Salt Effect on the Transfer of Ions at Liquid/Liquid Interface Studied by Polarography With Aqueous Electrolyte Dropping Electrode

Abstract

-The ion transfer polarography was applied to investigate the dissolved state of a diluted ion, i Z, which is rather hydrophobic, in an aqueous solution in the presence of concentrated hydrophilic ions. The effect of coexisting ions on the dissolved states of iZ was evaluated based on the electrostatic and non-electrostatic contributions on the solvation energy of i z and ion-pair formation of iz with a coexisting counter ion. The change in the electrostatic contribution was attributed to the dehydration of iz resulted from the decrease of water activity due to the hydration of the coexisting ions. The change in the non-electrostatic contribution was explained by the variation of the cavity formation energy and the dispersion force with the change of the concentration of coexisting ions. The ion-pair formation of iZ with the coexisting ion is more serious when iZ is more hydrophobic. Key wordsVoltammetry for ion transfer at liquid/liquid interface, salt effect, water activity, ion-pair formation, dispersion forceThe structure of concentrated electrolyte solutions has been studied using Raman spectrum [1-3], NMR [4], neutron diffraction [5] etc.. These methods have been applied to the investigation of the properties of the bulk of the solution, and the dehydration followed by the ion-pair formations in salt solution has been clarified. The methods are not adequate, however, for the investigation of the effect of coexisting salts on the behavior of a diluted ion, iZ, in a solution, The methods employed in such investigations so far have been colorimetry [6] or polarography at the dropping mercury electrode [7]. The voltammetry for the ion transfer at the interface of two immiscible electrolyte solutions, VITIES, has unmatched advantage that the transfer energy, and the number of ions transferred at the aqueous(w)/organic(org) interface can be measured simultaneously [$]. The VITIES is applicable to the transfer of diluted i Z from w to org in the presence of concentrated electrolyte in w.In the present work, half-wave potentials, zV112, for the transfer of various iZ from w to org have been investigated by current scan polarography with the electrolyte solution dropping electrode which is the one of the branch of VITIES, and the effects of kind of coexisting salt and their concentration, cs~lt, on the dissolved state of iZ are discussed based on AV112. EXPERIMENTAL Polarogruphic measurementThe current-scan polarograms were recorded according to the procedure described previously [S]. As coexisting salts which worked also as supporting electelectrolytes, SE, in w, M C12, Mg(NO )2: CaC12, CaBr2, Ca(N03)2 andSrCl2 were examined. The or ptoYd was 2-nitroProPane, NP, into which 0.03 M ,~ crystalviolet tetraphenylborate, CW•TPhB-, was added as SE. The dielectric constant (e) of NP is high enough to dissolve and dissociate the ions under investigation. Since the density of NP is smaller than w, the descending water dropping electrode [S] was employed in the present work. When iZ were alkali metal, alkaline eart...