medium effect

The medium effect on ionic species B due to transfer from solvent $S 1$ to solvent $S 2$ (number) is defined by

$R T ln γ S 1 S 2 B = μ B o , S 2 − μ B o , S 1$

where $R$ is the gas constant, $T$ is the thermodynamic temperature and $μ B o , S i$ is the standard chemical potential of B in solvent $S i$ (where $i$ = 1 or 2), the reference states being the same in both solvents. $γ S 1 S 2 B$ is not an exactly measurable quantity.

Source:

PAC, 1974, 37, 499 (Electrochemical nomenclature) on page 508