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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: electrogeek on September 05, 2022, 07:34:59 AM

Title: Measuring negative pKa values - is it possible?
Post by: electrogeek on September 05, 2022, 07:34:59 AM: 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.
Title: Re: Measuring negative pKa values - is it possible?
Post by: Babcock_Hall on September 05, 2022, 11:59:06 AM: 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.
Title: Re: Measuring negative pKa values - is it possible?
Post by: Borek on September 05, 2022, 01:36:37 PM: 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.
Title: Re: Measuring negative pKa values - is it possible?
Post by: electrogeek on September 06, 2022, 04:24:57 AM: Quote from: Babcock_Hall on September 05, 2022, 11:59:06 AM
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.
Title: Re: Measuring negative pKa values - is it possible?
Post by: electrogeek on September 06, 2022, 04:27:11 AM: Quote from: Borek on September 05, 2022, 01:36:37 PM
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?
Title: Re: Measuring negative pKa values - is it possible?
Post by: Babcock_Hall on September 06, 2022, 09:55:26 AM: I am not an expert, but isn't acid strength sometimes measured by comparing one acid to another?