The question states it all.

I'm working out special cases for a different problem and such an acid would create another special case.

Sulfuric Acid is the strongest that I remember. pKa's −3, 1.99

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    $\begingroup$ I guess any disulfonic acid with sufficiently long organic link in the middle will do. $\endgroup$ – Ivan Neretin Apr 30 '19 at 13:13
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    $\begingroup$ @IvanNeretin - Super fantastic! Thank you. $\endgroup$ – MaxW Apr 30 '19 at 13:38
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    $\begingroup$ @MaxW wants superacidic MHA buffer.... :-) $\endgroup$ – Poutnik Apr 30 '19 at 13:43
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    $\begingroup$ @Poutnik - Yes, the idea was to find an "amphoteric" salt of such an acid, eg Sodium hydrogen 1,2-Ethanedisulfonate. $\endgroup$ – MaxW Apr 30 '19 at 14:39
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    $\begingroup$ What about $$\ce{ HO-SO2-CF2-CF2-SO2-OH}$$ :-) ( See en.wikipedia.org/wiki/Triflic_acid pKa −14.7±2.0 ). $\endgroup$ – Poutnik Apr 30 '19 at 14:51

Ethanedisulfonic acid does indeed have $pK_{a1}=-2.06, pK_{a2}=-1.46$.

Using polycyano substutution can also work. This paper reports the table below, note that the tetracyano hydroquinone "makes it" (in the excited state produced by irradiation).

enter image description here

  • $\begingroup$ The Wikipedia link for Methanedisulfonic acid is only for the monosulfonic acid. The disulfonic acid does exist, but I couldn't find any pKa values for it. However both pKa values would surely be less than 0. $\endgroup$ – MaxW May 2 '19 at 14:09
  • $\begingroup$ Removed. Misread something back then, I guess. $\endgroup$ – Oscar Lanzi Sep 7 '20 at 18:41

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