structural:reformatting, stylistic:tone, structural:scholar_edition
Measures of acid strength

While the p K a {\displaystyle \mathrm {p} K_{{\ce {a}}}} value measures the tendency of an acidic solute to transfer a proton to a standard solvent (most commonly water or DMSO), the tendency of an acidic solvent to transfer a proton to a reference solute (most commonly a weak aniline base) is measured by its Hammett acidity function, the H 0 {\displaystyle H_{0}} value. Although these two concepts of acid strength often amount to the same general tendency of a substance to donate a proton, the p K a {\displaystyle \mathrm {p} K_{{\ce {a}}}} and H 0 {\displaystyle H_{0}} values are measures of distinct properties and may occasionally diverge. For instance, hydrogen fluoride, whether dissolved in water (p K a {\displaystyle \mathrm {p} K_{{\ce {a}}}} = 3.2) or DMSO (p K a {\displaystyle \mathrm {p} K_{{\ce {a}}}} = 15), has p K a {\displaystyle \mathrm {p} K_{{\ce {a}}}} values indicating that it undergoes incomplete dissociation in these solvents, making it a weak acid. However, as the rigorously dried, neat acidic medium, hydrogen fluoride has an H 0 {\displaystyle H_{0}} value of –15, making it a more strongly protonating medium than 100% sulfuric acid and thus, by definition, a superacid. (To prevent ambiguity, in the rest of this article, "strong acid" will, unless otherwise stated, refer to an acid that is strong as measured by its p K a {\displaystyle \mathrm {p} K_{{\ce {a}}}} value (p K a {\displaystyle \mathrm {p} K_{{\ce {a}}}} < –1.74). This usage is consistent with the common parlance of most practicing chemists.)