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 e xchange voltammetry to trace determination of small ionic species with low ion exchange selectivity coefficients, dissolved in electrolyte-f ree media, has been adopted to verify whether the degrees of preconcen tration achieved with such a voltammetric technique actually increase with the decrease of the sample ionic strength-in agreement with therm odynamic expectations-or whether they are otherwise conditioned by kin etic complications. With this purpose, a series of experiments has bee n conducted by accumulating either monovalent or divalent cationic ana lytes (Tl+ and Pb2+) at glassy carbon electrodes modified with Nafion( R) films loaded with either monovalent or divalent cations (Na+ and Ca 2+) from aqueous solutions containing different concentrations of both the analyte and the loading ion introduced as the cation of the suppo rting electrolyte. These preconcentrated ions were then assayed by ins erting the modified electrodes in the solid state cell mentioned. The results obtained indicate that the expected inverse proportional relat ion between the degree of preconcentration achieved and the ionic stre ngth of the sample analyzed is found only at fairly high concentration s of supporting electrolyte, the values of which depend on both the ch arge and concentration of the analyte as well as on the charge of the counterion from the supporting electrolyte. In contrast, at lower ioni c 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 e quilibrium constants, owing to cross-linking changes involved in the i on exchange processes. Also the outcome of such kinetic control on ana lytical determinations of trace ionic species in poorly conducting sam ples is considered.