Thermodynamics of ions in solution

By adopting the foregoing convention, therefore, it is possible to define the standard free energy of formation of a positive ion from the element M, as — z Em, where Em is the standard (oxidation) potential on the hydrogen scale. 

Similarly, for an electrode involving negative ions, the reaction in the cell 

Provided the potential of an electrode of the type M, or A, A, where M and A are elements, can be measured, the standard free energy of formation of the and A ions, respectively, can be readily determined. For this purpose the data in Table XXXIX may be used. If the ion is one which cannot form a stable electrode of the simple type indicated, indirect methods must be adopted sometimes electrode potential or equilibrium data can be employed, but in other cases recourse must be had to heat content and entropy values. For example, the potential of the Fe, Fe+ electrode cannot be determined experimentally because the system is unstable however, the potentials of the Fe, Fe++ and the Pt, Fe++, Fe++ + systems are known, viz., + 0.441 and — 0.771 volt at 25°, respectively, and from these the free energy of formation of the ferric ion can be determined. Thus, for the process 

The calculation of free energies of formation of ions from equilibrium data may be illustrated by reference to the iodate (lOi ) ion. At 25° C the equilibrium constant K for the system 

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The coordination number collective variable is a good choice for modeling solvent thermodynamics of ions in solution. The coordination number of species A with respect to species B is defined using a Fermi-Dirac function as... 

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