Comparative analysis of alkali and alkaline-earth cation transfer assistedby monensin across the water vertical bar 1,2-dichloroethane interface

In the first part of this paper the electrochemical transfer of alkali cati ons (M+) assisted by monensin (HX) across the water \ 1,2-dichloroethane (D CE) 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 t hese cations. At pH > 9 the current is voltammetrically negligible. An equa tion for the dependence of Delta (w)(o)phi (1/2) on pH and Na+ concentratio n is developed. In the second part of the gaper the transfer of alkaline ea rth 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 Delta E(p) = 0.030 V while at intermediate pH the electrochemical exchange reacti on (Me-(w)(2+) + HX(o) reversible arrow MeX(o)+ + H-(w)(+)) is proposed. Th e net charge transfer of + 1 at the interface accounts for DeltaE(p) = 0.06 0 V for the peak observed DeltaE(p) in agreement with the hyper-Nernstian s lope of 60 mV found for Ca2+ and Ba2+ ISE based on antibiotics with carboxy lic groups at intermediate pH values. The higher selectivity for Ba2+. and the tendency in selectivity at alkaline pH found in the ISEs are also obser ved and thus explained according to the mechanism proposed. (C) 2000 Elsevi er Science B.V. All rights reserved.