Comparative analysis of alkali and alkaline-earth cation transfer assisted by monensin across the water ∣ 1,2-dichloroethane interface

In the first part of this paper the electrochemical transfer of alkali cations (M+) assisted by monensin (HX) across the water ∣ 1,2-dichloroethane (DCE) interface at pH<5, combined with a chemical exchange reaction at 5<pH<9, is proposed as the only mechanism responsible for the transfer of these cations. At pH>9 the current is voltammetrically negligible. An equation for the dependence of Δowφ1/2 on pH and Na+ concentration is developed. In the second part of the paper the transfer of alkaline earth cations assisted by the same ionophore is studied. The electrochemical reactions (Me(w)2++HX(o)⇄MeHX2+(o)) and (Me(w)2++X−(o)⇄MeX+(o)) are responsible for the peaks observed at pH<5.0 and at pH>9.0, respectively. As expected, in both cases ΔEp=0.030 V while at intermediate pH the electrochemical exchange reaction (Me2+(w)+HX(o)⇄MeX+(o)+H+(w)) is proposed. The net charge transfer of +1 at the interface accounts for ΔEp=0.060 V for the peak observed ΔEp in agreement with the hyper-Nernstian slope of 60 mV found for Ca2+ and Ba2+ ISE based on antibiotics with carboxylic groups at intermediate pH values. The higher selectivity for Ba2+ and the tendency in selectivity at alkaline pH found in the ISEs are also observed and thus explained according to the mechanism proposed.