Superacids consisting of a mixture are going to have a certain range for the Hammett acidity function $H_0$. For example, $H_0$ 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 $H_0$ value of about $-25$ for Magic Acid. Fluoroantimonic acid is even stronger. As shown in Fig. 1, with $4$ mole percent $\ce{SbF5}$ the $H_0$ 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.
Fig. 1. Relative acidities of $\ce{HF}$ and $\ce{HSO3F}$ on addition of $\ce{SbF5}$.
That is why most textbooks (e.g. [2]) list $H_0$ from $-21$ to $-25$ for Magic Acid and $H_0$ from $-21$ to $-28$ for fluoroantimonic acid.
References
- 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.
- Miessler, G. L.; Fischer, P. J.; Tarr, D. A. Inorganic Chemistry, Fifth edition.; Pearson: Boston, 2014. ISBN 978-0-321-81105-9.