# pH adjustment - (Dec/07/2007 )

Hello!

I would like to acidify 300uL of a solution containing 3M ammonium hydroxide (i.e. NH3) with formic acid (pKa=3.7). Is there a quick calculation to get a rough idea of how many microliters of formic (100%) acid I have to add?

Thank you very much!!! 

What pH do you need first? It takes more time to calculate the pH of a mixture than you experimentally measure it.

If you have arough idea what molar ratio of the two you want to be, then get several parameters: the % of FA, MW and density then we can calculate using X g/ml/MW= y M of this solution, then mix both on ice, slowly, you are mixing acid and base!! And do it in the hood, both are nasty stuff.

-genehunter-1-

I was sure to have forgotten something... :-)
I need to acidify until pH 2.5... My question arose just to avoid measuring several times the pH with a pH paper as i only have 300uL... Thus I wanted to know whether there is a quick way to calculate the volume of FA I have to add... :-)

you can adapt the Henderson Hasselbach equation: pH = pka + log [salt]/[acid]

-mdfenko-

This calculation can be extreamely hard to do. You are dealing with a weak acid and a weak base at very high concentration ( which affect the dissociation behavior of both ions), the temperature would be an additional variable.

pH paper test to estimate would be the best thing to do, if the accuracy of the pH is not strictly required. You can't even measure it accurately with a pH meter equipped with a microprobe, because the concentration is too high.

This pH seems to be little extreme. It is near or I would guesstimate to be lower than the COOH group, are you sure you can do it with this combinantion?

Yes, the pKa for formic acid is 3.75, which means, you would have to use a stronger acid, like HCl to adjust the mixture to pH 2.5.

-genehunter-1-

QUOTE (genehunter-1 @ Dec 7 2007, 11:12 AM)
This calculation can be extreamely hard to do. You are dealing with a weak acid and a weak base at very high concentration ( which affect the dissociation behavior of both ions), the temperature would be an additional variable.

pH paper test to estimate would be the best thing to do, if the accuracy of the pH is not strictly required. You can't even measure it accurately with a pH meter equipped with a microprobe, because the concentration is too high.

This pH seems to be little extreme. It is near or I would guesstimate to be lower than the COOH group, are you sure you can do it with this combinantion?

Yes, the pKa for formic acid is 3.75, which means, you would have to use a stronger acid, like HCl to adjust the mixture to pH 2.5.

yeah, i know it would be harder to calculate than just checking with pH paper but sephadex asked for a way to calculate.

pH paper, spotting 1 or 2ul would probably be best, but a micro electrode should be able to be used. the concentration of ammonia shouldn't be a problem (do it under the hood!).

you can adjust it to below the pK of formate with formic acid (check the pH of fa). but, as with any buffering situation, it takes more (of any acid) to push it past the pK. and if using fa then you are also adding more buffering ion to the solution, increasing the solutions capacity to resist pH change.

-mdfenko-

He has 3M of NH3 to start with. Lets not to consider the complicated hydration process of weak amine and weak acid in water, just stick to the formular, you would need more than 10X of HCOO- over NH4+ to reach the ratio of 1 pH unit below its pKa, which will get to a concentration range of 30 M for FA. I think it is practically hard to achieve. He may really need some help from HCl to lower the pH to 2.5.

-genehunter-1-