It was proposed by the physical organic chemist Louis Plack Hammett 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.
Developed by Louis Plack Hammett, the Hammett acidity function is a prominent method for measuring acidity that extends the scope of Brønsted-Lowry acidity beyond the typical range measured using the pH scale in dilute aqueous solutions.