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 Delta (w)(o)phi (1/2) on pH and Na+ concentration is developed. In the second part of the gaper the transfer of alkaline earth cations assisted by the same ionophore is studied. The electrochemical reactions (Me-(w)(2+) + HX(o) reversible arrow MeHX(o)2+) and (Me-(w)(2+) + X-(o)(-) reversible arrow MeX(o)+) are responsible for the peaks observed at pH < 5.0 and at pH > 9.0, respectively. As expected, in both cases DeltaE(p) = 0.030 V while at intermediate pH the electrochemical exchange reaction (Me-(w)(2+) + HX(o) reversible arrow MeX(o)+ + H-(w)(+)) is proposed. The net charge transfer of + 1 at the interface accounts for DeltaE(p) = 0.060 V for the peak observed DeltaE(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. (C) 2000 Elsevier Science B.V. All rights reserved.