Acidity function

<h2 id="comparison-of-acidity-functions-with-aqueous-acidity">Comparison of acidity functions with aqueous acidity</h2>
In dilute aqueous solution, the predominant acid species is the <a href="/facts/Hydronium_ion/HzE3BGYJ">hydrated hydrogen ion</a> H3O+ (or more accurately [H(OH2)n]+). In this case H0 and H− are equivalent to pH values determined by the buffer equation or <a href="/facts/Henderson-Hasselbalch_equation/LrduPhHB">Henderson-Hasselbalch equation</a>.
However, an H0 value of −21 (a 25% solution of <a href="/facts/Antimony_pentafluoride/nTqasUkK">SbF5</a> in <a href="/facts/Fluorosulfonic_acid/HJng4DqN">HSO3F</a>)<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a> 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 <a href="/facts/Neutron_star/4enfoxBA">neutron star</a>. Rather, H0 = −21 implies that the reactivity (<a href="/facts/Protonating/q5Gf9z0M">protonating</a> 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. <a href="/facts/Br%C3%B8nsted_acid/bqxQnJoX">Brønsted acids</a>. The hydrogen ion H+ never exists on its own in a condensed phase, as it is always <a href="/facts/Solvation/JVM6j8R1">solvated</a> 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+.

<h2 id="references">References</h2>

<ol>
<li id="fn:1">IUPAC Commission on Physical Organic Chemistry (1994). "Glossary of Terms used in Physical Organic Chemistry." Pure Appl. Chem. 66:1077–1184. "Acidity function. Archived 2013-08-04 at the Wayback Machine" Compendium of Chemical Terminology. <a href="/wiki/IUPAC" target="_blank">/wiki/IUPAC</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Rochester, Colin H. (1970). Acidity functions. London: Academic Press. ISBN 0-12-590850-4. OCLC 93620. <a href="0-12-590850-4" target="_blank">0-12-590850-4</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Hammett, Louis Plack (1940). Physical Organic Chemistry: Reaction Rates, Equilibria, and Mechanisms. McGraw-Hill Book Company, Incorporated. <a href="https://books.google.com/books?id=Ns_QAAAAMAAJ" target="_blank">https://books.google.com/books?id=Ns_QAAAAMAAJ</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Hammett, Louis Plack (1940). Physical Organic Chemistry: Reaction Rates, Equilibria, and Mechanisms. McGraw-Hill Book Company, Incorporated. <a href="https://books.google.com/books?id=Ns_QAAAAMAAJ" target="_blank">https://books.google.com/books?id=Ns_QAAAAMAAJ</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Rochester, Colin H. (1970). Acidity functions. London: Academic Press. ISBN 0-12-590850-4. OCLC 93620. <a href="0-12-590850-4" target="_blank">0-12-590850-4</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Cox, Robin A.; Yates, Keith (2011-02-05). "Acidity functions: an update". Canadian Journal of Chemistry. 61 (10): 2225–2243. doi:10.1139/v83-388. <a href="https://cdnsciencepub.com/doi/abs/10.1139/v83-388" target="_blank">https://cdnsciencepub.com/doi/abs/10.1139/v83-388</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Jolly, William L. (1991). Modern Inorganic Chemistry (2nd Edn.). New York: McGraw-Hill. ISBN 0-07-112651-1. p. 234. <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
</ol>

Acidity function open-in-new

Acidity function