Gibbs free energies of transfer of individual ions from water into water-urea mixtures

Abstract

The use of a new method for the determination of the absolute electrode potentials and the thermodynamics of individual ions in various solvent systems has been extended to solvents of higher dielectric constant than water, formed by adding structure-breaking urea to water. Standard e.m.f. values are reported at 25 °C for the cell: Pt,H 2 (g, 1 atm)/HX (m), water-urea/AgX, Ag, where X = C1, Br and I, in eight different aqueous solvents containing up to 40 wt.% urea. The standard e.m.f. values were used to determine not only the standard transfer Gibbs free energy Δ G° for halogen acids from water into water-urea mixtures but also the standard absolute potentials E° of the hydrogen and the Ag,AgX electrodes in these solvents as well as the values of Δ G t° for the individual H + and X − ions. These data, together with the reported values of Δ G t° (MCl), enabled us to evaluate values of E° for the M/M + (M = Li, Na, K, Rb and Cs) electrodes, the radii of the solvated cations and the extent of their solvation in such media, and values of Δ G° t, for the individual M + ions. The results are discussed and compared with those in various aqueous solvent systems.