Hammett acidity function

The Hammett acidity function, H₀, measures the protonating power of highly concentrated acidic media using the equilibrium behavior of weak indicator bases, where ordinary pH is inadequate.

Acid–base indicator: A compound whose measurable property changes between protonated and unprotonated forms. H₀ is determined from the protonated-to-unprotonated ratio of a selected indicator base.
Brønsted–Lowry acid–base theory: A theory defining acids as proton donors and bases as proton acceptors. H₀ quantifies how strongly a medium can protonate a base.
Superacid: An acidic medium more protonating than pure sulfuric acid under the conventional acidity-function comparison. H₀ is commonly used to characterize the extreme protonating power of superacidic media.
Louis Plack Hammett: An American physical organic chemist known for developing quantitative relationships in organic chemistry. Hammett introduced the acidity function that bears his name.
Indicator ratio: The ratio of protonated to unprotonated indicator concentrations in an acid–base equilibrium. This concentration ratio supplies the experimental term used to calculate H₀.
pH: A logarithmic measure related to hydrogen-ion activity in aqueous solutions. Unlike ordinary pH, H₀ remains useful in highly concentrated and strongly acidic media.
Hammett acidity function H₋: An acidity function for basic media, defined using indicators that lose protons. H₋ applies the indicator approach to strongly basic media rather than strongly acidic ones.
Physical organic chemistry: The study of how molecular structure and physical principles govern organic reactions. The function belongs to efforts to quantify reaction behavior in strongly acidic solutions.
Activity coefficient: A factor that relates a species' effective thermodynamic concentration to its measured concentration. Nonideal activity coefficients explain why concentrated media cannot be described by concentration alone.
Acid dissociation constant: An equilibrium constant that quantifies an acid's dissociation into a proton and its conjugate base. The indicator equilibrium behind H₀ is expressed through an acidity constant.

Linked from 12 pages