The McMillan–Mayer framework and the theory of electrolyte solutions

In electrolyte thermodynamics one often speaks of two thermodynamic frameworks; the Lewis–Randall framework (characterised by temperature, pressure, and mole numbers) and the McMillan–Mayer framework (characterised by temperature, total volume, solute mole numbers, and solvent chemical potential). However, there is only one framework in thermodynamics; the apparent difference between the two ‘frameworks’ is, in electrolyte thermodynamics, due to the change in the pressure caused by the charging process at constant volume and solvent chemical potential.The so-called McMillan–Mayer framework is set in the context of the classical thermodynamics and the use of it is examplified by the Debye–Hckel theory. The so-called McMillan–Mayer framework is superfluous when the thermodynamics of the electrolyte solutions is described by the Helmholtz energy functions.