I know that when you have aqueous solutions, then the pKa and the pKb are linked to each other (pKa + pKb = 14). Is there an equivalent relationship between pKa and pKb in non aqueous environments? I'm currently looking at a paper where they have measured pKb values in glacial acetic acid (https://pubs.acs.org/doi/epdf/10.1021/ic50084a055), but I'm interested in finding the pKa values for a different set of compounds in glacial acetic acid. Thank you.
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$\begingroup$ Closely related, possibly duplicate: chemistry.stackexchange.com/q/28022/35806 $\endgroup$– PoutnikCommented Oct 12, 2023 at 10:47
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Bruckenstein and Kolthoff give some values in the abstract, which is available in the free preview. Notable among acids: perchloric acid, $pK_a=4.87$; hydrogen chloride, $pK_a=8.55$.
Also given is an autodissociation constant, whose base-10 logarithm is $-14.45$. Thus whereas we render $pK_a+pK_b=14$ in water under ambient conditions, the corresponding constant in glacial acetic acid would be $14.45$. As an example, citing the hydrogen chloride acid dissociation constant given above, we can compute the $pK_b$ of the conjugate base chloride ion as $14.45-8.55=5.90$ (significant zero).
Reference
- S. Bruckenstein and I. M. Kolthoff (1956). "Acid-Base Equilibria in Glacial Acetic Acid. III. Acidity Scale. Potentiometric Determination of Dissociation Constants of Acids, Bases and Salts". J. Am. Chem. Soc. 78, 13, 2974–2979. https://doi.org/10.1021/ja01594a014
answered Oct 12, 2023 at 10:53
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1$\begingroup$ Wow, that is unexpected. I have been assuming perchloric acid is still strong in acetic acid, considering it is used for titration of weak bases there. But I admit I have zero experience in the area. $\endgroup$– PoutnikCommented Oct 12, 2023 at 10:57
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1$\begingroup$ This is brilliant! Thank you for the reference. I'm wanting to use perchloric acid for the non-aqueous titration, so this is also a really useful resource for that value. $\endgroup$– JackCommented Oct 12, 2023 at 10:59
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$\begingroup$ From the autodissociation constant aso given in the abstract, we calculate that the neutral "pH" in acetic acid is about 7.2. Perchloric acid with $pK_a$ between two and three units less can reach that condition with most bases; hydrogen chloride can't (unless you go far beyond the equivalence point) and sulfuric acid is marginal at best. $\endgroup$ Commented Oct 12, 2023 at 12:16