4.5 Article

Meaning and Measurability of Single-Ion Activities, the Thermodynamic Foundations of pH, and the Gibbs Free Energy for the Transfer of Ions between Dissimilar Materials

Journal

CHEMPHYSCHEM
Volume 16, Issue 9, Pages 1978-1991

Publisher

WILEY-V C H VERLAG GMBH
DOI: 10.1002/cphc.201500044

Keywords

charge transfer; contact potential; Gibbs free energy; single-ion activity; thermodynamics

Funding

  1. ARUP Institute for Clinical and Experimental Pathology

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Considering the relationship between concentration and vapor pressure (or the relationship between concentration and fugacity) single-ion activity coefficients are definable in purely thermodynamic terms. The measurement process involves measuring a contact potential between a solution and an external electrode. Contact potentials are measurable by using thermodynamically reversible processes. Extrapolation of an equation to zero concentration and ionic strength enables determination of single-ion activity coefficients. Single-ion activities can be defined and measured without using any extra-thermodynamic assumptions, concepts, or measurements. This method could serve as a gold standard for the validation of extra-thermodynamic methods for determining single-ion activities. Furthermore, it places the concept of pH on a thermodynamically solid foundation. Contact potential measurements can also be used to determine the Gibbs free energy for the transfer of ions between dissimilar materials.

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