Ion selectivity coefficient

Both the normal and expanded forms of Nafion exhibit the expected order of selectivities of alkali metal ions for a sulfonate ion exchanger. Perhaps the most interesting feature of these values is their large spread compared to conventional sulfonate resins. For example, a 16% cross-linked polystyrene sulfonate has metal ion-hydrogen ion selectivity coefficients equal to 0.680 (Li+), 1.61 (Na+), 3.06 (K+), 3.14 (Rb+), and 3.17 (Cs+) (12). The selectivity coefficients of Ag+ and Tl+ for Nafion are lower than those of polystyrene sulfonate resins of... 

Table I. Univalent Ion Selectivity Coefficients and Water Contents for 1200 EW Nafion.

Although other sources of entropy change are to be found in AS°, these would remain relatively constant for various metal ions compared to ASjj2q. This relationship is shown in Figure 3 for normal and expanded forms of 1200 EW Nafion using both univalent and divalent ion selectivity coefficients. Lines are least squares fits for alkali metal ions and alkaline earth ions. 

R. B. Barrett It appears that the variation of the trace ion selectivity coefficient is a linear function of internal alcohol concentration. 

In some cases the distribution constants, as well as ion selectivity coefficients, have been measured independently, making the subsequent analysis of kinetic data less dependent on adjustable parameters. 

Several methods have been proposed for the experimental determination of ion-selectivity coefficient. They can be listed in the two groups separate solution methods and mixed solution methods. Applying both methods, the cell voltages (emf) are measured in a cell when the reference electrodes and the ISEs are dipped in solutions of known composition. When the separate solution method is used, pure primary ion solutions or pure solutions of the tested interfering ion are introduced into the measuring cell, while in mixed solution methods, both the primary and the selected interfering ions are introduced. 

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. 

Selectivity As described earlier, most ion-selective electrodes respond to more than one analyte. For many ion-selective electrodes, however, the selectivity for the analyte is significantly greater than for most interfering ions. Published selectivity coefficients for ion-selective electrodes (representative values are found in Tables 11.1 through 11.3) provide a useful guide in helping the analyst determine whether a potentiometric analysis is feasible for a given sample. 

Creager and colleagues designed a salicylate ion-selective electrode using a PVC membrane impregnated with tetraalkylammonium salicylate. To determine the ion-selective electrode s selectivity coefficient for benzoate,... 

The equilibrium constant for this ion-exchange reaction, which is also called the selectivity coefficient, is... 

Ion-exchange isotherms assume different shapes depending on the selectivity factor and the variations in with the level of exchange The rational selectivity coefficient includes the ionic charge and is given by... 

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. 

The value of for calcium hydroxyapatite can be defined by charge of Ca + and PO ions. From this point of view calcium hydroxyapatite can be used as high-selective adsorbents for high performance liquid chromatography because with increasing of will be rise a selectivity coefficient a. 

R indicates the organic portion of the resin and SO3 is the immobile portion of the ion active group. Two resin sites are needed for nickel ions with a plus 2 valence (Ni ). Trivalent ferric ions would require three resin sites. As shown, the ion exchange reaction is reversible. The degree the reaction proceeds to the right will depend on the resins preference, or selectivity, for nickel ions compared with its preference for hydrogen ions. The selectivity of a resin for a given ion is measured by the selectivity coefficient. K. which in its simplest form for the reaction... 

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

According to the nine assumptions and approach a) for the diffusion potential inside the membrane the selectivity coefficient Kg , can be expressed by other parameters. Table 3 shows the results for the different kinds of membranes 66). In some cases the expressions for K J i contain ion-mobilities inside the membrane... 

The process is a reversible one and for ions of like charge the selectivity coefficient, K, is defined by ... 

---