A New Method for the Estimation of Transfer Activity Coefficients of Ions From the E.M.F. of Cells With Liquid Junction

Abstract

Transfer activity coefficients of the tetraalkylammonium ions, log_mγ_R, where R⁺ = Me₄N⁺, Et₄N⁺, Pr₄N⁺ and Bu₄N⁺, and of the picrate ion, log_mγ_Pic, between pairs of dipolar aprotic solvents, S₁ → S₂, were evaluated by combining the e.m.f . of cells

Ag(cryst.)|AgClO₄|RX|AgClO₄|Ag( cryst .)

                                 S₁        S₂

with the calculated liquid-junction potentials, E_j. The salt-bridge electrolyte RX was RClO₄ and R Pic for the determinations of log_mγ_R and log_mγ_Pic, respectively, and the solvents were: acetonitrile ( MeCN ), N,N- dimethylformamide (HCONMe₂), dimethyl sulfoxide (Me₂SO) and propylene carbonate ( pcar ). The log_mγ_R values showed a systematic variation with the AgClO₄ concentration, apparently caused by the concentration dependence of the solvent component of the liquid-junction potential, E_j,_s, neglected in the calculations. However, the above concentration dependence was virtually eliminated from the values of log_mγ_Pic when they were calculated with the aid of our concentration-dependent log_mγ_R data. For a number of our cells it was possible to evaluate the E_j,_s, with the aid of literature data. The solvent component of E_j was found to be appreciable and varied both with the AgClO₄ and the RX concentration. The E_j,_s, was particularly large for cells containing pcar , with a maximum of -172 mV for the pcar | MeCN junction.