Evaluation of Ionic and Solvent Components of the Liquid-Junction Potential Between Aqueous and Several Aquo-Organic Solutions

Abstract

Values of Ej,ion, the ionic component of the liquid-junction potential Ej, were calculated for the HCl,H2O |HCl,S2 and KCl,H2O | KCl,S2 junctions, where S2 was EtOH-H2O, HCONMe2- H2O and Me2SO-H2O solvents, and for the AgNO3,H2O | AgNO3,MeCN-H2O junction, over the entire mixed-solvent ranges. Both the old approximate equation for Ej,ion and our improved equation, which accounts for the variation of the ionic transport numbers t and chemical potentials Go in the interphase region, were used. Significant differences between the two equations were observed for systems where the t and ΔtG° functions dnisplayed extrema against the mixed-solvent composition. The highest Ej,ion value was 395 mV, for the HCl,H2O | HCl,Me2SO junction. Values of Ej,s,the solvent component of Ej, were calculated by subtracting the corresponding Ej,ion values from the total Ej, which was evaluated in each case from the e.m.f , of a cell with the liquid junction of interest and the transfer activity coefficient of the electroactive ion, estimated by the tetraphenylborate assumption. The magnitude of Ej,s was significant for the junctions between H2O and most of the solvents, and was particularly large for those involving dipolar aprotic solvents and highly solvated ions. The maximum Ej,s value was -201 mV, observed for the junction HCl,H2O|HCl,100% Me2SO.