Standard potentials of M/M+(M = Li, Na, K, Rb or CS) electrodes in dimethyl sulphoxide + water mixtures at 25°C and related ion-solvent interactions in the solvents

Abstract

The standard electrode potentials (_sE°) of M/M⁺(M = Li, Na, K, Rb and Cs) electrodes in aqueous solutions of dimethyl sulphoxide (DMSO)(containing 10, 20, 40 and 60 wt. % DMSO) have been determined at 25°C from e.m.f. measurements of cells of the type: M(Hg)|MCl(m), solvent| AgCl—Ag, obtaining the activity of metals in amalgams with the help of a similar type of cell in water. The free energies of transfer of MCl, δG^°_t(MCl)(mole fraction scale) from water to each of these solvents computed from these values and those of _sE^°_Ag—AgCl obtainable from the literature, rise sharply from Li⁺ to Na⁺, and fall from Rb⁺ to Cs⁺ with a maximum between Na⁺ and Rb⁺. This is considered as due to the superimposition of “soft—soft” interactions on the “electrostatic interactions” between the ions and the negative charge centres of hydrogen-bonded solvent complexes. The ΔG^°_t(i) values for individual ions, obtained by a method of “simultaneous extrapolation” are shown to be largely determined by the “acid-base” type ion-solvent interactions as observed in methanol + water mixtures.