It was proposed by the physical organic chemist Louis Plack Hammett[1][2] and is the best-known acidity function used to extend 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In highly concentrated solutions, simple approximations such as the Henderson–Hasselbalch equation are no longer valid due to the variations of the activity coefficients.
Hammett originally used a series of anilines with electron-withdrawing groups for the bases.
The value H0 = -12 for pure sulfuric acid must not be interpreted as pH = −12 (which would imply an impossibly high H3O+ concentration of 10+12 mol/L in ideal solution).
[7] Take note that the Hammett acidity function clearly avoids water in its equation.