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I am confused about $\mathrm{H_0}$ value of fluoroantimonic acid. Wikipedia says $\mathrm{-23<H_0<-21}$. Others say around $-31$. There's a huge gap. I won't worry for $3-4$ fluctuations but worry for $10$.

Can anyone provide confirmed data?

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    $\begingroup$ I think it's an example mistaken data copied from Wikipedia, later (now) fixed there (with refs). $\endgroup$ – mykhal Nov 1 '18 at 10:13
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    $\begingroup$ In this question there was a similar confusion over how strongly basic tetramethylammonium hydroxide is. Apparently Wikipedia, or more precisely the references it used, confused TMAH with trimethylamine. The erroneous information is now on the cutting room floor. $\endgroup$ – Oscar Lanzi Nov 1 '18 at 23:26
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Superacids consisting of a mixture are going to have a certain range for the Hammett acidity function $\mathrm{H_0}$. For example, $\mathrm{H_0}$ for both Magic Acid $\ce{HSO3F-SbF5}$ and fluoroantimonic acid $\ce{HF-SbF5}$ relies on concentrations defined by the equilibrium of the acid's formation, which, in turn, is affected by how much $\ce{SbF5}$ has been added [1]:

$$ \begin{align} \ce{2 HF + 2 SbF5 &<=> H2F+ + Sb2F11-}\\ \ce{2 HSO3F + 2 SbF5 &<=> H2SO3F+ + Sb2F10(SO3F)-} \end{align} $$

The acidity function of $\ce{HSO3F}$ increases from $-15.6$ to $-21.0$ on addition of $25$ mole percent $\ce{SbF5}$ ... as shown in Fig. 1. Extrapolation of the $\ce{HSO3F-SbF5}$ curve in Fig. 1 would lead to an $\mathrm{H_0}$ value of about $-25$ for Magic Acid. Fluoroantimonic acid is even stronger. As shown in Fig. 1, with $4$ mole percent $\ce{SbF5}$ the $\mathrm{H_0}$ value for $\ce{HF-SbF5}$ is already $-21.0$, a thousand times stronger than the value for fluorosulfuric acid with the same $\ce{SbF5}$ concentration. At present it is difficult to estimate the acidity of $1:1$ $\ce{HF-SbF5}$, but a value of $-28$ can be predicted ... on the basis of isomerization kinetics data.

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Fig. 1. Relative acidities of $\ce{HF}$ and $\ce{HSO3F}$ on addition of $\ce{SbF5}$.

That is why most textbooks (e.g. [2]) list $\mathrm{H_0}$ from $-21$ to $-25$ for Magic Acid and $\mathrm{H_0}$ from $-21$ to $-28$ for fluoroantimonic acid.

References

  1. Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids. Science 1979, 206 (4414), 13–20. https://doi.org/10.1126/science.206.4414.13.
  2. Miessler, G. L.; Fischer, P. J.; Tarr, D. A. Inorganic Chemistry, Fifth edition.; Pearson: Boston, 2014. ISBN 978-0-321-81105-9.
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