Acidity function

Acids and bases
Acceptor number Acid Acid–base reaction Acid–base homeostasis Acid strength Acidity function Amphoterism Base Buffer solutions Dissociation constant Donor number Equilibrium chemistry Extraction Hammett acidity function pH Proton affinity Self-ionization of water Titration Lewis acid catalysis Frustrated Lewis pair Chiral Lewis acid
Acid types
Brønsted–Lowry Lewis Mineral Organic Oxide Strong Superacids Weak Solid
Base types
Brønsted–Lowry Lewis Organic Oxide Strong Superbases Non-nucleophilic Weak
v t e

An acidity function is a measure of the acidity of a medium or solvent system,^[1][2] usually expressed in terms of its ability to donate protons to (or accept protons from) a solute (Brønsted acidity). The pH scale is by far the most commonly used acidity function, and is ideal for dilute aqueous solutions. Other acidity functions have been proposed for different environments, most notably the Hammett acidity function, H₀,^[3] for superacid media and its modified version H₋ for superbasic media. The term acidity function is also used for measurements made on basic systems, and the term basicity function is uncommon.

Hammett-type acidity functions are defined in terms of a buffered medium containing a weak base B and its conjugate acid BH⁺:

${\displaystyle H_{0}={\rm {p}}K_{\rm {a}}+\log {{c_{\rm {B}}} \over {c_{\rm {BH^{+}}}}}}$

where pK_a is the dissociation constant of BH⁺. They were originally measured by using nitroanilines as weak bases or acid-base indicators and by measuring the concentrations of the protonated and unprotonated forms with UV-visible spectroscopy.^[3] Other spectroscopic methods, such as NMR, may also be used.^[2][4] The function H₋ is defined similarly for strong bases:

${\displaystyle H_{-}={\rm {p}}K_{\rm {a}}+\log {{c_{\rm {B^{-}}}} \over {c_{\rm {BH}}}}}$

Here BH is a weak acid used as an acid-base indicator, and B⁻ is its conjugate base.