Acidity function

An acidity function measures a strongly acidic or basic medium’s protonating ability, often from the equilibrium behavior of indicator molecules. Unlike pH, it can describe media too concentrated or nonaqueous for ordinary pH measurements.

Hammett acidity function: An acidity function, H₀, based on the protonation equilibria of weak indicator bases in concentrated acidic media. It is the best-known acidity function and the standard example of indicator-based measurement.
pH: A logarithmic measure related to hydrogen-ion activity in aqueous solutions. Acidity functions extend acidity comparisons into media where pH is difficult to interpret or measure.
Superacid chemistry: The study of reactions and species in media with exceptionally high protonating power. Acidity functions provide a practical scale for comparing the strengths of superacidic media.
Louis Plack Hammett: An American physical organic chemist known for work on reaction rates, substituent effects, and acidity functions. Hammett introduced the acidity function that became the best-known measure for concentrated acids.
Indicator method: A measurement method that infers a medium’s properties from a compound’s observable response. Acidity functions are commonly determined by measuring indicator protonation equilibria.
Basicity function: A measure of a medium’s ability to accept a proton, often inferred from indicator equilibria. It describes proton-accepting power, opposite to the protonating ability measured by acidity functions.
Carbocation: An organic ion in which a carbon atom carries a positive charge. Strong-acid media characterized by acidity functions can generate and stabilize carbocations.
Hammett equation: A linear free-energy relationship connecting substituent effects to reaction rates or equilibria in aromatic compounds. Hammett’s broader program in physical organic chemistry provided context for quantifying medium effects.
Protonation: The addition of a proton to an atom, molecule, or ion. The equilibrium between protonated and unprotonated indicators underlies the measurement.
Acid dissociation constant: An equilibrium constant that quantifies an acid’s tendency to donate a proton in a specified solvent. It characterizes a particular acid, whereas an acidity function characterizes the protonating power of a medium.

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