Hammett acidity function

To quantify the acidity of superacids led Louis P. Hammett and Alden J. Deyrup 1932, the so-called Hammett function a, since the pH scale by Sørensen suitable only for dilute solutions and the Autoprotolysekonstante is limited to water. ${\displaystyle H_{0}}$

The degree of protonation of a weak organic base in the super acid medium is regarded as a measure of the strength of a super acid. ${\displaystyle {\frac {\left[BH^{+}\right]}{\left[B\right]}}}$ ${\displaystyle B}$

From the law of mass action for equilibrium we can derive: ${\displaystyle BH^{+}\rightleftharpoons B+H^{+}}$

${\displaystyle K_{BH^{+}}={\frac {a_{H^{+}}\cdot a_{B}}{a_{BH^{+}}}}}$

with the activity ${\displaystyle a=c\cdot f}$

${\displaystyle \rightarrow {\frac {c_{BH^{+}}}{c_{B}}}={\frac {1}{K_{BH^{+}}}}\cdot a_{H^{+}}\cdot {\frac {f_{B}}{f_{BH^{+}}}}}$

${\displaystyle \rightarrow H_{0}\equiv -\log a_{H^{+}}\cdot {\frac {f_{B}}{f_{BH^{+}}}}=-\log K_{BH^{+}}+\log {\frac {c_{B}}{c_{BH^{+}}}}}$

If one assumes that is the same for different bases in a certain solution, then, if one has determined experimentally, the acid strength can be determined. The experimental determination of can be carried out spectrophotometrically , conductometrically , electrochemically , kinetically , by IR spectroscopy or by NMR spectroscopy . ${\displaystyle {\frac {f_{B}}{f_{BH^{+}}}}}$ ${\displaystyle {\frac {\left[BH^{+}\right]}{\left[B\right]}}}$ ${\displaystyle H_{0}=pK_{BH^{+}}-\log {\frac {\left[BH^{+}\right]}{\left[B\right]}}}$ ${\displaystyle {\frac {\left[BH^{+}\right]}{\left[B\right]}}}$

By Ronald J. Gillespie applies to super acids : . ${\displaystyle H_{0}\leq -12}$

Depending on the composition, values ​​from −15.1 (HSO ₃ F, aHF (a stands for anhydrous = anhydrous)) to −27 (HSO ₃ F + SbF ₅ ( magic acid ) in a ratio of 1: 9) are possible. ${\displaystyle H_{0}=}$ ${\displaystyle H_{0}\approx }$

See also

• PH value
• Law of mass action
• Dissociation constant
• Acid constant

Individual evidence

1. ↑ LP Hammett, AJ Deyrup, J. Am. Chem. Soc. 1932 , 54 , 2721. doi: 10.1021 / ja01346a015 .
2. ↑ ^{a b} G. A. Olah, GKS Prakash, J. Sommer, Superacids , John Wiley & Sons, New York, 1985 .
3. ↑ M. Kilpatrick, FE Luborsky, J. Am. Chem. Soc. 1954 , 76 , 5865. doi: 10.1021 / ja01651a098 .
4. ↑ M. Couzi, J.-C. Cornut, PV Huong, J. Chem. Phys. 1972 , 56 , 426. doi: 10.1063 / 1.1676884 .
5. ^ LP Hammett, Chem. Rev. 1935 , 16 , 67. doi: 10.1021 / cr60053a006 .
6. ↑ Gillespie, RJ, Peel TE, Robinson EA, J. Am. Chem. Soc. 1971 , 93 , 5083. doi: 10.1021 / ja00749a021 .
7. ^ C. Janiak, TM Klapötke , H.-J. Meyer, E. Riedel, Modern Inorganic Chemistry , 2nd edition, de Gruyter, Berlin, 2004 .