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 Δ_o^wφ_1/2 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 ΔE_p=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 ΔE_p=0.060 V for the peak observed ΔE_p 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.