About a Trillion Times More Acidic than Expected? On the Difference Between the Hammett <i>H</i> ₀ and the Unified pH Acidity of Sulfuric Acid

Abstract

Abstract We report on the considerable difference between the distinct pH and Hammett's H 0 acidity function in the entire H 2 O‐H 2 SO 4 mixing range. The Hammett acidity for 100 wt% H 2 SO 4 is accepted with H 0 = −11.9. We measured the pH acidity using two different electrochemical cells, operating with at least one hydrogen electrode, giving for 100% sulfuric acid as −24 ± 2. This value was confirmed by high‐level compound calculations using the DLPNO‐CCSD(T) level at the basis set limit and COSMO‐RS solvation. Thus, the difference between both scales amounts to − H 0 = −12 or 12 orders of magnitude. It results from the Gibbs transfer energies of the (protonated) Hammett‐indicator bases B (BH + ) used to define and determine H 0 . High‐level computations of these Gibbs transfer energies confirm our experimental as well as our computational findings. A general method to convert H 0 values of acids other than sulfuric acid into values is given. Hence, this huge (pH −  H 0 )‐difference clearly delineates that the often used but erroneous assumption of the H 0 values constituting an extension of pH acidity to the realm of negative pH values is wrong. The distinctive protochemical potential curve progression is discussed as a function of sulfuric acid concentration and physical properties, together with the assignment of the potential‐determining species in the respective mixtures. Finally, superacidity is defined in a universal manner, i.e., applicable to all media, i.e., liquid, solid, or gaseous.