R. Toniolo et al., EFFECT OF THE SAMPLE IONIC-STRENGTH ON THE PRECONCENTRATION ATTAINED IN ION-EXCHANGE VOLTAMMETRY, Journal of electroanalytical chemistry [1992], 356(1-2), 1993, pp. 67-80
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.