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rapid estimation of ion-pair dissociation constants

Project description


rapid estimation of ion-pair dissociation constants


quick-fuoss is an open-source Python 3 program designed to allow for quick estimation of ion-pair dissociation constants. To run, quick-fuoss requires only the name of the cation and anion and the dielectric constant of the solvent in question. Alternatively, .xyz files can be supplied.

quick-fuoss requires a working installation of cctk, which does most of the heavy lifting.


As the name implies, quick-fuoss uses the Fuoss model for modelling association/dissociation of idealized spherical ions in implicit solvent.1 This approach is purely Coulombic and neglects any specific ion/ion interactions, as well as inner-sphere solvent effects (e.g. solvent coordination to transition metals). Nevertheless, these approximations are reasonably satisfied for many common ion pairs.

This model has been used to correct pKa values in relatively non-polar solvents: the present implementation is based on that literature.2,3

Please note that the current implementation of quick-fuoss only supports singly charged ions!


quick-fuoss can be used starting from either the names of the ions in question (requires Internet access) or .xyz files. Some examples are shown below:

# Kd for sodium chloride in a solvent with dielectric constant 40
$ python sodium chloride 40

# the same, but at -78 ºC
$ python --temp 195 sodium chloride 40

# Kd of the chloride salt of in a solvent with dielectric constant 80
$ python chloride 80

# Kd of the salt formed by combining and in a solvent with dielectric constant 12
$ python 12


The calculated values are in reasonable agreement with reported values,4 as seen below:

$ python tetraisoamylammonium nitrate 8.5
Reading ion #1 from rdkit...
Reading ion #2 from rdkit...
Dissociation constant:	0.00004930 M
Ionization energy: 5.873 kcal/mol
$ python tetraisoamylammonium nitrate 11.9
Reading ion #1 from rdkit...
Reading ion #2 from rdkit...
Dissociation constant:	0.00094706 M
Ionization energy: 4.122 kcal/mol

To test that the program is working correctly, simply run:

$ python
All tests successful! (9 tests run in 14.91 seconds)


  1. Fuoss, R. M. Ionic Association III: The Equilibrium between Ion Pairs and Free Ions. J. Am. Chem. Soc. 1958, 80, 5059.
  2. Abdur-Rashid, K. et al. An Acidity Scale for Phosphorus-Containing Compounds Including Metal Hydrides and Dihydrogen Complexes in THF: Toward the Unification of Acidity Scales. J. Am. Chem. Soc. 2000, 122, 9155.
  3. Paenurk, E. et al. A unified view to Brønsted acidity scales: do we need solvated protons? Chem. Sci. 2017, 8, 6964.
  4. Fuoss, R. M; Kraus, C. A. Properties of Electrolytic Solutions. III. The Dissociation Constant. J. Am. Chem. Soc. 1933, 55, 1019.


quick-fuoss was written by Corin Wagen (Harvard University). Please email with any questions or bug reports.

How to Cite:

Wagen, C.C. quick-fuoss 2020,


This project is licensed under the Apache License, Version 2.0. Please see LICENSE for full terms and conditions.

Copyright 2020 by Corin Wagen

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