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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 at 13:13
  • $\begingroup$ @IvanNeretin - Super fantastic! Thank you. $\endgroup$ – MaxW Apr 30 at 13:38
  • $\begingroup$ @MaxW wants superacidic MHA buffer.... :-) $\endgroup$ – Poutnik Apr 30 at 13:43
  • $\begingroup$ @Poutnik - Yes, the idea was to find an "amphoteric" salt of such an acid, eg Sodium hydrogen 1,2-Ethanedisulfonate. $\endgroup$ – MaxW Apr 30 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 at 14:51
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Ethanedisulfinic acid does indeed have $pK_{a1}=-2.06, pK_{a2}=-1.46$.

Methanedisulfonic acid is reported by Wikipedia to have $pK_a = -1.9$, but Wikipedia does not give two separate dissociation constants.

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

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  • $\begingroup$ 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 at 14:09

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