Elena A. Zdrachek

Improved Separate Solution Method for Determination of Low Selectivity Coefficients

Simple, fast, and theoretically substantiated experimental method for determination of improved selectivity coefficients is proposed. The method is based on the well-known fact that low selectivity coefficients determined by the separate solution method (SSM) are time-dependent and, upon our finding, this dependence is a well-defined linear function of time raised to the certain negative power. In particular, the selectivity coefficients obtained for equally charged primary and foreign ions by SSM linearly depend on time to the minus one-fourth. It was found that extrapolation of experimental data using this function to the intersection with Y axes gives reliable values of rather low selectivity coefficients (down to n × 10(-7)), which strongly differ from those measured using SSM and correspond well with the values obtained using the modified separate solution method (MSSM) proposed by Bakker. At the same time, the new method is free of one very essential limitation inherent to MSSM, namely, it is applicable after the conditioning of electrodes in the primary ion solution and can be repeated many times.

Potentiometric selectivity coefficients: Problems of experimental determination

The main problems arising in the determination of selectivity coefficients of ion-selective electrodes using the methods recommended by IUPAC are considered. A method for the determination of the limiting (thermodynamically justified) selectivity coefficients based on the analysis of the time dependence of the selectivity coefficients found experimentally using the separate solutions method is justified theoretically and experimentally. It has been demonstrated that the proposed method ensures the reliable determination of low values of selectivity coefficients (as low as n × 10−6). The criteria of the applicability of the proposed method are formulated and a particular algorithm of its implementation is proposed.

Modeling of the effect of diffusion processes on the response of ion-selective electrodes by the finite difference technique: Comparison of theory with experiment and critical evaluation

Experimental data are compared with the results of calculations by the finite difference technique within the dynamic diffusion model of the interphase potential on an example of a picrate-selective electrode in real scenarios corresponding to the conditions of the determination of selectivity coefficients by the methods recommended by IUPAC. It was found that, in the majority of the considered cases, the calculated values of the potential and selectivity coefficients and also the dynamics of potential change at particular steps well agree with the experimental data. The model has principal restrictions, leading the failure of calculations, when the concentration of potential-determining ions in the near-electrode layer of the solution performed is low according to the algorithm of measurements, whereas the instant increase in its concentration in the surface membrane layer due to the replacement of the sample solution induces a flux of these ions from the surface deep into of the membrane.

Time-Dependent Determination of Unbiased Selectivity Coefficients of Ion-Selective Electrodes for Multivalent Ions

A new method for the determination of unbiased low selectivity coefficients for two of the most prevalent cases of multivalent ions (zi = 2, zj = 1 and zi = 1, zj = 2) was theoretically and experimentally substantiated. The method is based on eliminating the primary ion concentration near the membrane by extrapolating the linearized time dependencies of selectivity coefficients determined by the separate solutions method (KijPot(SSM) as a function of t-1/3 or t-1/6, depending on the charge combination of the two ions, to infinite time. The applicability of the method is demonstrated for ionophore-based Mg2+-, Ca2+-, and Na+-selective electrodes. It is shown that the high level of primary ion impurities in the salts of interfering ions can significantly limit the efficiency of the technique, as demonstrated with salts of different purity levels.

A Simple Dynamic Diffusion Model of the Response of Highly Selective Electrodes: The Effect of Simulation Parameters and Boundary Conditions on the Results of Calculations

For a tetrabutylammonium-selective electrode with a ion-exchange membrane, in the real-work scenario corresponding to the determination of selectivity coefficients by the IUPAC-recommended method of separate solutions, it is shown that of the results of calculations obtained within the framework of the dynamic diffusion model based on the use of the finite-difference technique substantially depend on of the chosen boundary conditions and the values of arbitrarily set simulation parameters. The key parameter that determines the quality of simulation results is the thickness of the elementary layer in the membrane phase, especially for low diffusion coefficients. It is found that the use of thin elementary layers in membranes and thick elementary layers in the aqueous phase makes it possible to combine the high quality with the high calculation rate. In simulating the long-term experiments, account should be taken of the accumulation of the potential-determining ion in the aqueous solution volume as a result of its displacement by a foreign ion from the membrane. A good correspondence between calculation data and experimental results is demonstrated.