Howard DeVoe developed a thermodynamic treatment of solutions that distinguishes chemical potentials, reference states, and activity coefficients. An ideal-mixture expression supplies a reference for representing nonideal behaviour.
An activity coefficient therefore compares a real chemical potential with a stated ideal convention. Changing the convention changes the numerical representation without changing the underlying equilibrium condition.[ref]
Identity source: www2.chem.umd.edu/thermobook/v10-screen.pdf
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