Chemical Potential, Reference State and Partial Molar Volume

The chemical potential of a component in a mixture is defined relative to that of a pure component reference state. The standard expression assumes that the partial molar volume of the component in a liquid or solid mixture is identical to the molar volume of the pure component in the liquid reference state. However, ample experimental evidence shows that partial molar volumes in different phases are not the same in most cases. It will be shown that, in this case, the standard expression for the chemical potential does not adhere to the requirement that the chemical potential be a conservative property. A modification of the standard chemical potential expression is proposed to resolve this issue. The modification affects the Poynting correction in the vapor–liquid equilibrium equation and the osmotic equilibrium equation.