have one form of the buffer.

[Vlad Orlovsky]

There are thousands of validated method with buffers and I don't get this mixing volumes approach. Never had a problem with traditional buffer preparation: dissolving salt in water, adjusting pH of aqueous solution and then mixing with organic. If person is not qualified no matter what approach he/she uses there is always a chance of screw up.

[HW Mueller]

Vlad, you saw this argument before: If you use a pH meter you have the accuracy of a pH meter, from your statement this is good enough for you. If you are an excellent titrator you get the same ionic strength (~within the accuracy of the pH meter), if not your ionic strength is different each time. If you use weighing your much more accurate scale/balance is the basis.
(Please no more stories about having trouble weighing stuff which has been quantified by weighing for the last 100 years. For every "story" on weighing there are who knows how many "stories" with pH meters.)

[Bruce Hamilton]

I agree with Vlad, but only because I'm lazy.

My problem with the "prepare two solutions, then mix and dilute to target concentration" approach is that the solutions are not stable over meaningful timescales ( weeks = more wildlife than a beer ).

Because my work is varied, I have to prepare the stock solutions fresh, and so it's much easier accurately weigh the solid and add dilute acid or base solution to pH. Modern pH systems are very stable if used/stored as recommended.

If I'm fool enough to mess up caring for a pH system, I'm equally as likely to screw up volumetric buffer preparation.

Please keep having fun,

Bruce Hamilton

[HW Mueller]

Please just state exactly how you prepared your buffers, so that it is known whether you are lazy or not.
Bruce, you prepare the dil. acid/base everytime also, or are they stable in your hands? Maybe you are not so lazy after all?
On a more serious vein: Some of the more stable modern pH meters can be less accurate than the older ones, where one has to have a bit of patience.
Also, I just wonder how you people do this adjusting the pH with an acid/base solution. When I did that I almost always overshot, then needed more buffer sol. then overshot again . . . . Do you hang the electrode into your solution?
I guess I am not only lazy, but inept to boot.

[Bryan Evans]

Hans,

I looked over my comments and I see how I may have botched this up.

The concern is for scientists developing methods. These methods
may then be transferred to other labs (perhaps to another country).
I believe this discussion has validated these concerns.

Perhaps the example below better explains what I was trying to say:

--------------------------------------------------------------------------------

Method 1 states: 25mM HCOONH4 Buffer, pH 3.5
(How does one go about preparing this buffer?)

(a) Weigh acid / conjugate base, dilute to mark (Hend. Hassselback, sophisticated software, ect.)
(b) Weigh salt to get 25mM, titrate with acid (using pH meter) to pH 3.5
(c) use formic acid - titrate with ammonium hydroxide (using pH meter) to pH 3.5

Method 2 states: 25mM HCOOH / 25mM HCOONH4 = 60 / 40

---------------------------------------------------------------------------------
Method 1 (because of bad habits developed in the industry) is open to interpretation.
Method 2 is not.