What is "acidity function"

n. (context chemistry English) The relationship between the acidity of a solution and the concentration of its solute.

An acidity function is a measure of the acidity of a medium or solvent system, 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, H, 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:

$H_0 = {\rm p}K_{\rm a} + \log {{c_{\rm B}}\over{c_{\rm BH^+}}}$

where pK 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. Other spectroscopic methods, such as NMR, may also be used. The function H is defined similarly for strong bases:

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

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