Acidity function

An acidity function is the degree of the acidity of a medium or solvent system, usually expressed in terms of its ability to donate protons to or accept protons from a solute Brønsted acidity. The pH scale is by far the most commonly used acidity function, and is ideal for dilute aqueous solutions. Other acidity functions realize been provided for different environments, almost notably the Hammett acidity function, H₀, for superacid media together with its modified report H₋ for superbasic media. The term acidity function is also used for measurements introduced 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 BH^+:

where pK_a is the dissociation constant of BH^{+. They were originally measured by using nitroanilines as weak bases or acid-base indicators and by measuring the concentrations of the protonated and unprotonated forms with UV-visible spectroscopy. Other spectroscopic methods, such(a) as NMR, may also be used. 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 kind is the Henderson-Hasselbalch equation. However, an H₀ good of −21 a 25% sum of neutron star. Rather, H₀ = −21 implies that the reactivity Brønsted acids. The hydrogen ion H^{+ never exists on its own in a condensed phase, as this is the always solvated to aextent. The high negative expediency 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+.}