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A superacid is an acid with an acidity greater than that of 100% pure sulfuric acid. The conventional pH scale is inadequate for these media. Instead, their acidity is measured using the Hammett acidity function, [latex]H_0[/latex]. This function is based on the protonation of weak indicator bases and is defined as [latex]H_0 = pK_{BH^+} – logleft(frac{[BH^+]}{[B]}}right)[/latex].
The Hammett acidity function, [latex]H_0[/latex], extends the concept of pH to highly concentrated or non-aqueous acidic solutions where the activity of the proton is no longer simply related to its concentration. It measures the medium’s ability to protonate a neutral weak base ‘B’. The value of [latex]H_0[/latex] is determined experimentally using a series of weak bases with overlapping [latex]pK_{BH^+}[/latex] values, typically aromatic amines, and measuring the ratio of their protonated ([latex]BH^+[/latex]) to unprotonated ([latex]B[/latex]) forms via UV-visible spectroscopy. For dilute aqueous solutions, [latex]H_0[/latex] is approximately equal to pH, but it diverges significantly in concentrated acids, taking on large negative values. For example, 100% [latex]H_2SO_4[/latex] has an [latex]H_0[/latex] of -12.
George Olah’s work with superacids, such as ‘Magic Acid’ ([latex]FSO_3H-SbF_5[/latex]), which has an [latex]H_0[/latex] around -23, was revolutionary. It allowed for the preparation and direct observation of stable carbocations, which were previously only considered transient intermediates. This opened up new frontiers in physical organic chemistry and earned him the Nobel Prize in Chemistry in 1994.
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