I've been wondering how other people prepare and pH their buffers. What is your favourite way and why? For example when preparing 3 M ammonium formate, pH 5.0 buffer, which option you would use.
(1) Prepare 3 M ammonium formate (not pH'd) and 3 M formic acid solutions. Mix in such a proportion that the final pH is desired. This conserves the total formic acid concentration but usually there is leftover initial solutions and thus waste. If you have stock solutions of both, this is the fastest way to go.
(2) Calculate the needed amounts of ammonium formate and formic acid by Henderson-Hasselbalch equation, fill up to desired volume and adjust pH if necessary by ammonium hydroxide or formic acid. Basically the same as (1) but no waste generated. Takes few extra minutes to google the pKas and do the math.
(3) Measure desired amount of ammonium formate in small volume of water, pH with formic acid and fill up to desired voume. The final formic acid concentration is higher than 3 M. Maybe the fastest way (together with approach (4)).
(4) Measure desired amount of formic acid in small volume of water and pH with ammonia, fill up to desired volume. The total formic acid concentration is well defined.
Are (1), (2) or (4) the most rigorous ways to go? Do you consider (3) to be cutting corners? Are there guidelines? Are there situations where it really doesn't matter? Are there other approaches?
In this ammonium formate example, because both formate and ammonium ion are (weakly) acid-base active, the amount of formate in approach (3) is large and hence I went with (1) since I had stock solutions of both.
My approach depends on the situation, except that I don't remember the last time I used Henderson-Hasselbalch equation... If I know that the amount of pHing agent needed is low, I go with (3).
I'd be interested in hearing comments, not just for this particular example, but in general as well.