The pH scale is by far the most commonly used acidity function, and is ideal for dilute aqueous solutions.
In dilute aqueous solution, the predominant acid species is the hydrated hydrogen ion H3O+ (or more accurately [H(OH2)n]+).
The actual reactive species are different in the two cases, but both can be considered to be sources of H+, i.e. Brønsted acids.
The hydrogen ion H+ never exists on its own in a condensed phase, as it is always solvated to a certain extent.
The high negative value of H0 in SbF5/HSO3F mixtures indicates that the solvation of the hydrogen ion is much weaker in this solvent system than in water.