Hammett acidity function
The Hammett acidity function (H0) is a measure of acidity that is used for very concentrated solutions of strong
In highly concentrated solutions, simple approximations such as the
Definition
The Hammett acidity function, H0, can replace the pH in concentrated solutions. It is defined using an equation[4][5][6] analogous to the Henderson–Hasselbalch equation:
where log(x) is the
Hammett also pointed out the equivalent form
where a is the activity, and the γ are thermodynamic
Although the Hammett acidity function is the best known acidity function, other acidity functions have been developed by authors such as Arnett, Cox, Katrizky, Yates, and Stevens.[3]
Typical values
On this scale, pure
H0 for some concentrated acids:[10]
- Helonium: −63
- Fluoroantimonic acid (1990): −23 > H0 > −28
- Magic acid (1974): −23
- Carborane superacids: H0 < −18.0
- Fluorosulfuric acid (1944): −15.1
- Hydrogen fluoride: −15.1
- Trifluoromethanesulfonic acid (1940): −14.9
- Perchloric acid: −13
- Sulfurochloridic acid: −13.8; −12.78[11]
- Sulfuric acid: −12.0
For mixtures (e.g., partly diluted acids in water), the acidity function depends on the composition of the mixture and has to be determined empirically. Graphs of H0 vs mole fraction can be found in the literature for many acids.[3]
References
- ^ L. P. Hammett and A. J. Deyrup (1932) J. Am. Chem. Soc. 54, 2721
- ^ L. P. Hammett (1940). Physical Organic Chemistry. (McGraw-Hill)
- ^ ISBN 0-85404-464-7.
- ^ a b William L. Jolly, Modern Inorganic Chemistry (McGraw-Hill 1984), p.202-3
- ^ F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (5th edition, Wiley-Interscience 1988), p.107-9
- ^ G. L. Miessler and D. A. Tarr, Inorganic Chemistry (2nd edition, Prentice-Hall 1999), p.170-1
- ^ What do you mean pH = -1? Super Acids Archived 2006-09-23 at the Wayback Machine
- ^ Liang, Jack Joan-Nan, "The Hammett Acidity Function for Hydrofluoric Acid and some related Superacid Systems" (1976). Open Access Dissertations and Theses. Paper 3850.
- )
- ^ The Chemistry of Nonaqueous Solvents VB: Acid and Aprotic Solvents Ed J.J. Lagowski, pp139, Academic Press, London, 1978