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

Abstract

Values of E_j,ion, the ionic component of the liquid-junction potential E_j, 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 E_j,ion and our improved equation, which accounts for the variation of the ionic transport numbers t and chemical potentials G^o in the interphase region, were used. Significant differences between the two equations were observed for systems where the t and Δ_tG° functions dnisplayed extrema against the mixed-solvent composition. The highest E_j,ion value was 395 mV, for the HCl,H₂O | HCl,Me₂SO junction. Values of E_j,s,the solvent component of E_j, were calculated by subtracting the corresponding E_j,ion values from the total E_j, 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 E_j,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 E_j,s value was -201 mV, observed for the junction HCl,H₂O|HCl,100% Me₂SO.