In dilute aqueous solution, the predominant acid species is the hydrated hydrogen ion HO (or more accurately [H(OH)]). In this case H and H are equivalent to pH values determined by the buffer equation or Henderson-Hasselbalch equation.<br>
However, an H value of &minus;21 (a 25% solution of SbF in HSOF) does not imply a hydrogen ion concentration of 10 mol/dm: such a "solution" would have a density more than a hundred times greater than a neutron star. Rather, H = &minus;21 implies that the reactivity (protonating power) of the solvated hydrogen ions is 10 times greater than the reactivity of the hydrated hydrogen ions in an aqueous solution of pH 0. The actual reactive species are different in the two cases, but both can be considered to be sources of H, i.e. Brønsted acids.  never exists on its own in a condensed phase, as it is always solvated to a certain extent. The high negative value of H in SbF/HSOF mixtures indicates that the solvation of the hydrogen ion is much weaker in this solvent system than in water. Other way of expressing the same phenomenon is to say that SbF·FSOH is a much stronger proton donor than HO.