Acidity functions: an update

Abstract

The subject of acidity functions is reviewed, with emphasis on modern developments. H0 and other scales determined in acid media, using the Hammett cancellation assumption (zero'th order approximation) are discussed briefly. The extension to strongly basic systems is also documented. Literature references to acidity function scales in aqueous acid, aqueous acid/organic solvent, and aqueous base media are tabulated, together with those in aqueous base/organic solvent mixtures at both variable and fixed base concentration. Over 400 acidity functions are referenced, including several electrochemical scales. Various approaches to developing universal functions from the data are discussed. Proton activity scales, based on the Yates–McClelland method of [Formula: see text] determination, and on electrochemical HGF scales, are prominent among these. Carpentier's method is mentioned, and the first-order approximation, assumed linearity among log activity coefficient ratios, is used to introduce the Bunnett–Olsen and excess acidity (X-function) methods. A brief critical discussion of which method to use is provided. Future developments anticipate the second-order approximation, with a quadratic dependence of log ionization ratios upon X-functions. The review cites 337 individual references.