[electrogeek]

Hi all,

I know of methods to measure the pKa of a substance which has a positive pKa, but how would you do it for a negative pKa value? Are there any experimental methods known which are capable of doing this? I assume there would be a method for doing this, as the pKa of things such as chloride, bromide and iodide are known and are all negative.

[Babcock_Hall]

It is HI, HBr, and HCl which have negative pK_a values.  Why do you think that measuring them is difficult?  I don't disagree, but I want to understand your reasons.

[Borek]

Definitely not trivial, as far as I am aware best approach is to look for the concentration of the undissociated acid with some spectroscopic method.

[electrogeek]

It is HI, HBr, and HCl which have negative pK_a values.  Why do you think that measuring them is difficult?  I don't disagree, but I want to understand your reasons.

I know that a negative pKa means that Ka has to be greater than 1, so either the product [H+][A-] is high or the value for [HA] is low, but I don't see how you would be able to determine this experimentally. I see everywhere that it can be calculated, but never have seen an experiment where someone has measured these values directly (or for other compounds which have a negative pKa). I guess you would measure [HA] to determine it, but I guess this is hard to measure because there would be so little of it present (strong acid and so large amount of dissociation). And I guess you would have to measure [H+] or [A-] (these should be the same for things like HCl, HBr etc), but then you would have a negative pH because [H+] is greater than 1.

[electrogeek]

Definitely not trivial, as far as I am aware best approach is to look for the concentration of the undissociated acid with some spectroscopic method.

Ah okay! Would you do something like NMR to do it? Is there a reference or something that I could have a look through? It amazes me because I know the pKa can be really negative for some things, and I guess you can't use a pH probe / normal titration methods to measure it?

[Babcock_Hall]

I am not an expert, but isn't acid strength sometimes measured by comparing one acid to another?