Unusual reference states for chemical potentials

The chemical potential of a component in a system is customarily defined as the Gibbs energy of transferring of one mole of this component from a unary reservoir to the system at constant temperature and pressure. The reservoir must contain the component of interest, but there are no physical justifications forbidding a presence of other components in it. A situation when a reservoir is multicomponent rather than unary warrants an analysis. The present study illustrates how the chemical potential of B in a binary A–B substitutional solution can be calculated with respect to the oxide BO. It is demonstrated that the usage of stoichiometric compounds as reference states leads to equilibrium criteria and necessitates an existence of at least one additional reservoir.