[MangoPlant]

Hi all, I had a question I was thinking of that I was hoping someone would have insights on.

If I add a certain amount of ethylamine hydrochloride to a solution of pH 7 buffer, and in a separate vial add an identical molar amount of ethylamine (free amine) to an identical solution of pH 7 buffer, will the ratio of protonated to unprotonated amine be the same in both cases? I know that the amine should reach equilibrium but I can't get an idea if the protonated:unprotonated ratio would be the same in these cases. Any help appreciated

[phth]

No it won't be the same.  The amount of free base will deprotonate more if there is 10 versus 100.  Even Na₂CO₃ will deprotonate methanol ~1/10 concentration.  What equation could you use to calculate the answer for the amine? 

[MangoPlant]

I didn't quite understand what you meant by 10 versus 100. I thought of Henderson-Hasselbach, but I don't have a feel for how the pkA of ethylamine hydrochloride would compare to that of ethylamine. If they have the same pKa then they should have identical protonated:unprotonated concentrations in pH 7 buffer. Also this is not a homework question or a question that I know the answer to, just a question I was pondering when I was looking at hydrochloride salts of drugs.

In the pH 7 buffer, the ethylamine hydrochloride will come into equilibrium with its unprotonated ethylamine form. Similarly, if free base ethylamine is added to pH 7 buffer it will come into equilibrium with its protonated ethylamine form (CH₂CH₂NH₃⁺). I am just wondering whether the unprotonated ethylamine concentrations in both of these cases is the same.

[Borek]

Assuming pH doesn't change (and stays at 7) ratio of concentrations of the acid and conjugate base would be identical. Easy to prove rearranging dissociation constant formula in such a way you have the ratio on one side and K_a and [H⁺] on the other - K_a is a constant, [H⁺] doesn't change, ratio stays the same.

That's never exactly the case - you add an acid or a base, so the pH shifts slightly, but the shift can be small enough to be negligible.

[MangoPlant]

Thanks a lot. This is what I was thinking as well but was not sure that the K_a would be the same. So would the K_a of CH₂CH₂NH₃⁺Cl^- be the same as the K_a of CH₂CH₂NH₃⁺(HCO₃)^- or any other ethylamine salt (assuming the pH does not change and constant temperature)?

[Borek]

K_a is a property of the C₂H₅NH₃⁺, counterion doesn't mater.