Hammett acidity function

The Hammett acidity function (H0) is a measure of acidity that is used for very concentrated solutions of strong <a href="/facts/Acid/FySU18n1">acids</a>, including <a href="/facts/Superacid/dg82EcNa">superacids</a>. It was proposed by the physical organic chemist <a href="/facts/Louis_Plack_Hammett/1659s4aL">Louis Plack Hammett</a> and is the best-known <a href="/facts/Acidity_function/GJQKGeNL">acidity function</a> used to extend the measure of <a href="/facts/Br%25C3%25B8nsted%25E2%2580%2593Lowry_acid%25E2%2580%2593base_theory/bqxQnJoX">Brønsted–Lowry</a> acidity beyond the dilute <a href="/facts/Aqueous_solutions/eRNr3MH2">aqueous solutions</a> for which the <a href="/facts/PH/RnSv8D7y">pH</a> scale is useful.
In highly concentrated solutions, simple approximations such as the <a href="/facts/Henderson%25E2%2580%2593Hasselbalch_equation/LrduPhHB">Henderson–Hasselbalch equation</a> are no longer valid due to the variations of the <a href="/facts/Activity_coefficient/ImxE0u7t">activity coefficients</a>. The Hammett acidity function is used in fields such as <a href="/facts/Physical_organic_chemistry/qQSFoCoS">physical organic chemistry</a> for the study of <a href="/facts/Acid-catalyzed/JkJ7LkT6">acid-catalyzed</a> reactions, because some of these reactions use acids in very high concentrations, or even neat (pure).

Hammett acidity function open-in-new

Hammett acidity function