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Geochemical Reaction ModelingConcepts and Applications$
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Craig M. Bethke

Print publication date: 1996

Print ISBN-13: 9780195094756

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195094756.001.0001

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Activity Coefficients

Activity Coefficients

Chapter:
7 Activity Coefficients
Source:
Geochemical Reaction Modeling
Author(s):

Craig M. Bethke

Publisher:
Oxford University Press
DOI:10.1093/oso/9780195094756.003.0011

Among the most vexing tasks for geochemical modelers, especially when they work with concentrated solutions, is estimating values for the activity coefficients of electrolyte species. To understand in a qualitative sense why activity coefficients in electrolyte solutions vary, we can imagine how solution concentration affects species activities. In the solution, electrical attraction draws anions around cations and cations around anions. We might think of a dilute solution as an imperfect crystal of loosely packed, hydrated ions that, within a matrix of solvent water, is constantly rearranging itself by Brownian motion. A solution of uncharged, nonpolar species, by contrast, would be nearly random in structure. The electrolyte solution is lower in free energy G than it would be if the species did not interact electrically because of the energy liberated by moving ions of opposite charge together while separating those of like charge. The chemical potentials of the species, for the same reason, are smaller than they would be in the absence of electrostatic forces.

Keywords:   Activity coefficient, CHILLER software, Coulomb's law, Davies equation, Deprotonation reactions, Evaporites, Fluid inclusions, GTPLOT software, Ion size parameter, Osmotic coefficient

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