2
$\begingroup$

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.

$\endgroup$
1

1 Answer 1

4
$\begingroup$

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

  1. 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
$\endgroup$
3
  • 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. $\endgroup$
    – Poutnik
    Oct 12, 2023 at 10:57
  • 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. $\endgroup$
    – Jack
    Oct 12, 2023 at 10:59
  • $\begingroup$ 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. $\endgroup$ Oct 12, 2023 at 12:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.