Acidity function
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An acidity function is a measure of the
acidity In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a ...
of a medium or solvent system, usually expressed in terms of its ability to donate protons to (or accept protons from) a
solute In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...
( Brønsted acidity). The pH scale is by far the most commonly used acidity function, and is ideal for dilute
aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be r ...
s. Other acidity functions have been proposed for different environments, most notably the
Hammett acidity function The Hammett acidity function (''H''0) is a measure of acidity that is used for very concentrated solutions of strong acids, including superacids. It was proposed by the physical organic chemist Louis Plack Hammett and is the best-known acidity fu ...
, ''H''0, for
superacid In chemistry, a superacid (according to the classical definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (), which has a Hammett acidity function (''H''0) of −12. According to the modern definition, a superaci ...
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 A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...
BH+: ::H_0 = K_ + \log where p''K''a is the
dissociation constant In chemistry, biochemistry, and pharmacology, a dissociation constant (K_D) is a specific type of equilibrium constant that measures the propensity of a larger object to separate (dissociate) reversibly into smaller components, as when a complex ...
of BH+. They were originally measured by using
nitroaniline The term nitroaniline in chemistry refers to a derivative of aniline (C6H5NH2) containing a nitro group (—NO2) There are three simple nitroanilines of formula C6H4(NH2)(NO2) which differ only in the position of the nitro group: * 2-Nitroaniline ...
s as weak bases or
acid-base indicator A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hence ...
s 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_- = K_ + \log Here BH is a weak acid used as an acid-base indicator, and B is its conjugate base.


Comparison of acidity functions with aqueous acidity

In dilute aqueous solution, the predominant acid species is the hydrated hydrogen ion H3O+ (or more accurately (OH2)nsup>+). In this case ''H''0 and ''H'' are equivalent to pH values determined by the buffer equation or Henderson-Hasselbalch equation.
However, an ''H''0 value of −21 (a 25% solution of SbF5 in HSO3F)Jolly, William L. (1991). ''Modern Inorganic Chemistry'' (2nd Edn.). New York: McGraw-Hill. {{ISBN, 0-07-112651-1. p. 234. does not imply a hydrogen ion concentration of 1021 mol/dm3: such a "solution" would have a density more than a hundred times greater than a
neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...
. Rather, ''H''0 = −21 implies that the reactivity (
protonating In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid, ...
power) of the solvated hydrogen ions is 1021 times greater than the reactivity of the hydrated hydrogen ions in an aqueous solution of pH 0. The actual reactive species are different in the two cases, but both can be considered to be sources of H+, i.e. Brønsted acids.
The hydrogen ion H+ ''never'' exists on its own in a condensed phase, as it is always solvated to a certain extent. The high negative value of ''H''0 in SbF5/HSO3F mixtures indicates that the solvation of the hydrogen ion is much weaker in this solvent system than in water. Other way of expressing the same phenomenon is to say that SbF5·FSO3H is a much stronger proton donor than H3O+.


References

Acids Chemical properties Solvents