Selectivity coefficients of ion-selective

V. P. Y. Gadzekpo and G. D. Christian, Determination of Selectivity Coefficients of Ion-Selective Electrodes by a Matched-Potential Method. Anal. Chim. Acta, 166 (1984) 279. 

Tohda, K. Pragoe. D. Hibata. M. Umezawa. Y. Studies on the matched potential method for determining the selectivity coefficients of ion-selective electrodes based on neutral ionophores Experimental and theoretical verification. Anal. Sci. 2001, 17, 733-743. 

Umezawa, Y. Biihlmann, P. Umezawa. K. Tohda, K. Amemiya, S. Potentiometric selectivity coefficients of ion-selective electrodes. Part I. Inorganic cations. Pure Appl. Chem. 2000. 72. 1852-2082. 

Umezawa Y, Umezawa K, Bublmann P, Hamada N, Aoki H, Nakanisbi J, Moritosbi S, Xia KP, and Nishimura Y (2002) Potentiometric selectivity coefficients of ion-selective electrodes. Part II. Inorganic anions. lUPAC, Pure and Applied Chemistry 74 923-994. 

Gadzekpo, V.P.Y. and Christian, G.D. (1984) Determination of selectivity coefficients of ion-selective electrodes by a matched-potential method. Anal. Chim. Acta, 164, 279-282. 

Here Yi and y2 are the activity coefficients of ions in solution, y, and y2 are the coefficients of resin activity, cx and c2 are ion concentrations in solution, ntj and m2 are fixed ion concentrations (exchange or weight concentrations) and Ks is the concentration constant of ion exchange, the selectivity constant. 

Table 26-3 Relative selectivity coefficients of ion-exchange resins... 

The fact that the selective uptake of metal ion by the ion exchanger is associated with highly coordinated species undetectable in the solution, even vtiien ligand concentrations in the two phases have been kept similar, has to be attributed to either the immobilization of these species by their interaction with the organic component of the exchanger matrix or to sizable reduction of activity coefficients of ion species in the resin phase because of the lowering of the dielectric constant of the resin phase media. 

In effect, ion exchange chromatography exploits differences in the degree of charge and affinity (measured by the reaction equilibrium constant or molar selectivity coefficient) of ions in solution for sites of opposite charge in the stationary phase to accomplish separation. 

This is a product of the ratio of the mobility of ion A to that of ion B in the membrane phase and the selectivity coefficient of ion A to ion B. In electrodialysis, diffusion boundary layers are formed at membrane surfaces, where the concentration at the membrane surfaces is not the same as that of the bulk solution. To obtain the real PAB of the ion exchange membrane, the diffusion boundary layer should be eliminated by vigorous agitation of the solution, which corresponds to the limiting value at zero current density,... 

Because the specific conductivity k (S/m) of an electrolyte is determined readily and easy, this property is widely used for optimizing the battery performance. In contrast, other parameters which are more difficult to obtain, e.g., diffusion coefficients of ions near to or in the electrode materials or transference numbers of ions, are seldom studied and not yet included in optimization. We expect that automated measurement systems will be used in the future to optimize this and other critical parameters of solutions as long as no valid theoretical approach is available. These systems should be able to measure selected quantities automatically as a function of temperature and composition of solutions according to proposals made by optimization methods such as simplex. First steps on this way were undertaken by Schweiger et al., who presented an equipment that is able to measure K(T(t)) and T(t) automatically in up to 32 cells [34-38]. 

Membrane compositions and selectivity coefficients of ion-exchanger-based ISEs... 

The more a deviates from unity for a given pair of ions, the easier it will be to separate them. If the selectivity coefficient is unfavorable for the separation of two ions of the same charge, no variation in the concentration of H+ (the eluant) will improve the separation. 

The main chemico-analytical properties of the designed ionoselective electrodes have been determined. The work pH range of the electrodes is 1 to 5. The steepness of the electrode function is close to the idealized one calculated for two-charged ions (26-29 mV/pC). The electrode function have been established in the concentration range from 0.1 to 0.00001 mole/1. The principal advantage of such electrodes is the fact that thiocyanate ions are simultaneously both complexing ligands and the ionic power. The sensitivity (the discovery limits), selectivity (coefficient of selectivity) and the influence of the main temporal factors (drift of a potential, time of the response, lifetime of the membranes) were determined for these electrodes. 

Recent publications indicate the cloud-point extraction by phases of nonionic surfactant as an effective procedure for preconcentrating and separation of metal ions, organic pollutants and biologically active compounds. The effectiveness of the cloud-point extraction is due to its high selectivity and the possibility to obtain high coefficients of absolute preconcentrating while analyzing small volumes of the sample. Besides, the cloud-point extraction with non-ionic surfactants insures the low-cost, simple and accurate analytic procedures. 

The so-called potentiometric selectivity coefficient K " reflects the non-ideal behavior of ion-selective membranes and determines the specificity of this electro-... 

One of the most rational means for displacing a broad zone is electrolyte desorption under the conditions of decreasing degree of ionization, i.e., when counterions are converted into dipolar ions, uncharged molecules and coions. This conversion corresponds to a sharp decrease in distribution coefficients of the desorbed substance. Hence, the displacement of equilibrium parame ters at a high rate of mass-exchange is one of the methods of selective stepwise chromatography. 

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