The Hammett Acidity Function for Some Superacid Media

Abstract

<p> The Hammett acidity function, Ho, has been extended from dilute solutions of water in sulfuric acid to the highly acidic HSO3F-SbF5•3SO3 system, a range of more than eight Ho units. The protonation behaviour of eleven aromatic nitro indicators was studied by means of a spectrophotometric technique. These indicators were found to behave in a similar fashion to the previously studied aniline indicators which have been used to define the Ho function from infinitely dilute solutions of acids in water to concentrated sulfuric acid solutions. The superacid systems studied were H2O-H2SO4-SO3, H2SO4-HSO3Cl, H2SO4-HSO3F, H2SO4-HB(HSO4)4, KSO3F-HSO3F, HSO3F-SO3, HSO3F-AsF5, HSO3F-SbF5, and HSO3F-SbF5•3SO3 of which the latter is the most acidic solvent system known.</p>