The Medium Effect for Single Ionic Species

Abstract

The relation of the “ medium effect ” or “ solvent activity coefficient ” to the directly measurable real free energy of transfer is discussed. Measurements of the latter quantity for solutions in aqueous solvents containing methanol, ethanol, dioxan, acetone, ethylene glycol, formamide or dimethyl sulphoxide are reported and discussed. The surface composition of the solvent mixture is accessible and this information can be used to set reasonable limits on the possible value of the “ medium effect ”. Thus it can be shown that some assignments of single ion medium effects based on oversimplified models of solvation are inadequate. The assignment based on the neglect of liquid junction potentials between solvents is often surprisingly good.