Acidity function

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Acids and bases
Diagrammatic representation of the dissociation of acetic acid in aqueous solution to acetate and hydronium ions.
Acid types
Base types

An acidity function is a measure of the

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

conjugate acid
BH+:

where pKa is the

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:

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

Henderson-Hasselbalch equation
.
However, an H0 value of −21 (a 25% solution of
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 H0 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

  1. Compendium of Chemical Terminology
    .
  2. ^
    OCLC 93620
    .
  3. ^ a b Hammett, Louis Plack (1940). Physical Organic Chemistry: Reaction Rates, Equilibria, and Mechanisms. McGraw-Hill Book Company, Incorporated.
  4. doi:10.1139/v83-388
    .
  5. ISBN 0-07-112651-1
    . p. 234.
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