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.
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.
ARUP Laboratories, 500 Chipeta Way, Salt Lake City, Utah 84108 (USA). alan.rockwood@aruplab.com.|Department of Pathology, University of Utah School of Medicine, Salt Lake City, Utah 84132 (USA). alan.rockwood@aruplab.com.
Alan L
AL
Rockwood
Gibbs free energy; charge transfer; contact potential; single-ion activity; thermodynamics