EFFECT OF THE SAMPLE IONIC-STRENGTH ON THE PRECONCENTRATION ATTAINED IN ION-EXCHANGE VOLTAMMETRY

A solid state cell proposed elsewhere, enabling the extension of ion exchange voltammetry to trace determination of small ionic species with low ion exchange selectivity coefficients, dissolved in electrolyte-free media, has been adopted to verify whether the degrees of preconcentration achieved with such a voltammetric technique actually increase with the decrease of the sample ionic strength-in agreement with thermodynamic expectations-or whether they are otherwise conditioned by kinetic complications. With this purpose, a series of experiments has been conducted by accumulating either monovalent or divalent cationic analytes (Tl+ and Pb2+) at glassy carbon electrodes modified with Nafion(R) films loaded with either monovalent or divalent cations (Na+ and Ca2+) from aqueous solutions containing different concentrations of both the analyte and the loading ion introduced as the cation of the supporting electrolyte. These preconcentrated ions were then assayed by inserting the modified electrodes in the solid state cell mentioned. The results obtained indicate that the expected inverse proportional relation between the degree of preconcentration achieved and the ionic strength of the sample analyzed is found only at fairly high concentrations of supporting electrolyte, the values of which depend on both the charge and concentration of the analyte as well as on the charge of the counterion from the supporting electrolyte. In contrast, at lower ionic strengths, the degree of preconcentration is kinetically controlled. This finding is discussed in terms of interdiffusion effects as well as of the decrease in both diffusion coefficients in the polymer and equilibrium constants, owing to cross-linking changes involved in the ion exchange processes. Also the outcome of such kinetic control on analytical determinations of trace ionic species in poorly conducting samples is considered.