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.
The Hammett acidity function, proposed by Louis Plack Hammett, is a widely recognized acidity measure used to extend the Brønsted–Lowry acidity concept beyond dilute aqueous solutions where the pH scale is applicable.