Superacid scale Hammett acidity

Measuring super acids requires something beyond the normal pH scale from 0 - 14. The superacid scale, or Hammett acidity function, is based on experimental evidence. For example, in this equation... 

Figure 8.1 Comparison of the acidity of selected liquid and solid acids on the Hammett acidity scale. Ho = — 2 for a superacid. (See text for an explanation of Ho) [Reproduced from reference 1 with permission. Copyright 2002 American Chemical Society.]...

The highest acidity level measured according to the Ho acidity scale is around -27 for 90% SbFs in HSO3F. However, based on rate measurements in superacidic reactions the fluoroantimonic acid system is predicted to be even stronger [Fig 2.3], Similar to BF3 in HF, SbFs also ionises in dilute solutions to produce the H2F cation. The lower Hammett acidity of the HF/BF3 system compared to SbFs and TaFs, can be attributed to the lower concentration of the H2F " cation produced in the HF/BF3 system. The HF/BF3... 

Up to a // value of — 10, all indicators are primary amines and are therefore suitable for the measurement of the Hammett H() function. For stronger acids, new indicators such as nitro compounds have to be used. Although the acidity function scale based upon nitro compounds as indicators may not be a satisfactory extension of the aniline indicator scale, Gillespie and Peel18 have shown that the most basic nitro compound indicator, para-nitrotoluene overlaps in a satisfactory manner with the weakest indicator in the aniline series, 2,4,6-trinitroaniline. Thus, the acidity measurements using the nitro compounds may be considered to give the best semiquantitative picture of the acidity of the various superacid systems. 

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