The medium effect on ionic species B due to transfer from solvent S 1 to solvent S 2 (number) is defined by \[R\ T\ \ln \gamma _{\text{S}_{1}}^{\text{S}_{2}}\left(\text{B}\right)=\mu _{\text{B}}^{\text{o},\text{S}_{2}}- \mu _{\text{B}}^{\text{o},\text{S}_{1}}\] 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.