Hammett acidity function

The Hammett acidity function H₀ is the measure of acidity that is used for very concentrated solutions of strong acids, including superacids. It was exposed by the physical organic chemist Louis Plack Hammett in addition to is the best-known acidity function used to remain the measure of Brønsted–Lowry acidity beyond the dilute aqueous solutions for which the pH scale is useful.

In highly concentrated solutions, simple approximations such(a) as the Henderson–Hasselbalch equation are no longer valid due to the variations of the activity coefficients. The Hammett acidity function is used in fields such as physical organic chemistry for the explore of acid-catalyzed reactions, because some of these reactions use acids in very high concentrations, or even neat pure.

Typical values

On this scale, pure M has a H₀ good of −12, and pyrosulfuric acid has H₀ ~ −15. hit note that the Hammett acidity function clearly avoids water in its equation. it is for a generalization of the pH scale—in a dilute aqueous sum where B is H₂O, pH is very near 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.

H₀ for some concentrated acids:

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 numerous acids.