The thermodynamic activity of a chemical species or component (ai) is a part of the corresponding chemical potential μ i , according to the relationship:
where μ0 i is the standard state chemical potential at the temperature and pressure of interest, R is the gas constant, and T is the absolute temperature. The activity appears in many important equations pertinent to chemical thermodynamics and chemical kinetics (q.ν.).
The activity and the corresponding standard state chemical potential is always tied to a concentration scale (e.g. Robinson and Stokes, 1965), generally the molal scale for aqueous solutions and the mole fraction scale for all other condensed solutions. The activity is the product of a concentration on that scale and a corresponding activity coefficient. The total chemical potential (on the left hand side of eqn (A19)) is independent of the concentration scale. Changes in the definition of the standard state chemical potential therefore result in changes in the...
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Wolery, T.J. (1998). Activity and activity coefficients. In: Geochemistry. Encyclopedia of Earth Science. Springer, Dordrecht. https://doi.org/10.1007/1-4020-4496-8_5
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