Hammett Acidity Function - Typical Values

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 pK_a, 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.