Chromatography Forum

Hi!!

I'm preparing a mobil phase of ammonium formiate, and I have to basify it...
I think I should use ammoniu.
Is it correct???

Thank you!!!

Yes. If you are starting with concentrated formic acid, then the easiest way to do this is to weigh it into a beaker (with the scale between you and a fume hood!). Adjust the amount you weigh to take account of the concentration of your formic acid reagent [example: If it's 96%, then add 4% more by weight]. Add a stirring bar and then some water, slowly, with stirring (about 70-80% of the volume that you will eventually need). Immerse a pH electrode and start adding NH4OH solution until you get to the pH you want. Transfer to a volumetric flask and add water to the callibration mark. Filter it (0.45 micron filter membrane) if you intend to use this as a mobile phase or sample solvent in HPLC.

OK, thank you a lot for the information!
The only think is that I'm preparing mobile phase using ammonium formiate, not concentrated formic acid.
But I don't think it is a problem!

Best regards,

yes you can start with ammonium formate salt; the only problem I find is that it's horribly hygroscopic, becoming gradually wetter and wetter, which makes it hard to make an accurate solution. Ammonium hydroxide solutions are as reliable as chocolate tea-pots and also gradually change concentration, so if you're lucky, the only constant thing about your buffer will be the pH. (If you are unlucky and your pH meter isn't up to much, even this will vary). Yes, I prepare buffers the way you do, and it hurts every time. I think I might try Andy Alpert's version one day.

Andy, do you filter your buffers when prepared from formic acid and NH4OH? We always filter anything that came from solid salts, but we don't filter anything that came in liquid form (on the grounds that everything we buy is hplc-grade or better and probably filtered better than we can filter anyway). Avoiding filtration would be another benefit.

I have had problems with contamination (dyes and salts from the calibration buffers and storage solution) when the pH probe is immersed directly in the bulk buffer solution. I always remove a small aliquot and check the pH of the aliquot (discard the aliquuot after measuring pH). This process is a bit more tedious the first time you make the buffer but once you have the recipe it adds little, if any, time and you avoid problems with contamination.

I second Carl's recommendation. A pH meter should *never* contact the bulk mobile phase.

Whether you start with ammonium formate salt and add acid to adjust the pH downwards or start with formic acid and add NH4OH to adjust pH upwards depends on the pH you want to get. The pKa of formate is 3.7. If you want a pH in the range 4.0-4.7, then I would consider starting with the salt and adding acid. Otherwise, I always start with the acid. I have peer-reviewed papers where people started with ammonium formate salt, added enough formic acid to get to pH 2.2, and then wondered why their chromatography didn't work. I helpfully pointed out that you need to add an equal number of moles of the unbuffered acid to the salt in order to get pH 3.7. To get to pH 2.7 you need 10x more moles of the acid. To get to pH 2.2 you need something like 70x more moles of the acid...
Re: Filtering the final buffer: I would consider not filtering it if both of the reagents were HPLC-grade and if I were confident in the inline filtration of the deionized water system in the lab.
Re: The warning against immersing a pH electrode directly in the solution while its pH is being adjusted: Fine, if you're making something like 1 L. of a solution of ammonium formate. If you're making a small volume of a concentrated solution of an expensive reagent, such as sodium methylphosphonate, then it would be impractical to keep removing aliquots. Also, if the final solution is a concentrated stock solution, then any solution prepared with an aliquot of this would presumably have the electrode solution components highly diluted and causing minimal problems.

I second Carl's recommendation. A pH meter should *never* contact the bulk mobile phase.

I've also been forever leery of adjusting pH of the actual mobile phase to be used by using pH meters and pH electrodes. I've seen publications where they adjust the pH of a mixed aqueous-organic mixture instead of just adjusting the aqueous part only. And I assume that first the pH meter must be cGMP-checked somehow.

What I prefer to do is measure how much pH-adjusting chemical (such as phosphoric acid) and then adding that chemical by volume or weight in my preparations, and therby not using a pH meter. Our QA and my supervisor would like guidance if my way is OK. In our robustness testing, we typically add 80% and 120% of the adjusting chemical, etc., to demonstrate robustness, and have had no issues. So, opinions please, Tom and others? Thanks.

Buffer preparation by weight is *far* more accurate and precise than trying to measure pH.

A bit over a decade ago, Bill Tindall posted some really great material on pH, which he subsequently expanded into three articles that appeared in John Dolan's "LC Troubleshooting" column:

http://chromatographyonline.findanalyti ... rticle.pdf

http://chromatographyonline.findanalyti ... rticle.pdf

http://chromatographyonline.findanalyti ... rticle.pdf

His suggestion was that pH meters be banned from mobile phase prep labs, and all buffers prepared by weight. The first time I talked with him about it, I mentioned the problem with hygroscopic salts. He had a one-word response: "dessicator".

If you want to have "fun" some evening, fire up the Henderson-Hasselbalch equation and figure out how large a weighing error you have to make in order to be off by 0.1 pH unit when preparing a buffer (Hint: it's greater than 10%). Any lab where the chemists or technicians can't weigh to within 10% of a target value has much greater problems than just pH!

Thanks, everyone, for very useful feedback. OK, so I can store my ammonium formate in a dessicator (good one-word answer!), but anybody got any thoughts on what to do with my ammonium hydroxide if I'm to make buffers entirely by weight/volume? Can I just buy fresh fairly regularly and hope it doesn't lose concentration too fast? Oh I wish I were not confined to volatile buffers...

To be fair to pH meters, they get a particularly bad name because they are very often ill-maintained things in a shared environment, left to fester in ancient solutions, and the people who buy them are rarely aware of the range of electrodes available, nor the fact that one electrode does not suit all applications. A properly cleaned and calibrated pH electrode is not necessarily a complete catastrophe for chromatography, especially if it has spent all of its life with such care and never been abused. After all, a Dionex ion chromatography system using pulsed amperometry electrochemical detection will quite probably include a pH electrode in the flow path. But I'd still prefer not to use one, and I'm very happy to hear tips on how to avoid them.