Conventions and General Formulation

In order to aid in the investigation of the dependence of the chemical potentials of the components of a liquid phase on composition, it is convenient to define an ideal substance. An ideal substance is defined in such a way that its chemical potential has a simple functional dependence on a concrete composition variable. Under appropriate conditions, the properties of a large class of real substances must be adequately represented by the properties of the ideal substance. It is found by experiment and from molecular considerations that a suitable form of the chemical potential of an ideal component i can be written 

The excess chemical potential of component /, /(j represents the difference between the chemical potential of component i in the real solution and the chemical potential of / considered as an ideal component. Thus, can be written as 

The activity coefficient of component i,fi, may be defined in terms of its excess chemical potential in the form 

It is sometimes convenient to make use of the activity of component i, a defined by the relation 

In order completely to determine the excess chemical potential, the activity coefficient, or the activity, the convention used to define the reference value of the chemical potential and the composition units used, must be specified. 

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