Free energies of transfer of alkali halides from water to urea-water mixtures at 25�C from EMF measurements

Abstract

The standard potentialssEo of M/M+ (M=Li, Na, K, Rb, and Cs) electrodes in aqueous urea solutions containing 12, 20, 30 and 37% by weight of urea have been determined at 25°C from emf measurements on the cell M(Hg)/MCl (m), solvent/AgCl−Ag, from the activities of metals in amalgams by use of a similar type of cell in water, and from the values ofsEo of the Ag/AgCl electrode determined earlier. The standard free energies of transfer of MCl, ΔG t o (MCl), from water to the mixed solvents, computed by use of these values and those for the Ag−AgCl electrode, rise sharply from Li+ to Na+ but fall from Na+ to K+ and rather sharply from K+ to Cs+ with a maximum at Na+ in all the solvent compositions. This has been attributed to the superimposition of “soft-soft” interactions on the “electrostatic interactions” between the ions and the negative charge centers of the possible hydrogen-bonded solvent complexes in the mixed solvents. Comparison of ΔG t o (i) values for individual ions, obtained by a “simultaneous extrapolation” procedure, with those in aqueous mixtures of methanol,t-butanol, and dimethyl sulfoxide leads to the conclusion that the solvation of these ions in all these solvents is chiefly dictated by the “acid-base” type of ion-solvent interactions.