Anacidity function is a measure of theacidity of a medium or solvent system,[1][2] usually expressed in terms of its ability to donate protons to (or accept protons from) asolute (Brønsted acidity). ThepH scale is by far the most commonly used acidity function, and is ideal for diluteaqueous solutions. Other acidity functions have been proposed for different environments, most notably theHammett acidity function,H0,[3] forsuperacid media and its modified versionH− forsuperbasic media. The term acidity function is also used for measurements made on basic systems, and the termbasicity function is uncommon.
Hammett-type acidity functions are defined in terms of abuffered medium containing a weak base B and itsconjugate acid BH+:
where pKa is thedissociation constant of BH+. They were originally measured by usingnitroanilines as weak bases oracid-base indicators and by measuring the concentrations of the protonated and unprotonated forms withUV-visible spectroscopy.[3] Other spectroscopic methods, such asNMR, may also be used.[2][4] The functionH− is defined similarly for strong bases:
Here BH is a weak acid used as an acid-base indicator, and B− is its conjugate base.
In dilute aqueous solution, the predominant acid species is thehydrated hydrogen ion H3O+ (or more accurately [H(OH2)n]+). In this caseH0 andH− are equivalent to pH values determined by the buffer equation orHenderson-Hasselbalch equation.
However, anH0 value of −21 (a 25% solution ofSbF5 inHSO3F)[5] 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 aneutron star. Rather,H0 = −21 implies that the reactivity (protonating 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 alwayssolvated to a certain extent. The high negative value ofH0 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+.
