SUCCESSIVE COMPLEX-FORMATION OF MULTIVALENT IONS WITH OCTAETHYLENE GLYCOL DODECYL ETHER AT THE NITROBENZENE WATER INTERFACE

Transfer of Mg2+ and La3+ ions across the nitrobenzene (NB)/water (W) interface facilitated by octaethylene glycol monododecyl ether (C12E8) has been studied using cyclic voltammetry under conditions where the concentration of C12E8 in NB((b)c(C12E8)NB) is much smaller than the concentration of Mg2+ or La3+ ions in W. The shape of the voltammograms is markedly dependent on (b)c(C12E8)NB. When (b)c(C12E8)NB greater-than-or-equal-to 1 mM, the voltammograms exhibit a clear shoulder on both forward and reverse scans of the potential. For La3+ transfer, the convolution voltammograms exhibit two-step waves. The dependence of wave shape on ligand concentration indicates the successive formation of 1:2 (metal:ligand) and 1:1 complexes in the vicinity of the interface. The first wave is ascribed to the formation of the 1:2 (metal: ligand) complex, and the second wave to the 1:1 complex. This double wave merges into a single peak, when (b)c(C12E8)NB less-than-or-equal-to 0.2 mM. From the midpoint potentials and the concentration dependence of the peak separation, stability constants for the 1:1 and 1:2 complexes in NB are determined to be log10(K1/M-1) = 8.8 and log10(K2/M-1) = 3.3 for Mg2+ and log10(K1/M-1) = 11.0 and log10(K2/M-1) = 4.4 for La3+, respectively.