Studies in some ‘isodielectric’ media. Part III. Standard potentials of M/M+(M = Li, Na, and K) electrodes in methanol–propylene glycol

Abstract

Standard potentials (_sE°) of M/M⁺(M = Li, Na, or K) electrodes have been determined at 25° in the ‘isodielectric’ methanol–propylene glycol solvent system from the measured e.m.f. values of the cell M_y(Hg)/MBr(m), solvent/AgBr–Ag. These potentials as well as the known (_sE°) values of the AgX–Ag (X = Cl or Br) electrode furnished the standard potentials (_sE_cell°) of the complete cells comprising the M/M⁺ and AgX–Ag electrodes. The standard free-energy changes ΔG_t°(MX) accompanying the transfer of MX from methanol to the other solvents have been computed and are found to depend chiefly on the ion–dipole interactions, as the ‘electrostatic effect’ is relatively small in this solvent system. The values of ΔG_t°(i) for the individual ions, computed from the ‘simultaneous extrapolation’ of the observed linear plots of ΔG_t°(MCl) and ΔG_t°(KCl–MCl) against (r_M⁺)^–1(M = Li, Na, or K), to (r_M⁺)^–1= 0, suggest that the cations are methanophilic and that the anions are glycophilic. The variation of ΔG_t°(i) values with the solvent composition is found to be in fairly good agreement with what is expected from the consideration of the interaction energies of these ionic species with positive or negative charge centres of the isolated solvent dipoles. Also the values of ΔG_t°(H⁺) for H⁺ seem to indicate that methanol is more ‘basic’ than propylene glycol.