Typical Values
On this scale, pure H2SO4 (18.4 M) has a H0 value of −12, and pyrosulfuric acid has H0 ~ −15. Take note that the Hammett acidity function clearly avoids water in its equation. It is a generalization of the pH scale—in a dilute aqueous solution (where B is H2O), pH is very nearly equal to H0. By using a solvent-independent quantitative measure of acidity, the implications of the leveling effect are eliminated, and it becomes possible to directly compare the acidities of different substances (e.g. using pKa, HF is weaker than HCl or H2SO4 in water but stronger than HCl in glacial acetic acid; and pure HF is "stronger" than H2SO4 because the H0 of pure HF is higher than that of pure H2SO4.)
H0 for some concentrated acids:
- Fluoroantimonic acid (1990): −31.3
- Magic acid (1974): −19.2
- Carborane superacid (1969): −18.0
- Fluorosulfuric acid (1944): −15.1
- Triflic acid (1940): −14.1
- Sulfuric acid −12.0
For mixtures (e.g., partly diluted acids in water), the acidity function depends on the composition of the mixture and has to be determined empirically. Graphs of H0 vs mole fraction can be found in the literature for many acids.
Read more about this topic: Hammett Acidity Function
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